Sex ratio at birth: causes of variation and narrative review of literature.

Introduction: The human sex ratio at birth (SRB) is the parameter which reflects the relationship between males and females at birth. It is not 50:50, but approximately 0.515 in favor of males. There are many factors that can influence SRB and its variations are not easily demonstrable.

Evidence Acquisition: Our aim was to investigate the environmental and human agents capable of causing variations in the SRB by providing a narrative review of the scientific literature.

Evidence Synthesis: Studies demonstrated that male and female embryos/fetuses' response to stress in different way and the male ones show less resistance. This phenomenon seems to persist for approximately six months after a stressful event, suggesting a purpose of reproductive regulation and protection of the perpetuation of the species. Although the relationship of dependence with parenting decisions, thanks to prenatal diagnosis and assisted fertility techniques is easily understood, it is more complex to correlate its variations with environmental pressures that act through secondary mechanisms. The Covid-19 pandemic, natural catastrophes, terroristic attacks, economic crises are some of the difficulties encountered in recent times.

Conclusions: The impact of all the factors reported is not explained only with a possible a priori determination of sex at conception, but also with secondary effects in the pregnancy outcomes. Thus, the determination of sex appears to be a complex and multifactorial mechanism, besides a matter of genetics. The pandemic and climate changes are the present important opportunities of research for possibly reaching more certain answers.