Sex Ratio Variation and Influencing Factors

The sex ratio at birth typically favours males, averaging around 1.05 male infants per female, yet small deviations emerge across populations and over time. Such variation reflects a complex interplay of genetic predispositions, parental hormonal milieus, maternal physiological status and environmental pressures. Evolutionary models propose that female conceptuses survive more readily under adverse conditions, producing cohort‐level shifts that may serve adaptive purposes. Maternal characteristics—age, parity and nutritional status—consistently shape sex outcomes, while external stressors such as pollution, natural disasters and pandemics can induce transient alterations through mechanisms including selection in utero and altered conception dynamics. Recent advances have elucidated the role of low-grade inflammation and micronutrient levels—particularly vitamin D—in modulating embryo viability. Large-scale demographic investigations further link cohort sex ratios to long-term health trajectories, underscoring potential applications in public health surveillance and policy.

Research from Nature Portfolio

In a retrospective study of over 11 000 births in Central Europe, maternal age and birth order emerged as the dominant predictors of seasonal fluctuations in the secondary sex ratio, overshadowing the contributions of air pollution during heating seasons in multivariable models. Older and higher-order pregnancies were disproportionately associated with female births under specific environmental conditions. Separately, a prospective analysis of women attempting conception demonstrated that preconception serum 25-hydroxyvitamin D concentrations were positively correlated with the probability of delivering a male infant. This association was most pronounced among individuals with elevated high-sensitivity C-reactive protein, suggesting that sufficient vitamin D status may mitigate inflammatory processes that preferentially compromise male conceptus survival.

Sex Ratio Variation and Influencing Factors publication trend

The graph below shows the total number of articles in sex ratio variation and influencing factors across all publications each year (not limited to Nature Index journals).

Technical terms

Sex ratio at birth (SRB): The proportion of male to female live births in a cohort.

Secondary sex ratio (SSR): The ratio of male to female live-born infants, typically measured at birth.

Selection in utero: The process by which male and female embryos or fetuses experience differential survival before birth due to biological or environmental influences.

Parity: The count of previous live births a woman has had, influencing subsequent reproductive outcomes.

References

1. Sex ratio at birth across 100 years in Sweden and risk of cardiovascular disease and all-cause mortality – a national register study. European Journal of Epidemiology (2024).
2. Factors influencing sex ratio at birth in Krosno, Poland. Scientific Reports (2024).
3. Sex Ratio at Birth after Concurrent Events of Earthquakes and the COVID-19 Pandemic in Croatia. International Journal of Environmental Research and Public Health (2024).
4. Selection in utero and population health: Theory and typology of research. SSM - Population Health (2018).
5. The role of maternal preconception vitamin D status in human offspring sex ratio. Nature Communications (2021).