Seasonal inconstancy of human sex ratio at birth

Impact of the Season of Birth on the Development of Juvenile Idiopathic Arthritis in the United States: A Nationwide Registry-based Study

OBJECTIVE

Autoimmune disorders result from the interplay of genetic and environmental factors. Many autoimmune disorders are associated with specific seasons of birth, implicating a role for environmental determinants in their etiopathology. We investigated if there is an association between the season of birth and the development of juvenile idiopathic arthritis ( JIA).

METHODS

Birth data from 10,913 children with JIA enrolled at 62 Childhood Arthritis and Rheumatology Research Alliance Registry sites was compared with 109,066,226 US births from the same period using a chi‑square goodness-of-fit test. Season of birth of the JIA cohort was compared to the US population estimate using a 2-sided 1-sample test for a binomial proportion and corrected for multiple comparisons. Secondary analysis was performed for JIA categories, age of onset, and month of birth.

RESULTS

A greater proportion of children with JIA were born in winter (January-March) compared to the US general population (25.72% vs 24.08%; corrected P < 0.0001). This observation was also true after stratifying for age of onset (≤ or > 6 yrs). When analyzed by the month of birth, a greater proportion of children with JIA were born in January compared to the US population (9.44% vs 8.13%; corrected P < 0.0001).

CONCLUSION

Relative to the general population, children with JIA are more often born in the winter, and specifically in the month of January. These observations support the hypothesis that seasonal variations in exposures during the gestational and/or early postnatal periods may contribute to development of JIA.

The effects of Thanksgiving, Christmas and Valentine's Day on the sex ratio at birth in the United States, 2003-2015

INTRODUCTION

Males are born slightly in excess of females and many factors have been shown to influence M/F, the sex ratio at birth. Seasonality has also been shown to impinge on M/F. This study was carried out in order to ascertain whether Thanksgiving Day, Christmas Day and Valentine's Day had any influence on M/F in the United States population, nine months later.

METHODS

Births by ethnicity, region and sex were obtained from the website of the Centres for Disease Control. Analyses were applied to seasonally adjusted time series and non-seasonally adjusted series. A seasonally adjusted indirect result was calculated for the sex ratio.

RESULTS

This paper studied 53,105,069 live births for 2003-2015 (27,178,968 males, 25,926,101 females). M/F rises exceeding 90th and 95th percentiles were strongest for the seasonally adjusted series for all births in regions: South (CENS-R3), West (CENS-R4) and the time series All-regions_All-race. When comparing unadjusted and seasonally adjusted series, a similar pattern was observed in the regions Northeast and Midwest for American Indian or Alaska Native and Asian or Pacific Islander. A similar pattern was observed for the region Midwest for the ethnic group White.

DISCUSSION

M/F rose above the 90th percentile in all the series and occasionally above the 95th percentile. Increased periconceptual coital rates increases M/F and this study thus lends further credence to the hypothesis that coital rates around the time of conception causally influences the sex ratio of subsequent births nine months later, possibly due to a hormonal mechanism.

Female Reproductive Performance and Maternal Birth Month: A Comprehensive Meta-Analysis Exploring Multiple Seasonal Mechanisms

Globally, maternal birth season affects fertility later in life. The purpose of this systematic literature review is to comprehensively investigate the birth season and female fertility relationship. Using PubMed, we identified a set of 282 relevant fertility/birth season papers published between 1972 and 2018. We screened all 282 studies and removed 131 non-mammalian species studies on fertility and 122 studies that were on non-human mammals. Our meta-analysis focused on the remaining 29 human studies, including twelve human datasets from around the world (USA, Europe, Asia). The main outcome was change in female fertility as observed by maternal birth month and whether this change was correlated with either temperature or rainfall. We found that temperature was either strongly correlated or anti-correlated in studies, indicating that another factor closely tied to temperature may be the culprit exposure. We found that rainfall only increases fertility in higher altitude locations (New Zealand, Romania, and Northern Vietnam). This suggests the possibility of a combined or multi-factorial mechanism underlying the female fertility - birth season relationship. We discuss other environmental and sociological factors on the birth season - female fertility relationship. Future research should focus on the role of birth season and female fertility adjusting for additional factors that modulate female fertility as discussed in this comprehensive review.

A systematic search of the factors that influence the sex ratio at birth

INTRODUCTION

The sex ratio at birth has been studied for decades. A systematic review of the factors that influence this has never been performed. This study conducted a systematic search of this topic.

METHODS

A scoping review of the literature was initially carried out in January 2012 and this was formally updated for contemporaneity in December 2017. A systematic search of the literature subsequently followed in May 2019 and was conducted based on PRISMA statement's flow diagram. The search was applied across five databases: Cumulative Index to Nursing and Allied Health Literature (CINAHL), MEDLINE, PubMed, The Cochrane Library and Web of Science and was restricted to January 1, 1990 to May 31, 2019. The systematic review stopped at screening stage of the PRISMA. A narrative approach was adopted to report study findings.

RESULTS

494 studies were included. Ten factors were identified from the literature as having an influence on M/F, with stress and sex-selective termination being the most predominant. Additional factors included the hormonal theory, geographical trends, coital rates, radiation, secular trends, seasonality and theoretical modeling.

DISCUSSION

Future studies addressing M/F should adopt similar methodologies in order to provide the possibility of comparison between findings and a more formal form of systematic review.

A review of terrorism and its reduction of the gender ratio at birth after seasonal adjustment

BACKGROUND

Males are born in excess of females, a ratio expressed as M/T (males:total births). The ratio exhibits seasonal variation. Furthermore, acute stressful events may result in a transient dip in male births due to excess foetal losses, reducing M/T.

AIMS

This study was carried out in order to identify significant M/T dips after adjusting for seasonality.

STUDY DESIGN

Live births by gender and month were sought for acute stressful events. After seasonal correction (where appropriate), M/T dips were sought.

SUBJECTS

Live births.

OUTCOME MEASURES

M/T dips.

RESULTS AND CONCLUSIONS

This paper studied 112,226,306 live births. The following events showed dips ≤5th percentile 3-5months after these acute episodes: the Brooklyn Bridge protests, Katrina Hurricane for all 4 states and for each individual state (Alabama, Florida, Louisiana, Mississippi), the Battle in Seattle, the London bombings, The Madrid bombings (for Madrid and for Spain), the Breivik shooting, the Oklahoma City bombing and the Sandy Hook Elementary School shooting. The Virginia Polytechnic Institute and State University shooting the Fukushima Daiichi nuclear disaster also showed dips albeit slightly later. Seasonal adjustments should be taken into consideration in order to avoid Type 1 or 2 error pitfalls.

EFFECTS OF STRESSFUL EVENTS IN FRANCE (1968) AND JAPAN (1995) ON THE SEX RATIO AT BIRTH

Males are usually born in excess of females. The sex ratio at birth (SR) is often expressed as the ratio of male to total births. A wide variety of factors have been shown to influence SR, including terrorist attacks, which have been shown to reduce SR. This paper reviews the effects on SR outcomes of the stressful events in France in 1968 (in association with the student and worker riots) and in Japan following the Aum Shinrikyo religious cult's attack on the Tokyo subway using sarin nerve gas in 1995. Both countries displayed seasonal variation in SR. France exhibited a decline in SR in 1968 (p=0.042), with a particularly strong dip in May of that year (p=0.015). For Japan, there was no statistically significant dip for 1995 but there was a significant dip in June of that year (p=0.026). The SR dips follow catastrophic or tragic events if these are perceived to be momentous enough by a given populace. It is believed that SR slumps may be caused by population stress, which is known to lead to the culling of frail/small male fetuses. It has been observed that these fluctuations are comparable in intensity to a substantial proportion of quoted values for perinatal mortality, potentially making this a public health issue.