The Danish National Birth Cohort--its background, structure and aim

Maternal intake of paracetamol during pregnancy and biomarkers of male fecundity in young adult sons

Paracetamol is suggested to have endocrine disrupting properties possibly affecting fetal programming of reproductive health that might lead to impaired semen quality and changes in reproductive hormones. In this longitudinal study, we included 1058 young adult men born 1998-2000 into the Danish National Birth Cohort with follow-up at 18-21 years of age. The exposure, maternal intake of paracetamol, was modelled in three ways: dichotomized, trimester-specific, and as duration of exposure categorized into: short (1-2 weeks), medium (3-9 weeks) or long duration (>9 weeks) vs. no intake. Outcomes included semen characteristics, self-measured testis volume, and reproductive hormone levels. We used negative binominal regression to estimate the percentage difference and 95% confidence interval (CI) for each outcome. In total, 547 (48%) sons were prenatally exposed to paracetamol due to maternal intake at least once. Maternal intake of paracetamol during pregnancy was not associated with any of the biomarkers in the dichotomized or trimester-specific exposure models. For duration of exposure, sons of mothers with long duration of maternal intake of paracetamol showed tendencies towards lower semen concentration (-14% [95% CI: -31%; 8%]), a higher proportion of nonprogressive and immotile spermatozoa (8% [95% CI: -4%; 21%]) and higher DNA Fragmentation Index (16% [95% CI: -1%; 36%]) compared to son of mothers with no intake. Maternal intake of paracetamol during pregnancy was not clearly associated with biomarkers of male fecundity in adult sons. However, it cannot be ruled out that long duration of maternal intake of paracetamol might be associated with impaired semen characteristics.

Sleep duration and biomarkers of fecundity in young men: a cross-sectional study from a population-based cohort

BACKGROUND

Poor male fecundity is of concern and warrants the identification of potential modifiable risk factors. Short and long sleep duration might be risk factors for poor male fecundity although evidence in this research field is inconsistent.

OBJECTIVES

To investigate the association between sleep duration and biomarkers of male fecundity in young men.

MATERIALS AND METHODS

We conducted a cross-sectional study of 1,055 young men from the Fetal Programming of Semen Quality (FEPOS) cohort, Denmark, 2017-2019. Sleep duration was obtained from an online survey answered by the participants prior to the clinical visit, where semen and blood samples were obtained, and testis volume was self-assessed using an Orchidometer. Percentage differences in semen characteristics, testes volume, and reproductive hormone levels were analysed according to sleep duration using multivariable negative binomial regression models. Sleep duration was dichotomised (recommended (6-9 h/night) versus deviant sleep) and visualised continuously as restricted cubic spline plots.

RESULTS

Deviation from recommended sleep duration was associated with higher high DNA stainability (HDS) of 5% (95% CI: -1%; 13%), higher testosterone of 3% (95% CI: 0%; 7%) and higher free androgen index (FAI) of 6% (95% CI: 0%; 13%). The spline plots overall supported these results, suggesting u-shaped associations between sleep duration and HDS, testosterone and FAI, a linear association between sleep duration and semen volume and sex hormone binding globulin (SHBG) and an inverse u-shaped association with normal morphology.

DISCUSSION

Information on sleep duration was obtained by self-report in broad categories with at least 3 h intervals. We were not able to investigate short or long sleep duration separately, since only few participants reported this.

CONCLUSION

Sleep duration was associated with some biomarkers of fecundity in young men. Maintaining a recommended sleep duration may thus be beneficial for young men with regard to reproductive health.

Adherence to different forms of plant-based diets and pregnancy outcomes in the Danish National Birth Cohort: A prospective observational study

INTRODUCTION

The number of people adhering to plant-based diets has been increasing dramatically in recent years, fueled by both environmental and animal welfare concerns. Beneficial or possible adverse consequences of such diets, particularly the most restrictive forms during pregnancy, have been minimally explored. The aim of this prospective observational study was to examine associations between different forms of plant-based diets during pregnancy with birth outcomes and pregnancy complications.

MATERIAL AND METHODS

The Danish National Birth Cohort included 100 413 pregnancies to 91 381 women in 1996-2002. The population consisted of 66 738 pregnancies, about which sufficient dietary data were available and included in the study. Dietary and supplemental intake was assessed by Food Frequency Questionnaire in gestational week 25 and women were characterized as fish/poultry-vegetarians, lacto/ovo-vegetarians, vegans or omnivorous, based on their self-report in gestational week 30. Main outcome measures were pregnancy and birth complications, birthweight and small for gestational age.

RESULTS

A total of 98.7% (n = 65 872) of participants were defined as omnivorous, whereas 1.0% (n = 666), 0.3% (n = 183) and 0.03% (n = 18) identified themselves as fish/poultry vegetarians, lacto/ovo-vegetarians or vegans, respectively. Protein intake was lower among lacto/ovo-vegetarians (13.3%) and vegans (10.4%) than among omnivorous participants (15.4%). Intake of micronutrients was also considerably lower among vegans, but when dietary supplements were taken into consideration, no major differences were observed. Compared with omnivorous mothers, vegans had a higher prevalence of preeclampsia and their offspring had on average -240 g (95% confidence interval -450 to -30) lower birthweight.

CONCLUSIONS

The women reporting that they adhered to vegan diets during pregnancy had offspring with lower mean birthweight and higher risk of preeclampsia compared with omnivorous mothers. Low protein intake might be one plausible explanation for the observed association with birthweight.

No association between long-chain n-3 fatty acid intake during pregnancy and risk of type 1 diabetes in offspring in two large Scandinavian pregnancy cohorts

AIMS/HYPOTHESIS

The aim of this study was to investigate whether higher dietary intake of marine n-3 fatty acids during pregnancy is associated with a lower risk of type 1 diabetes in children.

METHODS

The Danish National Birth Cohort (DNBC) and the Norwegian Mother, Father and Child Cohort Study (MoBa) together include 153,843 mother-child pairs with prospectively collected data on eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) intake during pregnancy from validated food frequency questionnaires. Type 1 diabetes diagnosis in children (n=634) was ascertained from national diabetes registries.

RESULTS

There was no association between the sum of EPA and DHA intake during pregnancy and risk of type 1 diabetes in offspring (pooled HR per g/day of intake: 1.00, 95% CI 0.88, 1.14), with consistent results for both the MoBa and the DNBC. Robustness analyses gave very similar results.

CONCLUSIONS/INTERPRETATION

Initiation of a trial of EPA and DHA during pregnancy to prevent type 1 diabetes in offspring should not be prioritised.

Mode of birth and long-term maternal mental health: A follow-up study in the Danish National Birth Cohort

BACKGROUND

Cesarean birth has been associated with increased risks of short-term mental health problems. Little is known about whether these associations persist in the long term. This study aimed to estimate the associations between mode of birth and maternal mental health in midlife while considering mental health before and during pregnancy.

METHODS

Cohort study among mothers in the Danish National Birth Cohort. Birth mode for each woman's entire reproductive history was obtained from Danish national registries. Symptoms of depression and stress in midlife were self-reported using validated scales. Log binomial regression was used to calculate risk ratios (RR) with 95% confidence intervals (CI) for the association between birth mode and depressive symptoms. Linear regression was used to calculate mean difference in stress score by birth mode.

RESULTS

Among 42,872 women, 15.5% reported depressive symptoms at follow-up, where they were, on average, 43.9 years and 11.2 years after their last birth. Compared with women who only ever had spontaneous vaginal births, women who only had cesarean births, or had both cesarean and vaginal births with the last birth by cesarean, reported slightly more symptoms of depression (RR 1.10, 95% CI 1.01;1.20) and stress (mean difference 0.68 on a 100-point scale, 95% CI 0.10;1.26).

CONCLUSION

Whether due to the birth experience or underlying factors, depression and stress in midlife were more frequent in women with only cesarean births or whose last birth was by cesarean compared with women with vaginal births.

Stress and anxiety during pregnancy and length of gestation: a federated study using data from five Canadian and European birth cohorts

While its etiology is not fully elucidated, preterm birth represents a major public health concern as it is the leading cause of child mortality and morbidity. Stress is one of the most common perinatal conditions and may increase the risk of preterm birth. In this paper we aimed to investigate the association of maternal perceived stress and anxiety with length of gestation. We used harmonized data from five birth cohorts from Canada, France, and Norway. A total of 5297 pregnancies of singletons were included in the analysis of perceived stress and gestational duration, and 55,775 pregnancies for anxiety. Federated analyses were performed through the DataSHIELD platform using Cox regression models within intervals of gestational age. The models were fit for each cohort separately, and the cohort-specific results were combined using random effects study-level meta-analysis. Moderate and high levels of perceived stress during pregnancy were associated with a shorter length of gestation in the very/moderately preterm interval [moderate: hazard ratio (HR) 1.92 (95%CI 0.83, 4.48); high: 2.04 (95%CI 0.77, 5.37)], albeit not statistically significant. No association was found for the other intervals. Anxiety was associated with gestational duration in the very/moderately preterm interval [1.66 (95%CI 1.32, 2.08)], and in the early term interval [1.15 (95%CI 1.08, 1.23)]. Our findings suggest that perceived stress and anxiety are associated with an increased risk of earlier birth, but only in the earliest gestational ages. We also found an association in the early term period for anxiety, but the result was only driven by the largest cohort, which collected information the latest in pregnancy. This raised a potential issue of reverse causality as anxiety later in pregnancy could be due to concerns about early signs of a possible preterm birth.

Reproductive history of parous women and urinary incontinence in midlife: A National Birth Cohort follow-up study

OBJECTIVE

To investigate how reproductive history was associated with urinary incontinence in midlife.

DESIGN

A follow-up study.

SETTING

Denmark.

POPULATION

A total of 39 977 mothers who participated in the Maternal Follow up (2013-2014) in the Danish National Birth Cohort. National registries provided their reproductive history.

METHODS

How parity, mode of birth and obstetric tears associated with urinary incontinence were estimated with adjusted odds ratios (OR) and 95% CI using logistic regression.

MAIN OUTCOME MEASURES

Self-reported urinary incontinence including subtypes stress, urge and mixed urinary incontinence.

RESULTS

At an average age of 44 years, the prevalence of any urinary incontinence was 32% (21% stress, 2% urge, and 8% mixed urinary incontinence). Women with two births more often had urinary incontinence than women with one birth (OR 1.20, 95% CI 1.10-1.31). Compared with women with only spontaneous births, a history of only caesarean sections was associated with much lower odds of urinary incontinence (OR 0.39, 95% CI 0.35-0.42) and a history of instrumental births with slightly lower odds (OR 0.92, 95% CI 0.86-0.98). Compared with no tear/first-degree tear as the largest tear, episiotomy was associated with less urinary incontinence (OR 0.91, 95% CI 0.86-0.97) whereas third/fourth-degree tears were associated with more (OR 1.14, 95% CI 1.04-1.25). Findings were mainly explained by similar associations with stress and mixed urinary incontinence.

CONCLUSIONS

Vaginal birth was associated with a higher risk of long-term urinary incontinence, but our results indicate that this risk may be reduced by shortening the second stage of birth.

Association of common maternal infections with birth outcomes: a multinational cohort study

PURPOSE

It is unclear whether common maternal infections during pregnancy are risk factors for adverse birth outcomes. We assessed the association between self-reported infections during pregnancy with preterm birth and small-for-gestational-age (SGA) in an international cohort consortium.

METHODS

Data on 120,507 pregnant women were obtained from six population-based birth cohorts in Australia, Denmark, Israel, Norway, the UK and the USA. Self-reported common infections during pregnancy included influenza-like illness, common cold, any respiratory tract infection, vaginal thrush, vaginal infections, cystitis, urinary tract infection, and the symptoms fever and diarrhoea. Birth outcomes included preterm birth, low birth weight and SGA. Associations between maternal infections and birth outcomes were first assessed using Poisson regression in each cohort and then pooled using random-effect meta-analysis. Risk ratios (RR) and 95% confidence intervals (CI) were calculated, adjusted for potential confounders.

RESULTS

Vaginal infections (pooled RR, 1.10; 95% CI, 1.02-1.20) and urinary tract infections (pooled RR, 1.17; 95% CI, 1.09-1.26) during pregnancy were associated with higher risk of preterm birth. Similar associations with low birth weight were also observed for these two infections. Fever during pregnancy was associated with higher risk of SGA (pooled RR, 1.07; 95% CI, 1.02-1.12). No other significant associations were observed between maternal infections/symptoms and birth outcomes.

CONCLUSION

Vaginal infections and urinary infections during pregnancy were associated with a small increased risk of preterm birth and low birth weight, whereas fever was associated with SGA. These findings require confirmation in future studies with laboratory-confirmed infection diagnosis.

Breastfeeding Duration Is Not Associated With Offspring Inflammatory Bowel Disease Risk in Three Population-Based Birth Cohorts

BACKGROUND & AIMS

Breastfeeding is critical for offspring health and development. Although many observational studies report a protective effect between breastfeeding and inflammatory bowel disease (IBD), the relationship is not well-understood.

METHODS

We used prospectively collected data from 3 population-based birth cohorts (Danish National Birth Cohort, Norwegian Mother, Father, and Child Cohort, and All Babies in Southeast Sweden) and cross-linked national registers to ascertain the impact of breastfeeding duration on offspring IBD risk in each country, using adjusted Cox proportional regression analyses. We performed meta-analyses to determine pooled estimates.

RESULTS

We included 148,737 offspring and 169,510 offspring in analyses of exclusive and any breastfeeding duration, respectively. During median follow-up of 16.3-22.3 years, between 1996 and 2021, 543 offspring were diagnosed with IBD. In each country, there was no association between exclusive breastfeeding duration and offspring IBD risk after adjusting for birth year (Denmark), offspring sex, parental IBD status, maternal education, smoking during pregnancy, age at delivery, mode of delivery, preterm birth, and small for gestational age. The pooled adjusted hazard ratio for IBD was 1.24 (95% confidence interval, 0.94-1.62; Q = 0.16, I2 = 0.0%) and 1.02 (95% confidence interval, 0.85-1.21; Q = 1.45, I 2= 0.0%) among offspring breastfed exclusively for ≥6 months and <4 months, respectively, compared with 4-5 months. Similarly, we found null associations in pooled analyses of any breastfeeding duration and IBD, subtypes Crohn's disease and ulcerative colitis, as well as in cohort-specific analyses.

CONCLUSIONS

In prospectively collected data from 3 population-based birth cohorts, the duration of exclusive or any breastfeeding was not associated with offspring IBD risk.

Characteristics of Puberty in a Population-Based Sample of Danish Adolescents

PURPOSE

To describe the duration, timing, tempo, and synchronicity of puberty, as well as the correlation between timing and tempo of puberty.

METHODS

Overall, 15,819 of 22,439 invited children participated in the Puberty Cohort within the Danish National Birth Cohort. Participants completed a web-based questionnaire every 6 months through maturation with questions on current pubertal status. Girls reported current Tanner stage of breast and pubic hair development, and timing of menarche. Boys reported current Tanner stage of genital and pubic hair development, timing of first ejaculation, and vocal changes. While accounting for this interval-censored puberty information, we estimated the duration of puberty. Then, we used a nonlinear mixed effect growth model to estimate timing, tempo, synchronicity of puberty, and correlation between timing and tempo of puberty.

RESULTS

In girls, the average duration of breast development was longer, whereas the average tempo was slower than pubic hair development. The average timing of breast development was earlier than the average timing of pubic hair development. The majority of girls had asynchronous puberty. In boys, the average duration was longer and average tempo slower for genital than pubic hair development. The average timing of genital and pubic hair development were comparable; hence, the majority had synchronous pubertal development. Adolescents who had earlier timing also tended to have a faster tempo.

DISCUSSION

Being one of the largest puberty cohorts worldwide, these unique contemporary data can help physicians, parents, and children to understand and anticipate expected progression through pubertal development.

Association of Fetal Lung Development Disorders with Adult Diseases: A Comprehensive Review

Fetal lung development is a crucial and complex process that lays the groundwork for postnatal respiratory health. However, disruptions in this delicate developmental journey can lead to fetal lung development disorders, impacting neonatal outcomes and potentially influencing health outcomes well into adulthood. Recent research has shed light on the intriguing association between fetal lung development disorders and the development of adult diseases. Understanding these links can provide valuable insights into the developmental origins of health and disease, paving the way for targeted preventive measures and clinical interventions. This review article aims to comprehensively explore the association of fetal lung development disorders with adult diseases. We delve into the stages of fetal lung development, examining key factors influencing fetal lung maturation. Subsequently, we investigate specific fetal lung development disorders, such as respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD), congenital diaphragmatic hernia (CDH), and other abnormalities. Furthermore, we explore the potential mechanisms underlying these associations, considering the role of epigenetic modifications, transgenerational effects, and intrauterine environmental factors. Additionally, we examine the epidemiological evidence and clinical findings linking fetal lung development disorders to adult respiratory diseases, including asthma, chronic obstructive pulmonary disease (COPD), and other respiratory ailments. This review provides valuable insights for healthcare professionals and researchers, guiding future investigations and shaping strategies for preventive interventions and long-term care.

Birth by caesarean section and semen quality in adulthood: a Danish population-based cohort study

BACKGROUND

The caesarean section (CS) rate has increased worldwide and there is an increasing public and scientific interest in the potential long-term health consequences for the offspring. CS is related to persistent aberrant microbiota colonization in the offspring, which may negatively interfere with sex hormone homeostasis and thus potentially affect the reproductive health. It remains unknown whether adult sons' semen quality is affected by CS. We hypothesize that CS is associated with lower semen quality.

METHODS

This study was based on the Fetal Programming of Semen Quality cohort (FEPOS, enrolled from 2017 to 2019) nested within the Danish National Birth Cohort (DNBC, enrolled from 1996 to 2002). A total of 5697 adult sons of mothers from the DNBC were invited to the FEPOS cohort, and 1044 young men participated in this study. Information on mode of delivery was extracted from the Danish Medical Birth Registry, and included vaginal delivery, elective CS before labor, emergency CS during labor and unspecified CS. The young men provided a semen sample for analysis of semen volume, sperm concentration, motility and morphology. Negative binomial regression models were applied to examine the association between CS and semen characteristics with estimation of relative differences in percentages with 95% confidence intervals (CIs).

RESULTS

Among included sons, 132 (13%) were born by CS. We found a slightly lower non-progressive sperm motility (reflecting higher progressive sperm motility) among sons born by CS compared to sons born by vaginal delivery [relative difference (95% CI): - 7.5% (- 14.1% to - 0.4%)]. No differences were observed for other sperm characteristics. When CS was further classified into elective CS, emergency CS and unspecified CS in a sensitivity analysis, no significant differences in non-progressive motility were observed among sons born by any of the three types of CS compared to sons born vaginally.

CONCLUSIONS

This large population-based cohort study found no significant evidence for an adverse effect on semen quality in adult sons born by CS.

Urinary Incontinence in Midlife According to Weight Changes Across and After Childbearing Years

INTRODUCTION AND HYPOTHESIS

The objective was to investigate how weight change across and after the childbearing years was associated with urinary incontinence (UI) in midlife.

METHODS

Data were obtained from 35,645 women responding to the Maternal Follow-up questionnaire in the Danish National Birth Cohort in 2013-2014. Outcome was self-reported UI and its subtypes. Exposures were changes in body mass index (BMI) across and after the childbearing years. Adjusted odds ratios were estimated using logistic regression.

RESULTS

At follow-up, the mean age was 44 years and 32% experienced UI. Compared with stable weight, weight gain across the childbearing years of > 1 to 3, > 3 to 5 or > 5 BMI units increased the odds of any UI by 15%, 27%, and 41% respectively. For mixed UI, the odds increased by 23%, 41%, and 68% in these groups. Weight gain after childbearing showed the same pattern, but with a higher increase in the odds of mixed UI (25%, 60%, and 95% in the respective groups). Women with any weight loss during this period had 9% lower odds of any UI than women with a stable weight.

CONCLUSIONS

Weight gain across and after childbearing increased the risk of UI in midlife, especially the subtype mixed UI. Weight loss after childbearing decreased the risk.

Residential exposure to mold, dampness, and indoor air pollution and risk of respiratory tract infections: a study among children ages 11 and 12 in the Danish National Birth Cohort

BACKGROUND

The burden of respiratory tract infections (RTIs) is high in childhood. Several residential exposures may affect relative rates.

OBJECTIVES

To determine risk of RTIs in children ages 11 and 12 by residential exposures.

METHODS

We included children in the Danish National Birth Cohort (DNBC) at ages 11 and 12. We estimated incidence risk ratios (IRR) and 95% confidence intervals (CI) for counts of RTIs within the last year by exposure to mold/dampness, gas stove usage, summer and winter candle-burning, fireplace usage, cats and dogs indoors, and farmhouse living. We also estimated IRR and 95% CI for RTIs for predicted scores of four extracted factors ('owned house', 'mold and dampness', 'candles', and 'density') from exploratory factor analyses (EFA).

RESULTS

We included 42 720 children with complete data. Mold/dampness was associated with all RTIs (common cold: IRRadj 1.09[1.07, 1.12]; influenza: IRRadj 1.10 [1.05, 1.15]; tonsillitis: IRRadj 1.19 [1.10, 1.28]; conjunctivitis: IRRadj 1.16 [1.02, 1.32]; and doctor-diagnosed pneumonia: IRRadj 1.05 [0.90, 1.21]), as was the EFA factor 'mold/dampness' for several outcomes. Gas stove usage was associated with conjunctivitis (IRRadj 1.25 [1.05, 1.49]) and with doctor-diagnosed pneumonia (IRRadj 1.14 [0.93, 1.39]). Candle-burning during summer, but not winter, was associated with several RTIs, for tonsillitis in a dose-dependent fashion (increasing weekly frequencies vs. none: [IRRadj 1.06 [0.98, 1.14], IRRadj 1.16 [1.04, 1.30], IRRadj 1.23 [1.06, 1.43], IRRadj 1.29 [1.00, 1.67], and IRRadj 1.41 [1.12, 1.78]).

CONCLUSION

Residential exposures, in particular to mold and dampness and to a lesser degree to indoor combustion sources, are related to the occurrence of RTIs in children.

Association of perinatal factors with neurodevelopmental referrals in a population-based cohort study in Japan

Although the causes of neurodevelopmental disorders remain unknown, several environmental risk factors have attracted considerable attention. We conducted a retrospective, longitudinal, population-based cohort study using data from infant health examinations of children born to mothers with pregnancies between April 1, 2014 and March 31, 2016 in Kobe City to identify the perinatal factors associated with neurodevelopmental referrals in 3-year-old children. There were 15,223 and 1283 children in the normal and referral groups, respectively. Neurodevelopmental referrals at the health checkup for 3-year-old children were significantly associated with the lack of social support during pregnancy (adjusted odds ratio [aOR] 1.99, 99% CI 1.14-3.45, p = 0.001), history of psychiatric consultation (aOR 1.56, 99% CI 1.10-2.22, p = 0.001), no social assistance post-delivery (aOR 1.49, 99% CI 1.03-2.16, p = 0.006), Edinburgh Post-natal Depression Scale (EPDS) score ≥ 9 (aOR 1.36, 99% CI 1.01-1.84, p = 0.008), infant gender (male) (aOR 2.51, 99% CI 2.05-3.06, p < 0.001), and cesarean delivery (aOR 1.39, 99% CI 1.11-1.75, p < 0.001). In conclusion, this exploratory study in the general Japanese population identified six perinatal factors associated with neurodevelopmental referrals in 3-year-old children: infant gender (male), cesarean section, maternal history of psychiatric consultation, EPDS score ≥ 9, lack of social support during pregnancy, and no social assistance post-delivery.

Maximising the potential of Chinese birth cohort studies: a systematic review of mother-baby cohorts in mainland China

OBJECTIVES

There is now a growing interest in early-life influences on adult diseases in China. A number of birth cohorts have been established. This systematic review provided a better understanding of the development of mother-baby cohorts in China.

STUDY DESIGN

Systematic review.

METHODS

We conducted a systematic review for research or profile papers in English/Chinese that reported data from mother-baby cohorts in mainland China, with ≥1y follow-up after birth. We identified 315 papers, corresponding to 31 cohorts from 19 provinces/megacities.

RESULTS

All cohorts started in 1999-2017 (21 after 2010) and were set up with broad objectives or specific scientific focus. The baseline sample size varied, from <500 to >300,000 mothers. A majority of cohorts were initiated during pregnancy and followed children to <10y, only six to adolescence and none into adulthood. These cohorts mostly collected samples from mothers and babies, in addition to using interviews/questionnaires to collect information about pregnancy, birth and child health. Most cohorts were recruited from a single province/city. The large western region was understudied.

CONCLUSIONS

Mother-baby cohorts have developed rapidly in China, but usually with a short follow-up duration. Extending the follow-up of children and developing cross-cohort collaboration will increase the diversity, size and coverage of the sample, allow studying early influences on life-course health and identify targets for early intervention in the Chinese population.

Integrating multiple lines of evidence to assess the effects of maternal BMI on pregnancy and perinatal outcomes

BACKGROUND

Higher maternal pre-pregnancy body mass index (BMI) is associated with adverse pregnancy and perinatal outcomes. However, whether these associations are causal remains unclear.

METHODS

We explored the relation of maternal pre-/early-pregnancy BMI with 20 pregnancy and perinatal outcomes by integrating evidence from three different approaches (i.e. multivariable regression, Mendelian randomisation, and paternal negative control analyses), including data from over 400,000 women.

RESULTS

All three analytical approaches supported associations of higher maternal BMI with lower odds of maternal anaemia, delivering a small-for-gestational-age baby and initiating breastfeeding, but higher odds of hypertensive disorders of pregnancy, gestational hypertension, preeclampsia, gestational diabetes, pre-labour membrane rupture, induction of labour, caesarean section, large-for-gestational age, high birthweight, low Apgar score at 1 min, and neonatal intensive care unit admission. For example, higher maternal BMI was associated with higher risk of gestational hypertension in multivariable regression (OR = 1.67; 95% CI = 1.63, 1.70 per standard unit in BMI) and Mendelian randomisation (OR = 1.59; 95% CI = 1.38, 1.83), which was not seen for paternal BMI (OR = 1.01; 95% CI = 0.98, 1.04). Findings did not support a relation between maternal BMI and perinatal depression. For other outcomes, evidence was inconclusive due to inconsistencies across the applied approaches or substantial imprecision in effect estimates from Mendelian randomisation.

CONCLUSIONS

Our findings support a causal role for maternal pre-/early-pregnancy BMI on 14 out of 20 adverse pregnancy and perinatal outcomes. Pre-conception interventions to support women maintaining a healthy BMI may reduce the burden of obstetric and neonatal complications.

FUNDING

Medical Research Council, British Heart Foundation, European Research Council, National Institutes of Health, National Institute for Health Research, Research Council of Norway, Wellcome Trust.

Maternal pre-pregnancy BMI and reproductive health in adult sons: a study in the Danish National Birth Cohort

STUDY QUESTION

Is maternal pre-pregnancy BMI associated with semen quality, testes volume, and reproductive hormone levels in sons?

SUMMARY ANSWER

Maternal pre-pregnancy BMI was associated with an altered reproductive hormone profile in young adult sons, characterized by higher levels of oestradiol, LH, and free androgen index (FAI) and lower levels of sex hormone-binding globulin (SHBG) in sons born of mothers with pre-pregnancy overweight and obesity.

WHAT IS KNOWN ALREADY

Evidence suggests that maternal pre-pregnancy BMI may influence reproductive health later in life. Only one pilot study has investigated the association between maternal pre-pregnancy BMI and reproductive health outcomes in sons, suggesting that a high BMI was associated with impaired reproductive function in the adult sons.

STUDY DESIGN, SIZE, DURATION

A population-based follow-up study of 1058 young men from the Fetal Programming of Semen Quality (FEPOS) cohort nested within the Danish National Birth Cohort (DNBC), 1998-2019, was carried out.

PARTICIPANTS/MATERIALS, SETTING, METHODS

In total, 1058 adult sons (median age 19 years, 2 months), born 1998-2000 by mothers included in the DNBC, participated in FEPOS. At a clinical examination, they provided a semen and blood sample, measured their testes volume, and had height and weight measured. Maternal pre-pregnancy BMI was obtained by self-report in early pregnancy. Semen characteristics, testes volume, and reproductive hormone levels were analysed according to maternal pre-pregnancy BMI categories and as restricted cubic splines using negative binomial and ordinary least square regression models. Mediation analyses examined potential mediation by the sons' birthweight, pubertal timing, fat mass, and BMI. Additional analyses investigated the role of paternal BMI in the potential associations between maternal BMI and reproductive health outcomes.

MAIN RESULTS AND THE ROLE OF CHANCE

We found no consistent associations between maternal pre-pregnancy BMI and semen characteristics or testes volume. Sons of mothers with higher pre-pregnancy BMI had higher oestradiol and lower SHBG levels, both in a dose-dependent manner. Sons of mothers with pre-pregnancy obesity (≥30 kg/m2) had higher LH levels and a higher FAI than sons born by mothers with normal pre-pregnancy BMI (18.5-24.9 kg/m2). The mediation analyses suggested that the effect of maternal pre-pregnancy BMI on higher levels of oestrogen, LH, and FAI was partly mediated by the sons' birthweight, in addition to adult fat mass and BMI measured at the clinical examination, whereas most of the effect on lower levels of SHBG was primarily mediated by the sons' own fat mass and BMI. Paternal BMI was not a strong confounder of the associations in this study.

LIMITATIONS, REASONS FOR CAUTION

This study was based in a population-based cohort with a low prevalence of overweight and obesity in both mothers and adult sons. Some men (10%) had blood for reproductive hormone assessment drawn in the evening. While several potential confounding factors were accounted for, this study's inherent risk of residual and unmeasured confounding precludes provision of causal estimates. Therefore, caution should be given when interpreting the causal effect of maternal BMI on sons' reproductive health.

WIDER IMPLICATIONS OF THE FINDINGS

Given the widespread occurrence of overweight and obesity among pregnant women, it is imperative to thoroughly examine the potential consequences for reproductive hormone levels in adult sons. The potential effects of maternal pre-pregnancy obesity on sons' reproductive hormone profile may potentially be partly avoided by the prevention of overweight and obesity in the sons.

STUDY FUNDING/COMPETING INTEREST(S)

The project was funded by the Lundbeck Foundation (R170-2014-855), the Capital Region of Denmark, Medical doctor Sofus Carl Emil Friis and spouse Olga Doris Friis's Grant, Axel Muusfeldt's Foundation (2016-491), AP Møller Foundation (16-37), the Health Foundation, Dagmar Marshall's Fond, Aarhus University, Independent Research Fund Denmark (9039-00128B), and the European Union (ERC, BIOSFER, 101071773). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible. The authors declare that they have no conflict of interest.

TRIAL REGISTRATION NUMBER

N/A.

Developmental psychologists should adopt citizen science to improve generalization and reproducibility

Widespread failures of replication and generalization are, ironically, a scientific triumph, in that they confirm the fundamental metascientific theory that underlies our field. Generalizable and replicable findings require testing large numbers of subjects from a wide range of demographics with a large, randomly-sampled stimulus set, and using a variety of experimental parameters. Because few studies accomplish any of this, meta-scientists predict that findings will frequently fail to replicate or generalize. We argue that to be more robust and replicable, developmental psychology needs to find a mechanism for collecting data at greater scale and from more diverse populations. Luckily, this mechanism already exists: Citizen science, in which large numbers of uncompensated volunteers provide data. While best-known for its contributions to astronomy and ecology, citizen science has also produced major findings in neuroscience and psychology, and increasingly in developmental psychology. We provide examples, address practical challenges, discuss limitations, and compare to other methods of obtaining large datasets. Ultimately, we argue that the range of studies where it makes sense *not* to use citizen science is steadily dwindling.

Prenatal exposure to per-and polyfluoroalkyl substances and child executive function: Evidence from the Shanghai birth cohort study

BACKGROUND

Per-and polyfluoroalkyl substances (PFAS) are ubiquitous in the environment and accumulate in humans. Toxicological studies have indicated the potential neurotoxicity of PFAS during the fetal development. However, in epidemiological studies, the association between prenatal exposure to PFAS and executive function in offspring remains unclear.

OBJECTIVES

To investigate the association between prenatal exposure to PFAS and executive function in offspring.

METHOD

This study included 1765 mother-child pairs in the Shanghai Birth Cohort, a prospective birth cohort enrolled during 2013-2016. The levels of 10 PFAS were measured in maternal plasma samples collected during early gestation. Child executive function was assessed at 4 years of age using the parent-reported Behavior Rating Inventory of Executive Function-Preschool version (BRIEF-P), which provided 4 composite measures: Inhibitory Self-Control Index, Flexibility Index, Emergent Metacognition Index, and Global Executive Composite. We used multivariable linear regression to examine the associations between individual PFAS and BRIEF-P scores. Bayesian kernel machine regression (BKMR) was employed to evaluate the joint effects. We also investigated whether these associations were modified by sex.

RESULT

We found no significant associations between prenatal PFAS exposure and BRIEF-P scores when the child was 4 years old. BKMR analysis showed no joint effect of the PFAS mixture on child executive function. RCS analysis indicated that the majority of relationships between PFAS and BRIEF-P did not deviate from the linear relationship, even though there was a nonlinear association between PFUA and EMI. Additionally, the associations were not modified by sex.

CONCLUSION

Overall, our findings showed that there were no associations between prenatal exposure to PFAS and child executive function at 4 years of age.

Exposure to different residential indoor characteristics during childhood and asthma in adolescence: a latent class analysis of the Danish National Birth Cohort

BACKGROUND

Many residential indoor environments may have an impact on children's respiratory health.

OBJECTIVES

The aims of this study were to identify latent classes of children from the Danish National Birth Cohort (DNBC) who share similar patterns of exposure to indoor home characteristics, and to examine the association between membership in the latent classes and asthma in adolescence.

METHODS

We included data on residential indoor characteristics of offspring from the DNBC whose mothers had responded to the child's 11-year follow-up and who had data on asthma from the 18-year follow-up. Number of classes and associations were estimated using latent class analysis. To account for sample selection, we applied inverse probability weighting.

RESULTS

Our final model included five latent classes. The probability of current asthma at 18 years was highest among individuals in class one with higher clustering on household dampness (9, 95%CI 0.06-0.13). Individuals in class four (with higher clustering on pets ownership and living in a farm) had a lower risk of current asthma at age 18 compared to individuals in class one (with higher clustering on household dampness) (OR 0.53 (95%CI 0.32-0.88), p = .01).

CONCLUSION

Our findings suggest that, in a high-income country such as Denmark, groups of adolescents growing up in homes with mold and moisture during mid-childhood might be at increased risk of current asthma at age 18. Adolescents who grew-up in a farmhouse and who were exposed to pets seem less likely to suffer from asthma by age 18.

Association between relative age at school and persistence of ADHD in prospective studies: an individual participant data meta-analysis

BACKGROUND

The youngest children in a school class are more likely than the oldest to be diagnosed with ADHD, but this relative age effect is less frequent in older than in younger school-grade children. However, no study has explored the association between relative age and the persistence of ADHD diagnosis at older ages. We aimed to quantify the association between relative age and persistence of ADHD at older ages.

METHODS

For this meta-analysis, we searched MEDLINE, Embase, CINAHL, PsycINFO, and PubPsych up to April 1, 2022, with terms related to "cohort" and "ADHD" with no date, publication type, or language restrictions. We gathered individual participant data from prospective cohorts that included at least ten children identified with ADHD before age 10 years. ADHD was defined by either a clinical diagnosis or symptoms exceeding clinical cutoffs. Relative age was recorded as the month of birth in relation to the school-entry cutoff date. Study authors were invited to share raw data or to apply a script to analyse data locally and generate anonymised results. Our outcome was ADHD status at a diagnostic reassessment, conducted at least 4 years after the initial assessment and after age 10 years. No information on sex, gender, or ethnicity was collected. We did a two-stage random-effects individual participant data meta-analysis to assess the association of relative age with persistence of ADHD at follow-up. This study was registered with PROSPERO, CRD42020212650.

FINDINGS

Of 33 119 studies generated by our search, we identified 130 eligible unique studies and were able to gather individual participant data from 57 prospective studies following up 6504 children with ADHD. After exclusion of 16 studies in regions with a flexible school entry system that did not allow confident linkage of birthdate to relative age, the primary analysis included 41 studies in 15 countries following up 4708 children for a period of 4 to 33 years. We found that younger relative age was not statistically significantly associated with ADHD persistence at follow-up (odds ratio 1·02, 95% CI 0·99-1·06; p=0·19). We observed statistically significant heterogeneity in our model (Q=75·82, p=0·0011, I2=45%). Participant-level sensitivity analyses showed similar results in cohorts with a robust relative age effect at baseline and when restricting to cohorts involving children with a clinical diagnosis of ADHD or with a follow-up duration of more than 10 years.

INTERPRETATION

The diagnosis of ADHD in younger children in a class is no more likely to be disconfirmed over time than that of older children in the class. One interpretation is that the relative age effect decreases the likelihood of children of older relative age receiving a diagnosis of ADHD, and another is that assigning a diagnostic label of ADHD leads to unexplored carryover effects of the initial diagnosis that persist over time. Future studies should be conducted to explore these interpretations further.

FUNDING

None.

Effects of prenatal alcohol exposure on infant lung function, wheeze, and respiratory infections in Australian children

BACKGROUND

Prenatal alcohol exposure (PAE) is a known risk factor for a range of adverse outcomes, such as facial dysmorphism, adverse birth outcomes, and neurodevelopmental changes. Preclinical research shows that PAE also inhibits lung development, lowers surfactant protein expression, has detrimental effects on alveolar macrophages, and decreases both T and B cell numbers. However, clinical evidence of respiratory impacts from PAE is limited. This study explored whether lung function, wheeze, and incidence of respiratory infections differ in children with PAE compared with unexposed children.

METHODS

Data from the Barwon Infant Study (n = 1074) were examined. PAE data were extracted from maternal questionnaires at trimesters 1 and 2 (combined), and trimester 3, and included as "total standard drinks" during each trimester and total pregnancy intake, a binary yes/no for PAE, and binge drinking (>5 standard drinks in one session). Respiratory outcomes were parent-reported wheeze, lung function (measured by multiple breath washout), and parent report and medical record indicators of health service attendances for respiratory conditions. Linear and logistic regressions were performed to quantify relationships between PAE and respiratory outcomes, controlling for socioeconomic status, birthweight, sex, gestational age, and maternal smoking.

RESULTS

Binge drinking was associated with increased health service attendance for respiratory condition(s) in the first 12 months of life (OR = 5.0, 95% CI (1.7, 20.7), p = 0.008). We did not find a relationship between binary PAE and binge drinking with lung function at 4 weeks of age or wheeze at 12 months. The number of standard drinks consumed in trimester two was associated with a lower lung clearance index (β = -0.011 turnovers, 95% CI (-0.0200, -0.0013), p = 0.03), and a small increase in functional residual capacity (β = 0.34 mL, 95% CI (0.02, 0.66), p = 0.04).

CONCLUSIONS

We found an association between binge drinking and health service utilization for respiratory conditions in infancy, but no evidence that low-level PAE was associated with adverse respiratory outcomes.

A critical examination of Danish norms for the Strengths and Difficulties Questionnaire (SDQ)

OBJECTIVE

The objective was to describe observed differences between the official Danish Strengths and Difficulties Questionnaire (SDQ)-norms and data from a sample based on the Danish National Birth Cohort (DNBC) including children born between 1996 and 2003. We compared the risk classification, cut-off bandings and the group sizes between the Danish norms and the values found in our sample at ages 7, 11 and 18 years.

RESULTS

Two sets of norms are used in Denmark: Arnfred's norms, based on a sample from one single Danish municipality and Niclasen's norms, based on multiple Danish cohorts, including the DNBC. Inconsistencies were found between banding scores in the two existing norms and the banding scores identified in our sample from DNBC: discrepancies included banding scores for several of the problem scales for children and preadolescents. For adolescents, we found less apparent inconsistencies between Arnfred's sample and the DNBC. Results demonstrate that the existing SDQ norms do not apply well to a large-scale cohort sample in Denmark. The usefulness of the SDQ as a screening instrument for mental health problems depends on appropriate norms. We therefore urge that the current Danish SDQ norms are used with caution, and preferably they should be revised.

The estimated effect of season and vitamin D in the first trimester on pubertal timing in girls and boys: a cohort study and an instrumental variable analysis

BACKGROUND

Season of birth has been associated with age at menarche. Maternal vitamin D levels in pregnancy may explain this effect. We investigated whether the season of first trimester or maternal 25-hydroxyvitamin D3 [25(OH)D3] levels were associated with pubertal timing in children.

METHODS

We conducted a follow-up study of 15 819 children born in 2000-03 from the Puberty Cohort, nested in the Danish National Birth Cohort (DNBC). Mean differences in attaining numerous pubertal markers, including a combined estimate for the average age at attaining all pubertal markers, were estimated for low (November-April) relative to high (May-October) sunshine exposure season in the first trimester using multivariable interval-censored regression models. Moreover, we conducted a two-sample instrumental variable analysis using season as an instrument for maternal first-trimester 25(OH)D3 plasma levels obtained from a non-overlapping subset (n = 827) in the DNBC.

RESULTS

For the combined estimate, girls and boys of mothers who had their first trimester during November-April had earlier pubertal timing than girls and boys of mothers whose first trimester occurred during May-October: -1.0 months (95% CI: -1.7 to -0.3) and -0.7 months (95% CI: -1.4 to -0.1), respectively. In the instrumental variable analysis, girls and boys also had earlier pubertal timing: respectively, -1.3 months (95% CI: -2.1 to -0.4) and -1.0 months (95% CI: -1.8 to -0.2) per SD (22 nmol/L) decrease in 25(OH)D3.

CONCLUSIONS

Both first pregnancy trimester during November-April and lower 25(OH)D3 were associated with earlier pubertal timing in girls and boys.

Timing of puberty in relation to semen characteristics, testicular volume, and reproductive hormones: a cohort study

OBJECTIVE

To investigate whether the timing of puberty is associated with semen characteristics, testicular volume, and reproductive hormone levels.

DESIGN

Cohort study.

SETTING

Not applicable.

PATIENTS

The Danish National Birth Cohort and its subcohort, the Fetal Programming of Semen Quality cohort of 1,058 young men.

INTERVENTION(S)

Self-reported information on the timing (younger, same age, older than peers) of the pubertal markers: voice break (primary exposure), pubic hair growth, regular shaving, and axillary hair growth.

MAIN OUTCOME MEASURES(S)

We estimated the relative differences with 95% confidence intervals for semen characteristics (semen volume, sperm concentration, total sperm count, sperm motility, percentage of morphologically normal spermatozoa), testicular volume, and reproductive hormones (follicle stimulating hormone [FSH], luteinizing hormone, sex hormone-binding globulin [SHBG], testosterone, estradiol, and free androgen index [FAI]) obtained at a median age of 19.2 years according to timing of pubertal development.

RESULT(S)

Compared with men reporting voice break "same age as peers," men reporting voice break "older than peers" tended to have lower total sperm count (-12% [-25%, 4%]) and lower percent morphologically normal spermatozoa (-10% [-20%, 2%]), whereas men reporting voice break "younger than peers" tended to have a lower proportion of nonprogressive and immotile spermatozoa (-6% [-13%, 1%]) and larger testicular volume (7% [1%, 13%]). The pattern was less consistent for the other pubertal markers. For reproductive hormones, voice break "older than peers" tended to have higher FSH levels (24% [-1%, 55%]), higher SHBG levels (7% [0, 15%]), lower estradiol levels (-14% [-23%, -5%]), and lower FAI (-8% [-14%, -1%]), whereas voice break "younger than peers" tended to have higher luteinizing hormone levels (4% [-2%, 11%]), higher testosterone levels (5% [0%, 11%]), higher estradiol levels (17% [6%, 29%]), and higher FAI (4% [-2%, 11%]). When the categorical pubertal markers were analyzed as a linear term to assess dose dependence, older age at pubertal development was associated with higher FSH levels, higher SHBG levels, lower testosterone levels, lower estradiol levels, and lower FAI for most pubertal markers.

CONCLUSION(S)

These results lend weak support to the hypothesis that older age at pubertal development is associated with markers of reduced male fecundity, especially reproductive hormone levels, although associations with semen characteristics and testicular volume were statistically insignificant.

Effect of maternal fish oil supplementation during lactation on socioemotional wellbeing and physical activity in 13-year-old children: A randomized clinical trial

Early dietary long-chain n-3PUFA (n-3LCPUFA) may affect brain development. We investigated if fish oil supplementation of lactating mothers affected socioemotional wellbeing in adolescents in a potentially gender-specific manner. At age 13, we invited 92 children of mothers who completed a randomized trial with 1.5 g/d n-3 LCPUFA or olive oil during the first 4 months of lactation and 48 children of mothers with a high habitual fish intake. Children and parents answered validated questionnaires regarding socioemotional wellbeing and physical activity was monitored by ActiGraph for 7 days. Participation rate was 71%. Univariate correlations between children's and parents' ratings on the individual scales were moderate-strong, but correlations across questionnaires indicated that parents might base their ratings on proxy markers. We found no group differences in self-rated socioemotional outcomes or physical activity. Although the study was small, it was the first follow-up on effects of perinatal n-3LCPUFA supply on socioemotional wellbeing in adolescence.

Maternal Intake of Vitamin D Supplements during Pregnancy and Pubertal Timing in Children: A Population-Based Follow-Up Study

Maternal vitamin D may be important for several organ systems in the offspring, including the reproductive system. In this population-based follow-up study of 12,991 Danish boys and girls born 2000-2003, we investigated if maternal intake of vitamin D supplements during pregnancy was associated with pubertal timing in boys and girls. Information on maternal intake of vitamin D supplements was obtained by self-report in mid-pregnancy. Self-reported information on the current status of various pubertal milestones was obtained every six months throughout puberty. Mean differences in months at attaining each pubertal milestone and an average estimate for the mean difference in attaining all pubertal milestones were estimated according to maternal intake of vitamin D supplements using multivariable interval-censored regression models. Lower maternal intake of vitamin D supplements was associated with later pubertal timing in boys. For the average estimate, boys had 0.5 months (95% CI 0.1; 0.9) later pubertal timing per 5 µg/day lower maternal vitamin D supplement intake. Maternal intake of vitamin D supplements was not associated with pubertal timing in girls. Spline plots and sensitivity analyses supported the findings. Whether the observed association with boys' pubertal timing translates into an increased risk of disease in adulthood is unknown.

Prenatal exposure to nitrosatable drugs and timing of puberty in sons and daughters: A nationwide cohort study

BACKGROUND

N-nitroso compounds (NOCs) can be formed by endogenous reactions between nitrosatable drugs and nitrite. Animal studies have found that several NOCs are teratogenic, and epidemiological studies report associations between prenatal exposure to nitrosatable drugs and adverse birth outcomes. It is unknown whether prenatal exposure to nitrosatable drugs is harmful to the child's reproductive health, including pubertal development.

OBJECTIVES

We investigated whether prenatal exposure to nitrosatable drugs was associated with timing of puberty and whether nitrate, nitrite and antioxidant intake modified any association.

METHODS

The population-based Danish National Birth Cohort (DNBC) Puberty Cohort, which includes 15,819 children, was used to investigate the association between prenatal exposure to nitrosatable drugs and timing of puberty. Around gestational week 11 and gestational week 18, mothers provided information about drug use during pregnancy. The children's self-reported information on onset of pubertal milestones was collected every six months from 11 years of age and throughout puberty. To investigate potential effect modification by nitrite, nitrate and antioxidant intake, information on these factors was obtained from a food frequency questionnaire completed by the mothers in gestational week 25, and information on nitrate concentration in maternal drinking water at her residential address was obtained from monitoring data from public waterworks. Data were analysed using a multivariable regression model for interval-censored data estimating difference in months in timing of puberty between exposure groups.

RESULTS

A total of 2,715 children were prenatally exposed to nitrosatable drugs. We did not find an association between prenatal exposure to nitrosatable drugs and timing of puberty. This finding was supported by null-findings in the following sub-analyses investigating: 1. subtypes of nitrosatable drugs (secondary and tertiary amines and amides), 2. dose-dependency (duration of drug intake), 3. effect modification by maternal intake of nitrate, nitrite, and antioxidants. 4. confounding by indication.

CONCLUSIONS

Prenatal exposure to nitrosatable drugs was not associated with timing of puberty. Nitrosatable drugs are commonly used drugs in pregnancy, and further research is needed to allow firm conclusions on the potential effect of prenatal exposure to nitrosatable drugs on the child's reproductive health.

Maternal intake of folate and folic acid during pregnancy and pubertal timing in girls and boys: A population-based cohort study

BACKGROUND

Folate is essential for normal foetal development as it plays an important role for gene expression during different periods of foetal development. Thus, prenatal exposure to folate may have a programming effect on pubertal timing.

OBJECTIVES

To study the association between maternal intake of folate during pregnancy and pubertal timing in girls and boys.

METHODS

We studied 6585 girls and 6326 boys from a Danish population-based Puberty Cohort, 2000-2021. Information on maternal intake of folate from diet and folic acid from supplements was obtained from a food-frequency questionnaire in mid-pregnancy, and total folate was calculated as dietary folate equivalents. Information on age at menarche in girls, age at first ejaculation and voice break in boys, and Tanner stages, acne and axillary hair in both girls and boys was obtained every 6 months throughout puberty. We estimated mean monthly differences according to exposure groups for each pubertal milestone in addition to a combined estimate for the average age at attaining all pubertal milestones using multivariable interval-censored regression models. Total folate was analysed in quintiles, continuous and as restricted cubic splines.

RESULTS

Maternal intake of total folate in mid-pregnancy was not associated with pubertal timing in girls (combined estimate for overall pubertal timing per standard deviation (SD 325 μg/day) decrease in maternal intake of total folate: -0.14 months (95% confidence interval [CI] -0.51, 0.22)). Boys had slightly later overall pubertal timing per standard deviation (SD 325 μg/day) decrease in maternal intake of total folate (combined estimate: 0.40 months, 95% CI 0.01, 0.72). Spline plots supported these findings.

CONCLUSIONS

Prenatal exposure to low maternal intake of total folate in mid-pregnancy was not associated with pubertal timing in girls but associated with slightly later pubertal timing in boys. This minor delay is likely not of clinical importance.

Maternal alcohol intake in early pregnancy and biomarkers of fecundity in adult sons: A cohort study

Male fecundity may be largely determined through fetal programming and therefore potentially be sensitive to exposure to maternal alcohol intake during pregnancy. We investigated whether maternal alcohol intake in early pregnancy was associated with biomarkers of fecundity in adult sons. In total, 1058 sons from the Fetal Programming of Semen Quality (FEPOS) cohort nested in the Danish National Birth Cohort (DNBC) provided blood and semen samples at around 19 years of age. Information on maternal weekly average alcohol intake (0 drinks [ref], >0-1 drinks, >1-3 drinks, >3 drinks) and binge drinking episodes (intake of ≥5 drinks on one occasion: (0 [ref], 1-2, ≥3 episodes)) was self-reported at around gestational week 17. Outcomes included semen characteristics, testes volume and reproductive hormones. We found some small tendencies towards lower semen characteristics and an altered hormone level profile in sons of mother who had an intake of > 3 drinks/week in early pregnancy and sons of mother who had ≥ 3 episodes of binge drinking in pregnancy. However, the effect estimates were overall small and inconsistent and with no indication of a dose dependent association. Due to the limited number of mothers with a high weekly alcohol intake, we cannot exclude whether prenatal exposure to higher doses than 4.5 drinks/week of alcohol in early pregnancy might have a detrimental effect on the biomarkers of fecundity in adult sons..

Prenatal antidepressant exposure and emotional disorders until age 22: a danish register study

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) are the most frequently prescribed antidepressants in pregnancy. Animal and some clinical studies have suggested potential increases in depression and anxiety following prenatal SSRI exposure, but the extent to which these are driven by the medication remains unclear. We used Danish population data to test associations between maternal SSRI use during pregnancy and children outcomes up to age 22.

METHODS

We prospectively followed 1,094,202 single-birth Danish children born 1997-2015. The primary exposure was ≥ 1 SSRI prescription filled during pregnancy; the primary outcome, first diagnosis of a depressive, anxiety, or adjustment disorder, or redeemed prescription for an antidepressant medication. We used propensity score weights to adjust potential confounders, and incorporated data from the Danish National Birth Cohort (1997-2003) to further quantify potential residual confounding by subclinical factors.

RESULTS

The final dataset included 15,651 exposed and 896,818 unexposed, children. After adjustments, SSRI-exposed had higher rates of the primary outcome than those of mothers who either did not use an SSRI (HR = 1.55 [95%CI:1.44,1.67] or discontinued the SSRI use ≥ 3 months prior to conception (HR = 1.23 [1.13,1.34]). Age of onset was earlier among exposed (9 [IQR:7-13] years) versus unexposed (12 [IQR:12-17] years) children (p < 0.01). Paternal SSRI use in the absence of maternal use during the index pregnancy (HR = 1.46 [1.35,1.58]) and maternal SSRI use only after pregnancy (HR = 1.42 [1.35,1.49]) were each also associated with these outcomes.

CONCLUSIONS

While SSRI exposure was associated with increased risk in the children, this risk may be driven at least partly by underlying severity of maternal illness or other confounding factors.

Detangling the Structural Neural Correlates Associated with Resting versus Dynamic Phantom Limb Pain Intensity Using a Voxel-based Morphometry Analysis

The management of phantom limb pain (PLP) is still challenging due to a partial understanding of its neurophysiological mechanisms. Structural neuroimaging features are potential biomarkers. However, only a few studies assessed their correlations with clinical severity and treatment response. This study aims to explore the association between brain gray matter volume (GMV) with phantom limb manifestations severity and PLP improvement after neuromodulatory treatments (transcranial direct current stimulation and mirror therapy). Voxel-based morphometry analyses and functional decoding using a reverse inference term-based meta-analytic approach were used. We included 24 lower limb traumatic amputees with moderate to severe PLP. We found that alterations of cortical GMV were correlated with PLP severity but not with other clinical manifestations. Less PLP severity was associated with larger brain clusters GMV in the non-affected prefrontal, insula (non-affected mid-anterior region), and bilateral thalamus. However, only the insula cluster survived adjustments. Moreover, the reverse inference meta-analytic approach revealed that the found insula cluster is highly functionally connected to the contralateral insula and premotor cortices, and the decoded psychological processes related to this cluster were "rating," "sustained attention," "impulsivity, " and "suffering." Moreover, we found that responders to neuromodulatory treatment have higher GMV in somatosensory areas (total volume of S1 and S2) in the affected hemisphere at baseline, compared to non-responders, even after adjustments.

Maternal vitamin D levels and male reproductive health: a population-based follow-up study

Maternal vitamin D levels during pregnancy may be important for reproductive health in male offspring by regulating cell proliferation and differentiation during development. We conducted a follow-up study of 827 young men from the Fetal Programming of Semen Quality (FEPOS) cohort, nested in the Danish National Birth Cohort to investigate if maternal vitamin D levels were associated with measures of reproductive health in adult sons. These included semen characteristics, testes volume, and reproductive hormone levels and were analysed according to maternal vitamin D (25(OH)D3) levels during pregnancy. In addition, an instrumental variable analysis using seasonality in sun exposure as an instrument for maternal vitamin D levels was conducted. We found that sons of mothers with vitamin D levels < 25 nmol/L had 11% (95% CI - 19 to - 2) lower testes volume and a 1.4 (95% CI 1.0 to 1.9) times higher risk of having low testes volume (< 15 mL), in addition to 20% (95% CI - 40 to 9) lower total sperm count and a 1.6 (95% CI 0.9 to 2.9) times higher risk of having a low total sperm count (< 39 million) compared with sons of mothers with vitamin D levels > 75 nmol/L. Continuous models, spline plots and an instrumental variable analysis supported these findings. Low maternal vitamin D levels were associated with lower testes volume and lower total sperm count with indications of dose-dependency. Maternal vitamin D level above 75 nmol/L during pregnancy may be beneficial for testes function in adult sons.

Congenital heart disease and the risk of impaired fertility: A Danish nationwide cohort study using time to pregnancy

BACKGROUND

The number of women with congenital heart disease (CHD) becoming pregnant are increasing. Although menstrual irregularities appear to occur more often in these patients, knowledge on their fertility is limited. In this nationwide cohort study, we evaluated the risk of impaired fertility in women with CHD compared with unaffected women using time to pregnancy (TTP).

METHODS

The Danish National Birth Cohort (DNBC) of pregnant women constituted the study population. Information on TTP and use of medically assisted reproduction (MAR) treatment was reported at a first trimester interview. Women with CHD were identified by linkage to the Danish National Patient Registry. TTP was divided into three categories; 0-5 months, 6-12 months (i.e. subfertile), and > 12 months or use of MAR treatment (i.e. infertile). Relative risk ratios (RRR) for subfertility and infertility with 95% confidence intervals were estimated using multinomial logistic regression.

RESULTS

Among 93,832 pregnancies in 84,922 women, CHD was diagnosed in 333 women (0.4%), contributing with 360 pregnancies. The CHD was of simple complexity in 291 women (87.4%). No association was found between CHD and longer TTP (RRR of 1.02 (95% CI: 0.75-1.40) for subfertility, and RRR of 0.86 (95% CI: 0.61-1.20) for infertility). Similar was observed when comparing women with simple CHD and unaffected women. The number of women with complex CHD was too low for evaluation.

CONCLUSIONS

Women with CHD had no increased risk of impaired fertility, assessed by TTP, when compared with unaffected women. Separate analysis of women with complex CHD was hampered by low numbers.

Motor skills at 7 years of age and spinal pain at 11 years of age: a cohort study of 26,000 preadolescents

This study aims to investigate the relationship between motor skills at age 7 and spinal pain at age 11. The study included participants from the Danish National Birth Cohort. Data on motor skills were obtained from the Developmental Coordination Disorder Questionnaire, completed by the mothers when the children were 7 years old, and spinal pain was self-reported at age 11 for frequency and intensity of neck, mid back, and low back pain. This was categorized into "no," "moderate," or "severe" pain, based on frequency and pain intensity. Associations were estimated using multinomial logistic regression models. Data on both motor skills and spinal pain was available for 25,000 children. There was a consistent pattern of reporting more neck or mid back pain at age 11 for those with lower levels of fine motor skills and coordination scores at age 11. The relationship was significant for severe pain (the highest relative risk ratio being 1.87 and the lowest 1.18), but not for moderate pain (the highest relative risk ratio being 1.22 and the lowest 1.07). Gross motor skills were not associated with spinal pain, and there was no relationship between low back pain and motor skills.  Conclusion: Our results indicate a link between motor development at 7 years of age and neck and mid back pain, but not low back pain, at 11 years of age. Improvement of motor skills in young children might reduce the future burden of neck and mid back pain and should be a target of future research. What is Known: • Spinal pain in preadolescence and adolescence is common and predisposes to spinal pain in adulthood. • Motor skills influence the biomechanics of movement and therefore has a potential impact on musculoskeletal health. What is New: • Poor fine motor- and coordination skills in childhood were associated with increased risk of severe neck- or mid back pain, but not low back pain, four 4 years later. • Poor gross motor skills were not associated with higher risk of later spinal pain.

When a birth cohort grows up: challenges and opportunities in longitudinal developmental origins of health and disease (DOHaD) research

High-quality evidence from prospective longitudinal studies in humans is essential to testing hypotheses related to the developmental origins of health and disease. In this paper, the authors draw upon their own experiences leading birth cohorts with longitudinal follow-up into adulthood to describe specific challenges and lessons learned. Challenges are substantial and grow over time. Long-term funding is essential for study operations and critical to retaining study staff, who develop relationships with participants and hold important institutional knowledge and technical skill sets. To maintain contact, we recommend that cohorts apply multiple strategies for tracking and obtain as much high-quality contact information as possible before the child's 18th birthday. To maximize engagement, we suggest that cohorts offer flexibility in visit timing, length, location, frequency, and type. Data collection may entail multiple modalities, even at a single collection timepoint, including measures that are self-reported, research-measured, and administrative with a mix of remote and in-person collection. Many topics highly relevant for adolescent and young adult health and well-being are considered to be private in nature, and their assessment requires sensitivity. To motivate ongoing participation, cohorts must work to understand participant barriers and motivators, share scientific findings, and provide appropriate compensation for participation. It is essential for cohorts to strive for broad representation including individuals from higher risk populations, not only among the participants but also the staff. Successful longitudinal follow-up of a study population ultimately requires flexibility, adaptability, appropriate incentives, and opportunities for feedback from participants.

Body mass index, waist circumference, and urinary incontinence in midlife: A follow-up of mothers in the Danish National Birth Cohort

BACKGROUND

Obesity is a modifiable risk factor for urinary incontinence, yet few studies have investigated how waist circumference as compared to body mass index (BMI) influences the risk of urinary incontinence.

OBJECTIVE

To estimate how BMI and waist circumference associates with risk of urinary incontinence in midlife and determine which of the two is the strongest predictor of urinary incontinence.

METHODS

Cohort study among mothers in the Danish National Birth Cohort. Weight and waist circumference were self-reported 7 years after cohort entry. Symptoms of urinary incontinence in midlife were self-reported using the International Consultation on Incontinence Questionnaire Female Lower Urinary Tract Symptoms (ICIQ-FLUTS) and analyzed continuously and as presence or absence of any, stress (SUI), urgency (UUI), and mixed (MUI) urinary incontinence. Linear and log binomial regressions were used to calculate mean differences and risk ratios (RR) with 95% confidence intervals (CI). Restricted cubic splines were generated to explore nonlinear relationships.

RESULTS

Among 27 254 women at a mean age of 44.2 years, any urinary incontinence was reported by 32.1%, SUI by 20.9%, UUI by 2.4%, and MUI by 8.6%. For all outcomes, increases in risk were similar with higher BMI and waist circumference. The estimates of association were strongest for MUI (RR 1.10, 95% CI 1.08;1.12 and RR 1.12, 95% CI 1.10;1.14 for half a standard deviation increase in BMI and waist circumference, respectively). While increases in risk of the other outcomes were seen across the entire range of BMI and waist circumference, the risk of SUI rose until BMI 28 kg/m² (waist circumference 95 cm), and then fell slightly.

CONCLUSIONS

Symptoms of urinary incontinence and prevalence of any urinary incontinence, SUI, UUI, and MUI increased with higher BMI and waist circumference. Self-reported BMI and waist circumference were equally predictive of urinary incontinence.

A Review of Major Danish Biobanks: Advantages and Possibilities of Health Research in Denmark

Biobank research may lead to an improved understanding of disease etiology and advance personalized medicine. Denmark (population ~5.9 million) provides a unique setting for population-based health research. The country is a rich source of biobanks and the universal, tax-funded healthcare system delivers routinely collected data to numerous registries and databases. By virtue of the civil registration number (assigned uniquely to all Danish citizens), biological specimens stored in biobanks can be combined with clinical and demographic data from these population-based health registries and databases. In this review, we aim to provide an understanding of advantages and possibilities of biobank research in Denmark. As knowledge about the Danish setting is needed to grasp the full potential, we first introduce the Danish healthcare system, the Civil Registration System, the population-based registries, and the interface with biobanks. We then describe the biobank infrastructures, comprising the Danish National Biobank Initiative, the Bio- and Genome Bank Denmark, and the Danish National Genome Center. Further, we briefly provide an overview of fourteen selected biobanks, including: The Danish Newborn Screening Biobank; The Danish National Birth Cohort; The Danish Twin Registry Biobank; Diet, Cancer and Health; Diet, Cancer and Health - Next generations; Danish Centre for Strategic Research in Type 2 Diabetes; Vejle Diabetes Biobank; The Copenhagen Hospital Biobank; The Copenhagen City Heart Study; The Copenhagen General Population Study; The Danish Cancer Biobank; The Danish Rheumatological Biobank; The Danish Blood Donor Study; and The Danish Pathology Databank. Last, we inform on practical aspects, such as data access, and discuss future implications.

Exposures to pesticides and risk of cancer: Evaluation of recent epidemiological evidence in humans and paths forward

Knowledge of the role in cancer etiology of environmental exposures as pesticides is a prerequisite for primary prevention. We review 63 epidemiological studies on exposure to pesticides and cancer risk in humans published from 2017 to 2021, with emphasis on new findings, methodological approaches, and gaps in the existing literature. While much of the recent evidence suggests causal relationships between pesticide exposure and cancer, the strongest evidence exists for acute myeloid leukemia (AML) and colorectal cancer (CRC), diseases in which the observed associations were consistent across several studies, including high-quality prospective studies and those using biomarkers for exposure assessment, with some observing dose-response relationships. Though high-quality studies have been published since the IARC monograph on organophosphate insecticides in 2017, there are still gaps in the literature on carcinogenic evidence in humans for a large number of pesticides. To further knowledge, we suggest leveraging new techniques and methods to increase sensitivity and precision of exposure assessment, incorporate multi-omics data, and investigate more thoroughly exposure to chemical mixtures. There is also a strong need for better and larger population-based cohort studies that include younger and nonoccupationally exposed individuals, particularly during developmental periods of susceptibility. Though the existing evidence has limitations, as always in science, there is sufficient evidence to implement policies and regulatory action that limit pesticide exposure in humans and, hence, further prevent a significant burden of cancers.

Impact of a manager-oriented intervention on occupational exposures and well-being among pregnant hospital and day-care employees: A cluster randomised trial

BACKGROUND

Pregnancy can make it difficult to cope with demands at work and may affect women's well-being. We investigated if a manager-targeted educational intervention reduced demanding occupational exposures and improved the psychosocial work environment and well-being among pregnant employees.

METHODS

Data came from a cluster randomised trial in Danish hospitals and day-care institutions. Work units were assigned randomly and were non-blinded to the intervention, where managers were either invited to participate in a three-hour seminar addressing job adjustment in pregnancy or assigned to a control group undertaking their usual practice. Self-reported outcomes by pregnant employees at the work units were the proportion of pregnant employees with demanding occupational exposures, good psychosocial work environment and good well-being. Mixed logistic regression was applied in the population of responders and in intention-to-treat analyses using multiple imputations.

RESULTS

We included 915 pregnant employees: 451 in the intervention group and 464 in the control group. Of 216 invited managers, 103 (48%) participated in the seminar. A total of 339 (37%) pregnant employees answered the questionnaire. The proportion of pregnant employees who had demanding occupational exposures, good psychosocial work environment and good well-being in the intervention versus the control group were 36% versus 39% (odds ratio (OR)=0.90, 95% confidence interval (CI) 0.57-1.44), 56% versus 59% (OR=1.01, 95% CI 0.60-1.71) and 77% versus 73% (OR=1.13, 95% CI 0.68-1.87), respectively.

CONCLUSIONS

This manager-targeted educational intervention did not reduce demanding occupational exposures and did not improve the psychosocial work environment or the well-being among pregnant employees in hospital and day-care settings. Comprehensive and participatory interventions may be considered in future studies.

Maternal intake of folate and folic acid during pregnancy and markers of male fecundity: A population-based cohort study

BACKGROUND

Poor male fecundity is of concern, and a prenatal origin has been proposed. Folate, a methyl donor involved in DNA methylation, is essential for normal fetal development by regulating gene expression during different periods of fetal development. Thus, prenatal exposure to low maternal folate intake might have a programing function of the developing reproductive organs.

OBJECTIVES

To examine the association between maternal intake of folate from diet and folic acid from supplements during pregnancy and markers of fecundity in young men.

MATERIALS AND METHODS

We conducted a follow-up study using a Danish mother-son cohort of 787 young men born 1998-2000. Percentage differences in semen characteristics, testes volume, and reproductive hormone levels were analyzed according to total folate calculated as dietary folate equivalents from diet and supplements in midpregnancy, using multivariable negative binomial regression models. Total folate was analyzed in quintiles, continuous per standard deviation decrease (SD: 318 μg/day) and as restricted cubic splines.

RESULTS

Low maternal intake of total folate was associated with lower total sperm count (-5% (95% confidence intervals [CI]: -11%; 2%)), a lower proportion of non-progressive and immotile spermatozoa (-5% [95% CI: -8%; -3%]), and lower testes volume (-4% [95% CI: -6%; -2%]) per SD decrease in total folate intake. Spline plots supported these findings.

DISCUSSION

The finding of a lower proportion of non-progressive and immotile spermatozoa, and hence a higher proportion of motile spermatozoa, in men of mothers with a lower intake of total folate in midpregnancy was surprising and may be a chance finding.

CONCLUSION

Lower maternal intake of total folate in midpregnancy was associated with lower sperm count and lower testes volume, however, also with a lower proportion of non-progressive and immotile spermatozoa in adult men. Whether this actually affects the ability to obtain a pregnancy warrants further investigation.

Maternal fever during pregnancy and male offspring reproductive health: A longitudinal cohort study in young Danish males

BACKGROUND

Maternal fever during pregnancy has been associated with an increased risk of genital malformations, but the implication for long-term reproductive health in the offspring is unknown.

OBJECTIVES

To investigate associations between timing, duration, and temperature of fetal exposure to maternal fever and sons' semen quality, testicular volume, and levels of reproductive hormones in early adulthood. Further, to examine whether concurrent use of antipyretics and/or antibiotics modified the effect.

MATERIALS AND METHODS

We used the Fetal Programming of Semen Quality cohort consisting of men born to women enrolled in the Danish National Birth Cohort. Self-reported information on maternal fever was collected twice during pregnancy (median 16 and 31 pregnancy weeks) and categorized as any fever during pregnancy, fever during early pregnancy (weeks 1-15), and fever exclusively during late pregnancy (weeks 16-42). Semen quality and concentrations of reproductive hormones were measured at a clinical examination at the age of 18.9 years. We used negative binomial regression to examine the associations, adjusting for maternal age at birth, maternal smoking, family occupational status, and precision variables related to semen quality and hormonal levels, for example, abstinence time.

RESULTS

986 men were included in the study, of which 23% had mothers reporting at least one episode of fever. We found no strong indications of associations between maternal fever during pregnancy and male reproductive health in young men. Concurrent use of antipyretics and antibiotics did not modify the association.

DISCUSSION

Strengths include the large sample size, prospectively collected data, and the adjustment for maternal factors during pregnancy and important precision variables. A limitation is the crude self-reported information on maternal fever.

CONCLUSION

We found no evidence to support that timing, duration, or temperature of maternal fever during pregnancy has a long-term impact on semen characteristics, testicular volume, or level of reproductive hormones in male offspring.

Self-injury, suicidality and eating disorder symptoms in young adults following COVID-19 lockdowns in Denmark

An aggravation in mental health during the COVID-19 lockdown has been suggested but the impact on self-injury, suicidality and eating disorders (EDs) are less elucidated. Using linear regression in different data set-ups that is longitudinal (n = 7,579) and repeated cross-sectional data (n = 24,625) from the Danish National Birth Cohort, we compared self-reported self-injury, suicidality and symptoms of EDs from before through different pandemic periods until spring 2021. The longitudinal data indicate a reduction in the proportion of self-injury in men (-3.2% points, 95% confidence interval (CI) = -4.3%; -2.2%, P < 0.001, d.f. = 2) and women (5.7% points, 95% CI = -6.6%; -4.8%, P < 0.001, d.f. = 2) and of suicide ideation in men (-3.0% points, 95% CI = -4.6%; -1.4%, P = 0.002, d.f. = 2) and women (-7.4% points, 95% CI = -8.7%; -6.0%, P < 0.001, d.f. = 2), as well as symptoms of EDs in women (-2.3% points, 95% CI = -3.2%; -1.4%, P < 0.001, d.f. = 2). For suicide attempt, indication of an increase was observed in men only (0.4% points, 95% CI = 0.1%; 0.7%, P = 0.019, d.f. = 2). In the repeated cross-sectional data, we observed no changes in any of the outcomes. Our findings provide no support for the increase in self-injury, suicidality and symptoms of EDs after the lockdowns. Key limitations are differential attrition and varying age in pre- and post-lockdown measures in the longitudinal data.

Association of self-reported mother-infant relationship with child and adolescent mental health

BACKGROUND

The quality of the relationship between mother and infant may have profound implications for the development of a child. Early indicators of psychological vulnerability may allow targeting of support for the child's cognitive, emotional and social development. A challenging mother-infant relationship could be one indicator of risk.

AIMS

This study examined variations in psychological well-being and psychopathology among boys and girls according to early maternal perception of the mother-infant relationship.

METHOD

This study is based on 64 663 mother-infant pairs from the Danish National Birth Cohort, for which data on the mother-infant relationship were collected at 6 months postpartum. Behavioural problems were assessed with the Danish version of the Strengths and Difficulties Questionnaire (SDQ) at child ages 7, 11 and 18 years, and we retrieved information on diagnosed childhood and adolescent psychiatric disorders and prescriptions of psychotropic drugs from Danish registries.

RESULTS

Children in the challenging mother-infant relationship group had higher odds of behavioural problems at age 7 among both boys and girls. The same pattern of elevated estimates was identified for boys across all SDQ domains and for girls in three of five SDQ domains. All associations were attenuated at age 18, but increased odds of behavioural problems still existed. A challenging early mother-infant relationship increased the offspring's risk of being diagnosed with a psychiatric disorder or being prescribed a psychotropic drug before the age of 18.

CONCLUSION

A challenging self-reported mother-infant relationship was associated with later psychopathological difficulties. Routine clinical enquiry may be useful in identification of future vulnerability.

Selection bias in a male-offspring cohort investigating fecundity: is there reason for concern?

STUDY QUESTION

Is there risk of selection bias in etiological studies investigating prenatal risk factors of poor male fecundity in a cohort of young men?

SUMMARY ANSWER

The risk of selection bias is considered limited despite a low participation rate.

WHAT IS KNOWN ALREADY

Participation rates in studies relying on volunteers to provide a semen sample are often very low. Many risk factors for poor male fecundity are associated with participation status, and as men with low fecundity may be more inclined to participate in studies of semen quality, a risk of selection bias exists.

STUDY DESIGN, SIZE, DURATION

A population-based follow-up study of 5697 young men invited to the Fetal Programming of Semen Quality (FEPOS) cohort nested within the Danish National Birth Cohort (DNBC), 1998-2019.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Young men (age range: 18 years, 9 months to 21 years, 4 months) born 1998-2000 by mothers included in the DNBC were invited to participate in FEPOS. In total, 1173 men answered a survey in FEPOS (n = 115 participated partly); of those, 1058 men participated fully by also providing a semen and a blood sample at a clinical visit. Differential selection according to parental baseline characteristics in the first trimester, the sons' own characteristics from the FEPOS survey, and urogenital malformations and diseases in reproductive organs from the Danish registers were investigated using logistic regression. The influence of inverse probability of selection weights (IPSWs) to investigate potential selection bias was examined using a predefined exposure-outcome association of maternal smoking in the first trimester (yes, no) and total sperm count analysed using adjusted negative binomial regression. A multidimensional bias analysis on the same association was performed using a variety of bias parameters to assess different scenarios of differential selection.

MAIN RESULTS AND THE ROLE OF CHANCE

Participation differed according to most parental characteristics in first trimester but did not differ according to the prevalence of a urogenital malformation or disease in the reproductive organs. Associations between maternal smoking in the first trimester and male fecundity were similar when the regression models were fitted without and with IPSWs. Adjusting for other potential risk factors for poor male fecundity, maternal smoking was associated with 21% (95% CI: -32% to -9%) lower total sperm count. In the bias analysis, this estimate changed only slightly under realistic scenarios. This may be extrapolated to other exposure-outcome associations.

LIMITATIONS, REASONS FOR CAUTION

We were unable to directly assess markers of male fecundity for non-participants from, for example an external source and therefore relied on potential proxies of fecundity. We did not have sufficient power to analyse associations between prenatal exposures and urogenital malformations.

WIDER IMPLICATIONS OF THE FINDINGS

The results are reassuring when using this cohort to identify causes of poor male fecundity. The results may be generalized to other similar cohorts. As the young men grow older, they can be followed in the Danish registers, as an external source, to examine, whether participation is associated with the risk of having an infertility diagnosis.

STUDY FUNDING/COMPETING INTEREST(S)

The project was funded by the Lundbeck Foundation (R170-2014-855), the Capital Region of Denmark, Medical doctor Sofus Carl Emil Friis and spouse Olga Doris Friis's Grant, Axel Muusfeldt's Foundation (2016-491), AP Møller Foundation (16-37), the Health Foundation, Dagmar Marshall's Fond, Aarhus University and Independent Research Fund Denmark (9039-00128B). The authors declare that they have no conflict of interest.

TRIAL REGISTRATION NUMBER

N/A.