Time trends in biological fertility in Britain

Factors influencing natural fecundity in fertile couples: a survey of puerperae and their partners

RESEARCH QUESTION

What is the fecundity rate among fertile couples, and which factors influence it?

DESIGN

Retrospective study of all puerperae attending Cruces University Hospital Human Reproduction Unit over 9 months. An anonymous questionnaire was circulated to all patients, and 2510 valid completed questionnaires were collected. The main inclusion criterion was natural conception resulting in delivery. Pregnancies resulting from ART and contraceptive method failure were excluded. Investigated parameters were time to pregnancy, age and smoking (in women and men), previous pregnancies and intercourse frequency. A mathematical formula was developed to predict the per-month fecundity rate (PMFR).

RESULTS

The cumulative fecundity rate was 29.08%, 54.26%, 68.61%, 89.88%, 96.95% and 98.63% (at 1, 3, 6, 12, 24 and 36 months); between 12 and 36 months, the average PMFR ranged from 8.53-7.48%. Only 1.68% of pregnancies occurred between 24 and 36 months, and only 1.37% thereafter. The best fecundity markers were obtained in the group who had sexual intercourse seven to eight times a week. Women and men younger than 25 years had lower fecundity markers than those aged between 25 and 40 years.

CONCLUSIONS

Fertile couples have a non-negligible per-month fecundity rate between 12 and 36 months, which should be considered when planning fertility studies. The lower fecundity rate observed in women and men aged younger than 25 years deserves more study. Coital frequencies of more than two or three times a week did not affect the fecundity rate and was better with frequencies of seven to eight times a week.

An evaluation of changes over time in the semen parameters data used for the World Health Organization semen analysis reference ranges

BACKGROUND

Previous meta-analyses concluded that there is a decline in sperm parameters over time. This conclusion might be incorrect due to inherent biases or focusing only on a single parameter - sperm concentration.

OBJECTIVE

To study trends in sperm parameters over the past twenty years using data from the trials that defined the reference ranges of the World Health Organization manual.

MATERIALS AND METHODS

Retrospective evaluation of the data used to define the World Health Organization reference ranges. The data from 11 studies, including 3589 participants between 1996-2016, were divided into three period groups based on the decade of study. Differences in semen parameters' distribution were presented in boxplot. P-values were calculated by the Kruskal Wallis rank-sum test followed by Dunn post-hoc test. Analyses were conducted using the R programming language.

RESULTS

A small decrease was noted in mean sperm concentrations (88.1 million/ml, 87.6 million/ml and 77.2 million/ml for the first second and third decades respectively) (p<0.01). However, the 5^th percentile of sperm concentration for the third decade was higher than the first or second decades (18 million/ml vs. 14.9 million/ml and 15 million/ml respectively). No significant differences were noted in progressive motility over the years (p = 0.32). The percent of morphologically normal sperm decreased between the first (24.2%) and the second (12.6%) periods of the study (p<0.001) and then increased in the third decade (14.2%) (p<0.01). While TMC declined between the second and third decades (189 million and 153.9 million respectively, p<0.001), at levels unlikely to decrease fertility. However, the 5^th percentile of the TMC remained stable at 24.9, 20.8 and 20.6 million, for the first, second and third decades respectively (p = 0.36).

DISCUSSION AND CONCLUSION AND RELEVANCE

Trends in sperm parameters over the last three decades do not seem to be clinically significant. This article is protected by copyright. All rights reserved.

Trends in time-to-pregnancy in the USA: 2002 to 2017

STUDY QUESTION

Has there been there a temporal change in time-to-pregnancy (TTP) in the USA.

SUMMARY ANSWER

Overall, TTP was stable over time, but a longer TTP for women over 30 and parous women was identified.

WHAT IS KNOWN ALREADY

Fertility rates in the USA have declined over the past several years. Although these trends have been attributed to changing reproductive intentions, it is unclear whether declining fecundity (the biologic ability to reproduce measured by TTP in the current report) may also play a role. Indeed, trends based on declining sperm quality and higher utilisation of infertility treatment suggest fecundity may be falling.

STUDY DESIGN, SIZE, DURATION

This cross-sectional survey data from the National Survey of Family Growth was administered from 2002 to 2017. The surveys are based on nationally representative samples of reproductive-aged women in the USA. Interviews were conducted in person or through computer-assisted self-administration of sensitive questions.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The study included women who self-reported time spent trying to become pregnant allowing utilisation of the current duration approach to estimate the total duration of pregnancy attempt (i.e. TTP). In all, 1202 participants were analysed over each study period. To estimate a TTP distribution overall and by parity, we used a piecewise constant proportional hazards model that accounts for digit preference. Accelerated-failure-time regression models, which were weighted to account for the sampling design, were used to estimate time ratios (TRs). Models were adjusted for age, BMI, race, education, relationship status, parity, pelvic inflammatory disease treatment and any reproductive problems.

MAIN RESULTS AND THE ROLE OF CHANCE

Of the participants analysed, the average age was 31.8 and BMI was 28.6, which was similar across the survey periods. Relationship status was the only demographic characteristic that changed over time. All other variables remained constant across the study periods. Overall, TRs comparing TTP between 2002 and 2017 increased slightly (TR: 1.02, 95% CI: 0.99, 1.04). When stratified by parity, parous women had a longer TTP over the later years of the study (TR: 1.04, 95% CI: 1.01, 1.06). TTP remained constant for nulliparous women. Similarly, TTP also increased over time for women over age thirty (TR: 1.02, 1.00, 1.05) but not for women under age thirty.

LIMITATIONS, REASONS FOR CAUTION

Small changes in data collection over time may have impacted the findings. We accounted for this in sensitivity analyses using imputed data. Overall, TRs were slightly attenuated using the imputed data, but represented similar patterns to the original data. Results for parous women and women over 30 remained consistent in the sensitivity analyses.

WIDER IMPLICATIONS OF THE FINDINGS

Consistent with reports of falling fertility rates and sperm counts, this study suggests parous and older couples in the USA may be taking longer to become pregnant. Although trends were suggestive of a small overall increase in TTP, particularly for parous women and women over age thirty, additional data are needed to attempt to understand these trends given the societal, economic and public health implications related to fecundity.

STUDY FUNDING/COMPETING INTEREST(S)

Funding was provided by National Institutes of Health grant R03HD097287 to A.C.M. There are no competing interests.

TRIAL REGISTRATION NUMBER

N/A.

The association between iodine intake and semen quality among fertile men in China

BACKGROUND

Iodine intake is essential in the production of thyroid hormone but very few foods are rich in it. Iodine deficiency or excess iodine level may both lead to thyroid disorders, which further affects human fertility function. The objective of this study is to investigate the relationship between iodine intake and seminal parameters among fertile men in China.

METHODS

A total of 1098 couples were recruited by trained physicians at different family planning service stations in 2015. Semen and iodine samples were obtained from male respondents. A questionnaire survey inquired about demographic information from couples. The main outcome variables of semen quality were semen volume, semen concentration, semen motility, and sperm count, and time to pregnancy. Urinary iodine concentration (UIC) was used to measure iodine levels for male respondents. Ordinary least squared regressions and logistic regressions were performed to estimate the association between iodine intake level and semen quality parameters.

RESULTS

Male respondents with deficient or excess iodine levels had a 5% higher semen volume relative to those with optimal iodine intake (p < 0.1). Suboptimal iodine intake was negatively associated with semen concentration and semen counts (p < 0.01). Longer time of pregnancy was observed in iodine deficiency and excess group than those in the optimal group (p < 0.01).

CONCLUSION

In general, iodine deficiency and excess were both associated with decreasing semen quality parameters in male respondents.

The Role of Number of Copies, Structure, Behavior and Copy Number Variations (CNV) of the Y Chromosome in Male Infertility

The World Health Organization (WHO) defines infertility as the inability of a sexually active, non-contracepting couple to achieve spontaneous pregnancy within one year. Statistics show that the two sexes are equally at risk. Several causes may be responsible for male infertility; however, in 30–40% of cases a diagnosis of idiopathic male infertility is made in men with normal urogenital anatomy, no history of familial fertility-related diseases and a normal panel of values as for endocrine, genetic and biochemical markers. Idiopathic male infertility may be the result of gene/environment interactions, genetic and epigenetic abnormalities. Numerical and structural anomalies of the Y chromosome represent a minor yet significant proportion and are the topic discussed in this review. We searched the PubMed database and major search engines for reports about Y-linked male infertility. We present cases of Y-linked male infertility in terms of (i) anomalies of the Y chromosome structure/number; (ii) Y chromosome misbehavior in a normal genetic background; (iii) Y chromosome copy number variations (CNVs). We discuss possible explanations of male infertility caused by mutations, lower or higher number of copies of otherwise wild type, Y-linked sequences. Despite Y chromosome structural anomalies are not a major cause of male infertility, in case of negative results and of normal DNA sequencing of the ascertained genes causing infertility and mapping on this chromosome, we recommend an analysis of the karyotype integrity in all cases of idiopathic fertility impairment, with an emphasis on the structure and number of this chromosome.

Mitochondrial DNA copy number in peripheral blood: a potential non-invasive biomarker for female subfertility

PURPOSE

Low mitochondrial DNA (mtDNA) content in oocytes and in cumulus cells is an indicator of poor oocyte quality. Moreover, initial evidence showed a correlation between mtDNA content in cumulus cells and mtDNA copy number in peripheral blood cells. On these bases, we deemed of interest investigating the correlation between mtDNA copy number in peripheral blood and natural fecundity.

METHODS

This is a nested case-control study drawn from a prospective cohort of pregnant women referred for routine first trimester screening for aneuploidies (from 11 + 0 to 12 + 6 weeks of gestation) between January 2012 and March 2013 at the "Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico" of Milan, Italy. Cases were subfertile women who attempted to become pregnant for 12-24 months. Controls were the two subsequently age-matched women who became pregnant in less than 1 year. MtDNA was quantified using real-time PCR and normalized to nuclear DNA.

RESULTS

One hundred and four subfertile women and 208 controls were selected. The median (IQR) mtDNA copy number was 95 (73-124) and 145 (106-198), respectively (p < 0.001). The area under the ROC curve was 0.73 (95% CI 0.67-0.79) (p < 0.001). The Youden index was 105 mtDNA copy number. The crude OR for subfertility in women with mtDNA copy number below this threshold was 5.72 (95% CI 3.43-9.55). The accuracy of mtDNA copy number assessment in peripheral blood progressively decreased with increasing female age.

CONCLUSIONS

Low mtDNA copy number in peripheral blood is associated with an increased risk of subfertility and may represent a biomarker of natural fecundity.

Is human fecundity changing? A discussion of research and data gaps precluding us from having an answer

Fecundity, the biologic capacity to reproduce, is essential for the health of individuals and is, therefore, fundamental for understanding human health at the population level. Given the absence of a population (bio)marker, fecundity is assessed indirectly by various individual-based (e.g. semen quality, ovulation) or couple-based (e.g. time-to-pregnancy) endpoints. Population monitoring of fecundity is challenging, and often defaults to relying on rates of births (fertility) or adverse outcomes such as genitourinary malformations and reproductive site cancers. In light of reported declines in semen quality and fertility rates in some global regions among other changes, the question as to whether human fecundity is changing needs investigation. We review existing data and novel methodological approaches aimed at answering this question from a transdisciplinary perspective. The existing literature is insufficient for answering this question; we provide an overview of currently available resources and novel methods suitable for delineating temporal patterns in human fecundity in future research.

Macroscopic and microscopic morphology of first trimester miscarriage and subsequent pregnancy outcome - An exploratory study

INTRODUCTION

Reduced chorionic villous vascularization is associated with first trimester miscarriage and second trimester fetal loss. Differences in villous vascularization have been observed in combination with complications in the third trimester of pregnancy. The aim of this study was to investigate whether abnormal morphology and reduced chorionic villous vascularization in first trimester miscarriages are associated with an increased risk on adverse outcome and/or pregnancy complications in subsequent pregnancy. Secondly, to assess the influence of these parameters on the length of the interpregnancy interval and infertility.

METHODS

In a retrospective cohort study 134 consecutive women who underwent dilatation and curettage for a miscarriage were included. The degree of chorionic villous vascularization in miscarriage tissue was determined by a pathologist. Ultrasound details of these miscarriages and clinical data on the subsequent pregnancy of these women were obtained.

RESULTS

Neither reduced vascularization nor early embryonic arrest in first trimester miscarriages are associated with an increased risk of a subsequent miscarriage or adverse obstetric and perinatal outcome of subsequent pregnancy. Abnormal morphology of the first trimester miscarriage did not influence the time to subsequent pregnancy. A shorter mean interpregnancy interval between miscarriages was observed after miscarriages with reduced chorionic villous vascularization (5.5 vs. 10.7 months; p = 0.051), showing a trend towards an association.

DISCUSSION

Chorionic villous vascularization and morphology have no influence on subsequent pregnancy outcome. Therefore it remains unknown what aspects of miscarriage are causing the increased risk on subsequent miscarriage and complications in the third trimester of the subsequent pregnancy.

Improved Fecundity in Northern China: A Secular Trend from 1980 to 2003

Objective

This study aimed to assess the trend of human fecundity over time in China.

Methods

This retrospective study was conducted in Tongliao, China. Couples who were married during the time period between January 1, 1981 and December 31, 2003 were considered eligible for this study. A total of 27,413 individuals provided valid information via house-to-house interviews. The 12-month cumulative pregnancy rate (CPR) and annual percentage change were used as the outcome measurements.

Results

There was a significant increase in the CPR over the five successive time groups. A break point in 1988 divided the entire study period into two distinct segments: 1981–1988, during which the CPR increased from 72.2% to 84.2%, and 1988–2003, during which the CPR increased from 84.2% to 87.2%.

Conclusions

The findings were unlikely to be the result of biases, and could not be explained by increased medical treatment for infertility and changes in the prevalence of sexually transmitted diseases. Dramatic societal and behavioral changes due to the unique family planning policy and economic reform policies in China might have been the plausible reason for the results.

Paternal exposures to environmental chemicals and time-to-pregnancy: overview of results from the LIFE study

Published findings from the Longitudinal Investigation of Fertility and the Environment (LIFE) Study regarding the relation between environmental chemicals and couple fecundity, as measured by time-to-pregnancy (TTP), are reviewed with a particular focus on role of the male partner. The LIFE Study recruited 501 couples from 16 counties in two U.S. states upon discontinuing contraception for purposes of becoming pregnant. Upon enrollment, couples provided a blood and urine sample for the quantification of persistent and non-persistent environmental chemicals, respectively, and then completed daily journals until pregnant or up to one year of trying. Female partners used fertility monitors to aid the timing of intercourse relative to ovulation, and digital home pregnancy test kits on the day of expected menses. Chemical classes included: metals, persistent organic pollutants, environmental phenols, and phthalates that were quantified using inductively coupled plasma mass spectrometry or isotope dilution high-resolution or tandem mass spectrometry. Time-to-pregnancy (TTP) was defined as the number of prospectively observed menstrual cycles required for pregnancy. Fecundability odds ratios (FORs) and 95% confidence intervals (CIs) were estimated for each chemical and partner after adjusting for potential confounders and accounting for right censoring and time off contraception. FORs < 1 are suggestive of diminished fecundity or a longer TTP. Significant reductions (ranging from 17-31%) in couple fecundity were observed for male partners' concentration of lead (0.83; 0.70, 0.98), 2,2',4,4'-tetrahydroxybenzophenone (0.69; 0.49, 0.97), monobenzyl (0.80; 0.67, 0.97), and monomethyl (0.81; 0.70, 0.94) phthalates after adjusting for the female partners' concentrations. Seven PCB congeners quantified in men's serum were associated with a 17-29% reduction in couple fecundity. Our findings underscore the importance of a couple-based exposure design, inclusive of the male partner, when assessing couple-dependent outcomes such as TTP to avoid misinterpretation of results based only upon the female partner.

Serum Levels of 25-Hydroxyvitamin D and Time to Natural Pregnancy

AIM

The aim of this study was to evaluate whether natural fertility is related to serum 25-hydroxyvitamin D (25-OH-vitamin D) levels.

METHODS

A nested case-control study was designed from a prospective cohort of pregnant women undergoing first trimester screening for aneuploidies. Cases included women seeking pregnancy for 12-24 months. Controls were the subsequent age-matched women conceiving in less than 1 year. We excluded women aged ≥40 or <18 years, those assuming supplementary products that included vitamin D before or during pregnancy, those with irregular menstrual cycles or known causes of subfertility, those conceiving through assisted reproductive techniques or requiring ovarian stimulation and those who were overweight or obese. A quantitative detection of serum 25-OH-vitamin D and patients' interview were performed.

RESULTS

Seventy-three cases and 73 matched controls were selected. The mean ± SD serum 25-OH-vitamin D was 21.2 ± 6.8 and 19.7 ± 7.3 ng/ml, respectively (p = 0.16). The number (%) of women with serum levels <20 ng/ml (vitamin D insufficiency) was 34 (47%) and 37 (51%), respectively (p = 0.73). The adjusted OR of longer time to pregnancy in women with vitamin D insufficiency was 0.84 (95% CI 0.42-1.66).

CONCLUSIONS

Our study does not support a crucial role of 25-OH-vitamin D in natural fertility.

Selection bias due to parity-conditioning in studies of time trends in fertility

BACKGROUND

Studies of couple fertility over time have often examined study populations with broad age ranges at a cross-section of time. An increase in fertility has been observed in studies that followed episodes of fertility events either prospectively among nulliparous women or retrospectively among parous women. Fertility has a biological effect on parity. If defined at a cross-section of time, parity will also be affected by year of birth, and thus becomes a collider. Conditioning (stratifying, restricting, or adjusting) on a collider may cause selection bias in the studied association.

METHODS

A study with prospective follow-up was taken as the model to assess the validity of fertility studies. We demonstrate the potential for selection bias using causal graphs and nationwide birth statistics and other demographic data. We tested the existence of parity-conditioning bias in data including both parous and nulliparous women. We also used a simulation approach to assess the strength of the bias in populations with prior at-risk cycles. Finally, we evaluated the potential for selection bias due to conditioning on parity in various sampling frames.

RESULTS

Analyses indicate that the observed increase in fertility over time can be entirely explained by selection bias due to parity-conditioning.

CONCLUSION

Heterogeneity in fertility and differential success in prior at-risk cycles are the ultimate factors behind the selection bias. The potential for selection bias due to parity-conditioning varies by sampling frame. A prospective multidecade study with representative sampling of birth cohorts and follow-up from menarche to menopause would bypass the described bias.

Reproductive outcomes and nononcologic complications after radioactive iodine ablation for well-differentiated thyroid cancer

BACKGROUND

Radioactive iodine (RAI) ablation is frequently performed after initial surgery for well-differentiated thyroid cancer (WDTC). We examined the frequency and timing of childbirth as well as nononcologic complications after RAI ablation for WDTC on a population level.

METHODS

A retrospective cohort study of 25,333 patients (18,850 women) with WDTC was performed using the California Cancer Registry and California Office of Statewide Health Planning and Development database, 1999-2008. The primary outcomes were birthrate and median time to first live birth among women of childbearing age. Secondary outcomes were nononcologic diagnoses occurring outside the acute setting (>30 days) after ablation.

RESULTS

RAI ablation did not affect birthrate among women in the full dataset. However, in subgroup analyses, birthrate among women age 35-39 was significantly decreased in those who received RAI versus those who did not (11.5 versus 16.3 births per 1000 woman-years, p<0.001). Median time to first live birth after diagnosis of WDTC was prolonged among women who received RAI compared to those who did not (34.5 versus 26.1 months; p<0.0001). When 5-year age groups were examined individually, delay to first live birth was observed in women age 20-39 (p<0.05). This remained significant after adjustment for tumor characteristics, socioeconomic status, and marital status. The only nononcologic, nonreproductive adverse effect associated with RAI ablation was an increased rate of nasolacrimal stenosis (RR 3.44, p<0.0001).

CONCLUSIONS

RAI ablation is associated with delayed childbearing in women across most of the reproductive lifespan, and with decreased birthrate in the late reproductive years. The underlying mechanism likely involves physician recommendation to delay pregnancy, as well as a potential impact of RAI on both reproductive choice and reproductive health. Further investigation is merited.

Lipid concentrations and couple fecundity: the LIFE study

CONTEXT

A role of lipids in human fecundity is hypothesized as cholesterol is the main substrate for steroid synthesis and has also been shown to affect the hormonal milieu and steroidogenesis in both men and women.

OBJECTIVE

The objective of the study was to evaluate the association between male and female serum lipid concentrations and time to pregnancy (TTP).

DESIGN/SETTING

A population-based prospective cohort study recruiting couples from 16 counties in Michigan and Texas (2005-2009) using sampling frameworks allowing for identification of couples planning pregnancy in the near future.

PARTICIPANTS

Five hundred one couples desiring pregnancy and discontinuing contraception were followed up for 12 months or until a human chorionic gonadotropin pregnancy was detected.

MAIN OUTCOME AND MEASURES

Fecundability odds ratios (FORs) and 95% confidence intervals (CIs) were estimated after adjusting for age, body mass index, race, and education in relationship to female, male, and joint couple lipid concentrations.

RESULTS

Serum free cholesterol levels were higher on average among male and female partners of couples who did not became pregnant during the study follow-up (female, P = .04; male, P = .009), and levels in female partners were associated with significantly longer TTP in models based on both individual and couples concentrations (individual models: FOR 0.98, 95% CI 0.97, 0.99; couple models: FOR 0.98, 95% CI 0.97, 0.99). Male free cholesterol concentrations were associated with TTP only in the couple-based models (FOR 0.98, 95% CI 0.97, 0.99). Sensitivity analyses suggested that the observed associations are unlikely to be explained by potential unmeasured confounding such as diet.

CONCLUSIONS

Our results suggest that serum free cholesterol concentrations in both men and women have an effect on TTP, highlighting the importance of cholesterol and lipid homeostasis for male and female fecundity.

Semiparametric modeling of grouped current duration data with preferential reporting

Current duration data arise in cross-sectional studies from questions on the length of time from an initiating event to the time of interview. For example, in the National Survey on Family Growth, women who were considered at risk for pregnancy were asked (i) 'Are you currently attempting pregnancy?' and (ii) 'If yes, how many months have you been attempting to get pregnant?' The responses to (ii), referred to as the current durations, are length-biased because women with longer durations are more likely to answer yes to question (i) and therefore be included in the sample. Previous methods to analyze such data include continuous time nonparametric and parametric approaches. In this article, we propose a semiparametric Cox model and a piecewise constant baseline model (used to account for digit preference) to analyze grouped current duration data. We discuss and investigate through simulation studies, the robustness properties of the proposed methods when digit preference is present. Lastly, we present an analysis of the current duration data resulting from the 2002 National Survey on Family Growth. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Lipid concentrations and semen quality: the LIFE study

The decline in sperm count rates over the last 50 years appears to parallel the rising prevalence of obesity. As lipid levels are strongly associated with obesity, high lipids levels or hyperlipidaemia may thus play an important role in the decline in fertility in addition to other environmental or lifestyle factors. The objective of this population based cohort study was to evaluate the association between men's serum lipid concentrations and semen quality parameters among 501 male partners of couples desiring pregnancy and discontinuing contraception. Each participant provided prospectively up to two semen samples (94% of men provided one or more semen samples, and 77% of men provided a second sample approximately 1 month later). Linear mixed effects models were used to estimate the associations between baseline lipid concentrations and semen quality parameters, adjusted for age, body mass index and race. We found that higher levels of serum total cholesterol, free cholesterol and phospholipids were associated with a significantly lower percentage of spermatozoa with intact acrosome and smaller sperm head area and perimeter. Our results suggest that lipid concentrations may affect semen parameters, specifically sperm head morphology, highlighting the importance of cholesterol and lipid homeostasis for male fecundity.

Endocrine disruption: fact or urban legend?

Endocrine disruptors (EDs) are substances that cause adverse health effects via endocrine-mediated mechanisms in an intact organism or its progeny or (sub) populations. Purported EDCs in personal care products include 4-MBC (UV filter) or parabens that showed oestrogenic activity in screening tests, although regulatory toxicity studies showed no adverse effects on reproductive endpoints. Hormonal potency is the key issue of the safety of EDCs. Oestrogen-based drugs, e.g. the contraceptive pill or the synthetic oestrogen DES, possess potencies up to 7 orders of magnitude higher than those of PCP ingredients; yet, in utero exposure to these drugs did not adversely affect fertility or sexual organ development of offspring unless exposed to extreme doses. Additive effects of EDs are unlikely due to the multitude of mechanisms how substances may produce a hormone-like activity; even after uptake of different substances with a similar mode of action, the possibility of additive effects is reduced by different absorption, metabolism and kinetics. This is supported by a number of studies on mixtures of chemical EDCs. Overall, despite of 20 years of research a human health risk from exposure to low concentrations of exogenous chemical substances with weak hormone-like activities remains an unproven and unlikely hypothesis.

Time trends in biological fertility in Western Europe

We investigated trends in biological fertility in a comprehensive analysis of 5 major European data sets with data on time to pregnancy (TTP) and proportion of contraceptive failures. In particular, we distinguished a period effect from a birth cohort effect (lifelong tendency) in both sexes. Attempts at conception not resulting in birth were excluded. We analyzed data on pregnancies occurring in 9,247 couples between 1953 and 1993 and performed sensitivity analyses to check the robustness of findings. Separate analyses of each time effect showed an increasing fertility trend. Mutually adjusted analyses demonstrated that this rise was visible as a male cohort effect for both TTP and contraceptive failure. On the other hand, the female birth cohort effect showed a slight fall in the first half of the study period for both TTP and contraceptive failure. As a period effect, fertility remained generally stable, the slight trends in TTP and contraceptive failure being in opposite directions, likely indicating an artifact. The rising trend accords with most previous evidence. The increasing trend in male fertility does not contradict the previously reported semen quality deterioration, the effects of which are calculated to be small. The declining female fertility accords with a falling dizygotic twinning rate during the same period.

Prenatal famine, birthweight, reproductive performance and age at menopause: the Dutch hunger winter families study

STUDY QUESTION

Is there an association between acute prenatal famine exposure or birthweight and subsequent reproductive performance and age at menopause?

SUMMARY ANSWER

No association was found between intrauterine famine exposure and reproductive performance, but survival analysis showed that women exposed in utero were 24% more likely to experience menopause at any age.

WHAT IS KNOWN ALREADY

Associations between prenatal famine and subsequent reproductive performance have been examined previously with inconsistent results. Evidence for the effects of famine exposure on age at natural menopause is limited to one study of post-natal exposure.

STUDY DESIGN, SIZE, DURATION

This cohort study included men and women born around the time of the Dutch famine of 1944-1945. The study participants (n = 1070) underwent standardized interviews on reproductive parameters at a mean age of 59 years.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The participants were grouped as men and women with prenatal famine exposure (n = 407), their same-sex siblings (family controls, n = 319) or other men and women born before or after the famine period (time controls, n = 344). Associations of famine exposure with reproductive performance and menopause were analysed using logistic regression and survival analysis with competing risk, after controlling for family clustering.

MAIN RESULTS AND THE ROLE OF CHANCE

Gestational famine exposure was not associated with nulliparity, age at birth of first child, difficulties conceiving or pregnancy outcome (all P> 0.05) in men or women. At any given age, women were more likely to experience menopause after gestational exposure to famine (hazard ratio 1.24; 95% CI 1.03, 1.51). The association was not attenuated with an additional control for a woman's birthweight. In this study, there was no association between birthweight and age at menopause after adjustment for gestational famine exposure.

LIMITATIONS, REASON FOR CAUTION

Age at menopause was self-reported and assessed retrospectively. The study power to examine associations with specific gestational periods of famine exposure and reproductive function was limited.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings support previous results that prenatal famine exposure is not related to reproductive performance in adult life. However, natural menopause occurs earlier after prenatal famine exposure, suggesting that early life events can affect organ function even at the ovarian level.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by the NHLBI/NIH (R01 HL-067914).

TRIAL REGISTRATION NUMBER

Not applicable.

Has fertility declined from mid-1990s to mid-2000s?

OBJECTIVE

To assess changes in self-reported fertility from the mid-1990s to the mid-2000s.

DESIGN

The study is a retrospective population-based study.

SETTING

The study applied a dataset from two cross-sectional surveys conducted in the mid-1990s and some 10 years later, inviting all women in a county in Norway.

POPULATION

Women aged 50-59 years enrolled in either survey constituted two cohorts. Data on 4468 women in the first survey (Cohort 1940) and 4951 women in the latter survey (Cohort 1950) were collected by structured questionnaires.

MAIN OUTCOME MEASURES

Prevalence of fertility, infertility with subcategories subfertility and involuntary childlessness, as well as childlessness was estimated and compared between the surveys. Possible sociodemographic and lifestyle predictors of fertility were assessed at different points in time.

RESULTS

Fertility declined over the two successive surveys; 87.8% of the women in Cohort 1940 were fertile compared with 84.2% of the women in Cohort 1950 (p = 0.000). The prevalence of infertility increased over time due to an increase in subfertility from 7.8 to 10.6% (p = 0.000). The level of education increased with time, as did at-risk alcohol consumption and smoking, and these factors were adversely associated with fertility. The proportion of childless women increased across surveys (p = 0.004) but relatively fewer women were involuntarily childless in Cohort 1950 than in Cohort 1940 (p = 0.543).

CONCLUSION

Fertility, measured at 10-year intervals, declined significantly. The decline in fertility was related to changes in subfertility. Adjustments for sociodemographic and lifestyle factors did not fully explain the decline in fertility.

Endocrine-disrupting chemicals: associated disorders and mechanisms of action

The incidence and/or prevalence of health problems associated with endocrine-disruption have increased. Many chemicals have endocrine-disrupting properties, including bisphenol A, some organochlorines, polybrominated flame retardants, perfluorinated substances, alkylphenols, phthalates, pesticides, polycyclic aromatic hydrocarbons, alkylphenols, solvents, and some household products including some cleaning products, air fresheners, hair dyes, cosmetics, and sunscreens. Even some metals were shown to have endocrine-disrupting properties. Many observations suggesting that endocrine disruptors do contribute to cancer, diabetes, obesity, the metabolic syndrome, and infertility are listed in this paper. An overview is presented of mechanisms contributing to endocrine disruption. Endocrine disruptors can act through classical nuclear receptors, but also through estrogen-related receptors, membrane-bound estrogen-receptors, and interaction with targets in the cytosol resulting in activation of the Src/Ras/Erk pathway or modulation of nitric oxide. In addition, changes in metabolism of endogenous hormones, cross-talk between genomic and nongenomic pathways, cross talk with estrogen receptors after binding on other receptors, interference with feedback regulation and neuroendocrine cells, changes in DNA methylation or histone modifications, and genomic instability by interference with the spindle figure can play a role. Also it was found that effects of receptor activation can differ in function of the ligand.

The effect of pulsed 900-MHz GSM mobile phone radiation on the acrosome reaction, head morphometry and zona binding of human spermatozoa

Several recent studies have indicated that radiofrequency electromagnetic fields (RF-EMF) have an adverse effect on human sperm quality, which could translate into an effect on fertilization potential. This study evaluated the effect of RF-EMF on sperm-specific characteristics to assess the fertilizing competence of sperm. Highly motile human spermatozoa were exposed for 1 h to 900-MHz mobile phone radiation at a specific absorption rate of 2.0 W/kg and examined at various times after exposure. The acrosome reaction was evaluated using flow cytometry. The radiation did not affect sperm propensity for the acrosome reaction. Morphometric parameters were assessed using computer-assisted sperm analysis. Significant reduction in sperm head area (9.2 ± 0.7 μm² vs. 18.8 ± 1.4 μm²) and acrosome percentage of the head area (21.5 ± 4% vs. 35.5 ± 11.4%) was reported among exposed sperm compared with unexposed controls. Sperm-zona binding was assessed directly after exposure using the hemizona assay. The mean number of zona-bound sperm of the test hemizona and controls was 22.8 ± 12.4 and 31.8 ± 12.8 (p < 0.05), respectively. This study concludes that although RF-EMF exposure did not adversely affect the acrosome reaction, it had a significant effect on sperm morphometry. In addition, a significant decrease in sperm binding to the hemizona was observed. These results could indicate a significant effect of RF-EMF on sperm fertilization potential.

Male athletic activities and their effects on semen and hormonal parameters

Infertility is generally defined as the inability to conceive a pregnancy or the failure to do so within a reasonable period (typically 12 months). Approximately 85% of couples conceive a first pregnancy within 12 months. The prevalence of infertility has increased over the past 10 years, with approximately 10 million affected couples in the United States. Roughly 40% to 50% of infertility is either due to, or is contributed by, a male factor. Given how common the condition is, men and their partners are understandably concerned and interested in identifying and eliminating risk factors for male infertility. This article reviews the available literature on various aspects of male infertility related to athletic pursuits. These include the effects of exercise on semen parameters, hormonal axes, and testicular health. Due to the prevalence and particular relevance of anabolic steroid use by athletes and the impact of steroid use on fertility, this topic is also reviewed.

Fecundability among newly married couples in agricultural villages in Palestine: a prospective study

BACKGROUND

The validity of studies on fecundability in Western countries has been questioned. The complexity of societal and cultural factors makes it difficult to dissect pure biological impact. Our aim was to assess couple fecundability in a population which to a large degree is unaffected by the same socio-cultural influences.

METHODS

We conducted a prospective study on time-to-pregnancy (TTP), with a complete follow-up between 2005 and 2007, among 205 newly married couples in two Palestinian agricultural villages. The couples had never had premarital sex and all planned to become pregnant. We followed the couples from the date of marriage until pregnancy was recognized by a pregnancy test, or at maximum 12 months.

RESULTS

Overall fecundability was 0.17. Unexpectedly, cycle fecundability increased during the first cycles from 0.16 (cycle 1) to 0.25 (cycle 5), after which the expected decline started. The initial increase in fecundability was restricted to couples with teenage brides. A total of 70.7% of the couples conceived within 6 cycles, 13.4% did not conceive during follow-up. Prolonged TTP was associated with the oldest age category for both genders. Educated women appeared to be highly fecund.

CONCLUSIONS

The fecundability result is probably uninfluenced by the societal and cultural factors seen in Western populations, because premarital sex is a taboo in this Muslim population. The increase in fecundability during the first months following marriage is difficult to interpret, but could be due to either behavioural or biological influences.

Is human fecundity declining in Western countries?

Since Carlsen and co-workers reported in 1992 that sperm counts have decreased during the second half of the last century in Western societies, there has been widespread anxiety about the adverse effects of environmental pollutants on human fecundity. The Carlsen report was followed by several re-analyses of their data set and by many studies on time trends in sperm quality and on secular trends in fecundity. However, the results of these studies were diverse, complex, difficult to interpret and, therefore, less straightforward than the Carlsen report suggested. The claims that population fecundity is declining and that environmental pollutants are involved, can neither be confirmed nor rejected, in our opinion. However, it is of great importance to find out because the possible influence of widespread environmental pollution, which would adversely affect human reproduction, should be a matter of great concern triggering large-scale studies into its causes and possibilities for prevention. The fundamental reason we still do not know whether population fecundity is declining is the lack of an appropriate surveillance system. Is such a system possible? In our opinion, determining total sperm counts (as a measure of male reproductive health) in combination with time to pregnancy (as a measure of couple fecundity) in carefully selected populations is a feasible option for such a monitoring system. If we want to find out whether or not population fecundity will be declining within the following 20-30 years, we must start monitoring now.

Personal experiences of women with diabetes who do not attend pre-pregnancy care

AIMS

To explore the views of women who did not attend pre-pregnancy care (PPC), in particular their accounts of contraception, previous pregnancies and the influence of healthcare advice.

METHODS

We conducted semi-structured interviews with 29 pregnant women (21 with Type 1 diabetes, eight with Type 2 diabetes) at three UK specialist diabetes antenatal clinics. Interviews explored women's journeys to becoming pregnant, including use of contraception, their views regarding diabetes and pregnancy and the factors which encouraged and discouraged them from attending PPC.

RESULTS

All women had some understanding of the issues concerning diabetes during pregnancy, predominantly regarding the benefits of PPC (90%) and optimal glycaemic control (80%) and risks of malformation (48%) and macrosomia (35%). Most were not regularly using contraception (70%), having stopped deliberately (45%), become unintentionally less rigorous (28%) or experienced side effects/contraindications (14%). Knowledge concerning the risks of pregnancy (90%) and past pre-conception counselling (38%) did not encourage women to attend PPC, and neither did personal experience of miscarriage, malformation or stillbirth in women with previous poor pregnancy outcome (41%). Barriers included conceiving faster than anticipated (45%), fertility concerns (31%), negative experiences with health professionals (21%), desire for a 'normal' pregnancy (17%) and the logistics of attending (10%).

CONCLUSIONS

More integrated diabetes and reproductive health/contraceptive advice, increased awareness of the potentially short time between stopping contraception and conception and more intensive support between pregnancies are required, particularly for women with previously poor outcomes. Research is also needed into how communication between health professionals and women with diabetes can be improved.

What has happened to human fertility?

Semen quality appears to have declined in recent decades in some populations, e.g. north-western Europe. At the same time, couple fertility may have increased. Hypotheses are suggested for this apparent inconsistency. Alongside the deterioration of spermatogenesis there is clear evidence of an increase in other related problems, notably testicular cancer. The sharply rising trend in this condition started a century ago--decades earlier than sometimes thought. This and other evidence clearly indicates an environmental origin, but there is also a definite genetic component. The relationship of genetics and environment is discussed in the context of the puzzle that infertility is inherited, which appears to be impossible from an evolutionary standpoint. Poor semen quality is related not only to testicular cancer but also to zygote development, in which cancer-like disruption of the genetic apparatus is observed, with serious implications for offspring health. This needs to be seen in the context that human reproduction is prone to a higher degree of impairment than that of other mammalian species, in relation to spermatogenesis, couple fertility, early pregnancy loss and embryonic aneuploidy; female- and male-mediated pathways are both implicated. It is unclear whether such human specificity originated on an evolutionary/genetic or a historico-social timescale, which is important in relation to pathogenesis. The evidence clearly indicates that the currently most popular explanation for male reproductive system impairment, the endocrine disruption hypothesis, cannot explain the main features of the descriptive epidemiology. An alternative pathogenesis is outlined, and some possible exposures considered that could be responsible.

The epidemiology of infertility in the North East of Scotland

BACKGROUND

There is a perception that the prevalence of infertility is on the rise. This study aimed to determine the current prevalence of infertility in a defined geographical population, ascertain changes in self-reported infertility over time and identify risk factors associated with infertility.

METHODS

A postal questionnaire survey of a random population-based sample of women aged 31-50 years was performed in the Grampian region of Scotland. Questions addressed the following areas: pregnancy history, length of time taken to become pregnant each time, whether medical advice had been sought and self-reported exposure to factors associated with infertility.

RESULTS

Among 4466 women who responded, 400 (9.0%) [95% CI 8.1, 9.8] had chosen not to have children. Of the remaining 4066 women, 3283 (80.7%) [95% CI 79.5, 82.0] reported no difficulties in having children and the remaining 783 (19.3%) [95% CI 18.1, 20.5] had experienced infertility, defined as having difficulty in becoming pregnant for more than 12 months and/or seeking medical advice. In total 398 (9.8%) [95% CI 8.9, 10.7] women had primary infertility, 285 (7.0%) [95% CI 6.2, 7.8] had secondary infertility, 100 (2.5%) [95% CI 2.0, 2.9] had primary as well as secondary infertility. A total of 342 (68.7%) and 208 (73.0%) women with primary and secondary infertility, respectively, sought medical advice and 202 (59.1%) and 118 (56.7%) women in each group subsequently conceived. History of pelvic surgery, Chlamydial infection, endometriosis, chemotherapy, long-term health problems and obesity were associated with infertility. In comparison with a similar survey of women aged 46-50 from the same geographical area, the prevalence of both primary infertility (>24 months) [70/1081, (6.5%) versus 68/710 (9.6%) P = 0.02] and secondary infertility [29/1081 (2.7%) versus 40/710 (5.6%) P = 0.002] were significantly lower.

CONCLUSIONS

Nearly one in five women attempting conception sampled in this study experienced infertility, although over half of them eventually conceived. Fertility problems were associated with endometriosis, Chlamydia trachomatis infection and pelvic surgery, as well as obesity, chemotherapy and some long-term chronic medical conditions. There is no evidence of an increase in the prevalence of infertility in this population over the past 20 years.

Women's compliance with nutrition and lifestyle recommendations before pregnancy: general population cohort study

OBJECTIVE

To examine the extent to which women planning a pregnancy comply with recommendations for nutrition and lifestyle.

DESIGN

Prospective cohort study.

SETTING

Southampton, United Kingdom.

PARTICIPANTS

12 445 non-pregnant women aged 20-34 recruited to the Southampton Women's Survey through general practices, 238 of whom became pregnant within three months of being interviewed.

MAIN OUTCOME MEASURES

Folic acid supplement intake, alcohol consumption, smoking, diet, and physical activity before pregnancy.

RESULTS

The 238 women who became pregnant within three months of the interview were only marginally more likely to comply with recommendations for those planning a pregnancy than those who did not become pregnant in this period. Among those who became pregnant, 2.9% (95% confidence interval 1.2% to 6.0%) were taking 400 microg or more of folic acid supplements a day and drinking four or fewer units of alcohol a week, compared with 0.66% (0.52% to 0.82%) of those who did not become pregnant. 74% of those who became pregnant were non-smokers compared with 69% of those who did not become pregnant (P=0.08). Women in both groups were equally likely to consume five or more portions of fruit and vegetables a day (53% in each group, P=1.0), but only 57% of those who became pregnant had taken any strenuous exercise in the past three months compared with 64% in those who did not become pregnant (P=0.03).

CONCLUSION

Only a small proportion of women planning a pregnancy follow the recommendations for nutrition and lifestyle. Greater publicity for the recommendations is needed, but as many pregnancies are unplanned, improved nutrition and lifestyles of women of childbearing age is also required.

Methodological issues in analyzing time trends in biologic fertility: protection bias

One method of assessing biologic fertility is to measure time to pregnancy (TTP). Accidental pregnancies do not generate a valid TTP value and lead to nonrandom missing data if couples experiencing accidental pregnancies are more fertile than the general population. If factors affecting the rate of accidental pregnancies, such as availability of effective contraception and induced abortion, vary over time, then the result may be protection bias in the estimates of fertility time trends. Six European data sets were analyzed to investigate whether evidence of protection bias exists in TTP studies of fertility trends in Europe over the past 50 years. Couples experiencing accidental pregnancies tended to be more fertile than the general population. However, trends in accidental pregnancy rates were inconsistent across countries and were insufficient to produce substantial bias in fertility trends in simulated data. Where protection bias is suspected, the authors demonstrate use of 2 multiple imputation methods to generate realizations for the missing TTP values for accidental pregnancies. Simulation studies show that both methods successfully reduce or eliminate protection bias. The authors also demonstrate that standard sensitivity analyses for dealing with accidental pregnancies provide an upper bound on the extent of any bias.

Regional differences in time to pregnancy among fertile women from five Colombian regions with different use of glyphosate

The objective of this study was to test whether there was an association between the use of glyphosate when applied by aerial spray for the eradication of illicit crops (cocaine and poppy) and time to pregnancy (TTP) among fertile women. A retrospective cohort study (with an ecological exposure index) of first pregnancies was undertaken in 2592 fertile Colombian women from 5 regions with different uses of glyphosate. Women were interviewed regarding potential reproductive, lifestyle, and work history predictors of TTP, which was measured in months. Fecundability odds ratios (fOR) were estimated using a discrete time analogue of Cox's proportional hazard model. There were differences in TTP between regions. In the final multivariate model, the main predictor was the region adjusted by irregular relationship with partner, maternal age at first pregnancy, and, marginally, coffee consumption and self-perception of water pollution. Boyaca, a region with traditional crops and. recently, illicit crops without glyphosate eradication spraying (manual eradication), displayed minimal risk and was the reference region. Other regions, including Sierra Nevada (control area, organic agriculture), Putumayo and Narino (illicit crops and intensive eradication spray program), and Valle del Cauca, demonstrated greater risk of longer TTP, with the highest risk for Valle del Cauca (fOR 0.15, 95% CI 0.12, 0.18), a sugar-cane region with a history of use of glyphosate and others chemicals for more than 30 yr. The reduced fecundability in some regions was not associated with the use of glyphosate for eradication spraying. The observed ecological differences remain unexplained and may be produced by varying exposures to environmental factors, history of contraceptive programs in the region, or psychological distress. Future studies examining these or other possible causes are needed.

Geographical differences in semen quality in a population of young healthy volunteers from the different regions of Spain

Numerous studies have shown geographical differences in semen quality even within a given country. We have previously reported a low semen quality in volunteers from the province of Barcelona compared with the general population. The objective of this study was to determine the semen quality in a population of young healthy volunteers from the different regions of Spain. A total of 1239 volunteers between 18 and 30 years of age were enrolled. The parameters evaluated were semen volume, sperm concentration and progressive sperm motility. The results indicate that while there were no differences in semen volume or sperm motility, there were statistically significant differences in the rate of oligospermia in volunteers from the different regions studied. The prevalence of oligospermia was highest in Valencia (22.7%), Barcelona (22.7%) and Pais Vasco (18.7%), which are the regions of Spain with the highest degree of industrialisation for the last 50 years, and lowest in Galicia (8.5%) and Andalucía (13.7%), regions with a more recent industrial development. There were no differences in the rate of oligospermia as a function of age. These results support the hypothesis that exposure to environmental toxicants may affect the process of spermatogenesis leading to meiotic alterations, maturational arrest and oligospermia.

Estimating the relative hazard by the ratio of logarithms of event-free proportions

Clinical trials typically examine associations between an intervention and the occurrence of a clinical event. The association is often reported as a relative risk, more rarely as an odds ratio. Unfortunately, when the scientific interest lies with the ratio of incidence rates, both these statistics are inaccurate: the odds ratio is too extreme, and the relative risk too conservative. These biases are particularly strong when the outcomes are common. This paper describes an alternative statistic, the ratio of logarithms of event-free proportions (or relative log survival), which is simple to compute yet unbiased vis-à-vis the relative hazard. A formula to compute the sampling error of this statistic is also provided. Multivariate analysis can be conducted using complementary log-log regression. Precise knowledge of event occurrence times is not required for these analyses. Relative log survival may be particularly useful for meta-analyses of trials in which the proportion of events varies between studies.

Declining trends in conception rates in recent birth cohorts of native Danish women: a possible role of deteriorating male reproductive health

Recent findings of poor semen quality among at least 20% of normal young men in Denmark prompted us to use unique Danish registers on births and induced abortions to evaluate a possible effect of the poor male fecundity on pregnancy rates among their presumed partners--the younger cohorts of women. We have analysed data from the Danish birth and abortion registries as well as the Danish registry for assisted reproduction (ART) and defined a total natural conception rate (TNCR), which is equal to fertility rate plus induced abortion rate minus ART conception rate. A unique personal identification number allowed the linkage of these databases. Our database included 706,270 native Danish women born between 1960 and 1980. We used projections to estimate the fertility of the later cohorts of women who had not yet finished their reproduction. We found that younger cohorts had progressively lower TNCR and that in terms of their total fertility rate, the declining TNCR is compensated by an increasing use of ART. Our hypothesis of an ongoing birth cohort-related decline in fecundity was also supported by our finding of increasing and substantial use of ART in the management of infertility of relatively young couples in the later cohorts. Furthermore, the lower rates of induced abortion among the younger birth cohorts, often viewed as a success of health education programs, may not be fully explained by improved use of contraception. It seems more likely that decreased fecundity because of widespread poor semen quality among younger cohorts of otherwise normal men may explain some of the observed decline in conception rates. This may imply increasing reproductive health problems and lower fertility in the future, which is difficult to reverse in the short term. The current and projected widespread use of ART in Denmark may be a sign of such an emerging public health problem.

Environmental contaminants and human infertility: hypothesis or cause for concern?

Throughout the 1980s and 1990s the crude human birth rate (live births per 1000 population) declined, indicating reduced fertility and suggesting a potential decline in fecundity (the potential to conceive). Detection of environmental contaminants in human tissues, together with reports of a global decline in semen quality, further fueled speculation that human infertility rates are increasing and environmental toxicants are potentially important causal agents associated with this change. However, there is little compelling evidence to suggest that infertility rates amongst the general population have changed over time. Moreover, recent studies suggest a rise in the fertility rates. While several studies documented increased time to pregnancy (TTP) in exposed study populations, other investigators were not able to replicate these findings. Nevertheless, studies involving occupational exposure together with results from animal experiments lend support to the conclusion that environmental contaminants potentially adversely affect fertility. Consequently, the impact of exposure to environmental contaminants on human fertility remains controversial. To test the hypothesis that environmental contaminant exposure was associated with enhanced risk of infertility, data concerning trends in fertility and infertility rates were examined to assess the impact of exposure of developing gametes to environmental contaminants. The relationship between exposure and reproductive outcomes was then examined to illustrate the range of adverse effects for reproductive toxicants with data sets of divergent depth and reliability. Data showed that only a weak association between exposure to environmental contaminants and adverse effects on human fertility exists. However, it is postulated that evidence of chemical exposure and potential health consequences of these exposures highlight the need for further research in this area.