The male to female ratio at birth

The sex ratio at birth (2)

This second of two Best Practise Guidelines will continue to explore facets pertaining to factors that influence the sex ratio at birth (M/F). The papers include a quantitative study in the United States, investigating the effects of Thanksgiving Day, Christmas Day and Valentine's Day on M/F (which rises). A second paper is devoted to the factor that most influences M/F worldwide: male offspring preference. The third paper focusses on radiation which appears to be the only toxin/stressor that increases M/F by culling female in excess of male foetuses. The paper investigates the effects of the Mainz research nuclear reactor in Germany. Yet another paper reviews seasonality of M/F. Two more theoretical papers discuss the significance of low M/F of offspring and of siblings of probands with systemic lupus erythematosus or rheumatoid arthritis, implicating a common role for testosterone. In yet another paper, high levels of maternal first-trimester intrauterine testosterone are also implicated in male sex, autism, gender dysphoria and non-right-handedness. Finally, a paper reviews the effects of toxoplasmosis on M/F since may be one of the most important environmental factors on this ratio. It is hoped that this second set of papers will further provide stimulation and provocation for research on this intriguing topic.

The sex ratio at birth (1)

This and the next series of Best Practise Guidelines will revisit the main theories that attempt to explain the influences on the sex ratio at birth (M/F), which was similarly reviewed in this journal in 2015. The papers include a historical review of the classical theories and initial work pertaining to the study of M/F, an analysis of 88,875,750 live births in an attempt to quantify M/F, the effect of earthquakes and climate change and an attempt to approximate the primary sex ratio (at conception). Another theoretical paper provides several potential explanations for autism spectrum disorder that may be correlated with M/F. Yet paper tests the hypothesis that RhD positive heterozygous mothers have a male-skewed sex ratio. A final paper provides a systematic review of the factors that influences M/F. It is hoped that this set of papers will continue to provide stimulation and food for thought on this intriguing topic.

Secondary sex ratio in relation to exposures to polychlorinated biphenyls, dichlorodiphenyl dichloroethylene and methylmercury

This study was undertaken to assess the potential impact of maternal exposures to polychlorinated biphenyl (PCB), dichlorodiphenyl dichloroethylene (DDE) and methylmercury on the secondary sex ratios (the ratio of male to female live births) over a span of 23 years. The study includes prospective data from three Faroese birth cohorts, with a total of 2,152 healthy mother–child dyads recruited between 1986 and 2009. The Faroe Islands is a subarctic fishing community, where pilot whale meat and blubber are part of the traditional marine diet. Exposures were measured in maternal hair, serum or umbilical cord blood. Confounder adjusted logistic regression models were used to assess the associations between maternal exposures and the secondary sex ratio. A doubling in ΣPCB, p,p’-DDE and mercury concentrations were associated with increased odds by 8% (95% CI = 0–16%), 7% (95% CI = 0–14%) and 9% (95% CI = 2–17%), respectively, of giving birth to a boy. In conclusion, maternal exposure to ΣPCB, DDE and methylmercury was associated with a slightly increased secondary sex ratio. The impact of paternal exposures could not be taken into account and deserves attention.

Pregravid hypertension may have different secondary sex ratio effects in different races in the United States

BACKGROUND

Males are born in excess of females and the ratio is expressed as M/F (male/female births=secondary sex ratio, also known as secondary sex ratio). This is expected to approximate 1.048. Racial M/F disparities are known. A recent study in China showed that pregravid systolic hypertension is higher in women who delivered a boy than in those who had a girl. This study was carried out in order to identify the effect of pregravid hypertension in the United States on M/F by race.

METHODS

Monthly male and female live births by race for the entire US along with the presence/absence of hypertension were obtained from the website of the Centers for Disease Control and Prevention for 2007-2015 for the four racial groups: American Indian or Alaska Native, Asian or Pacific Islander, Black or African American and White.

RESULTS

This study analysed 36,364,253 live births. For White births, mothers who had chronic hypertension were likelier to have male than female offspring when compared to non-hypertensives (p=0.003). Conversely, Black or African American mothers who had hypertension were less likely to have male than female offspring when compared to non-hypertensives (p=0.022). There were F differences for/F differences for the presence or absence of hypertension for the other two races or for the total.

CONCLUSIONS

It is possible that hypothesised innate interracial periconceptual hormonal differences may modulate M/F responses to hypertension in different races.

The sex ratio at birth in France was unchanged 9months after the 1998 FIFA World Cup

INTRODUCTION

Positive psychological phenomena such as increased feelings of belonging linked to hosting the FIFA World Cup were observed in France 1998 and South Africa 2010. Approximately nine months after South Africa hosted the 2010 World Cup the sex ratio at birth increased significantly. Given the similarity of positive psychological phenomena between the two countries, this study sought to determine if the sex ratio at birth increased in France circa nine months after the 1998 World Cup.

METHODS

Anonymized publicly available live birth data from 1994 to 2004 inclusive was obtained from the Insee (Institut National de la Statistique et des Etudes/Économiques - National Institute of Statistics and Economic Studies). For statistical analysis, chi-squared tests were used.

RESULTS

The sex ratio at birth for March and April 1999 did not differ significantly from that of the rest of same period from 1996 to 2000 (p=0.558), there was also no significant difference for February and March 1999.

CONCLUSION

The sex ratio at birth did not increase in France after it hosted the 1998 World Cup. Possible reasons why the sex ratio at birth did not increase as it did in South Africa include a higher French prevalence of modern contraceptive use and a different kind/level of excitement for the World Cup in France since it has previously hosted large international sports tournaments. The influence of the World Cup on the sex ratio at birth depends on the context of a specific country.