The impact of catastrophic events on the sex ratio at birth: A systematic review.

OBJECTIVE: The impact of maternal stress on birth outcomes is well established in the scientific research. The sex ratio at birth (SRB), namely the ratio of male to female live births, shows significant alteration when mothers experience acute stress conditions, as proposed by the Trivers-Willard Hypothesis. We aimed to synthetize the literature on the relationship between two exogenous and catastrophic stressful events (natural disasters and epidemics) and SRB. METHODS: A systematic search was run in Scopus, PubMed, Web of Science, and Cochrane Library, until March 9, 2023. The search produced 1336 articles and 25 articles met the inclusion criteria. We found seven case-control studies and 18 observational studies. Most of studies investigated the impact of earthquakes and other natural disasters. Only seven studies examined the effect of epidemics or pandemics. RESULTS: The results of the studies seem inconsistent, as 16 studies found a decline in SRB, three found a rise, four did not record any change and two studies gave contradictory results. The period and population analyzed, the source of information, the method of variance analysis in the SRB, and the failure to assess confounding variables may have influenced the incongruence of the results. CONCLUSION: Our findings contribute to improve the knowledge about the relationship between socio-ecological factors and SRB. Future studies should investigate the mechanisms by which this relationship impacts public health, in particular the health of pregnant women and their newborn, through an accurate and consistent methodology that also includes confounding factors.