Experiencing heat during pregnancy results in fewer male babies: study Premium

Study finds higher temperatures during pregnancy lead to fewer male births, highlighting urgent maternal health risks in vulnerable populations.

When pregnant women experience higher ambient temperatures during gestation, fewer males are born, a recent analysis of demographic and health surveys in sub-Saharan Africa and India, showed.  

A paper titled ‘Temperature and sex ratios at birth’ in the journal Demography, by Jasmin Abdel Ghany et al., concludes after a detailed analysis that experiencing higher ambient temperatures during pregnancy is associated with changes in the natural sex ratio at birth across India and sub-Saharan Africa. The paper analyses over five million births drawn from more than 90 Demographic and Health Surveys, containing local temperature data, to examine how heat exposure across trimesters shapes sex ratio at birth. Sex ratios at birth shape population composition and are closely linked to maternal health and gender discrimination. 

The researchers write: “We find that days with a maximum temperature above 20 °C are negatively associated with male births in both regions. In sub-Saharan Africa, we observe fewer male births after high first-trimester temperature exposure, consistent with increased spontaneous abortions from maternal heat stress… By contrast, in India, we find that second-trimester temperature exposure is associated with fewer male births.” These reductions are focused among older mothers in rural areas who have had a number of children, the study highlights.

The daily maximum temperature in the month of birth is 30.0 °C in sub-Saharan Africa and 30.3 °C in India. In India, in the second trimester, the results indicate a negative relationship between temperature exposure and birth sex. The effect of a 25 to 30 °C indicates a lower male birth probability by 0.014 percentage points.  

To account for both biophysical health and behavioral mechanisms, researchers chose two regions with vastly different experiences with son preference and sex-selective abortion: India (where several regions have high son preference and sex selective abortions) and sub-Saharan Africa (where there is little evidence of son preference and sex-selective abortions are minimal).  

The hypothesis is that these heat-induced pregnancy losses are male-biased, in line with Trivers and Willard’s “frail male” hypothesis. “According to this evolutionary argument, weak males may have a lower chance of surviving to birth under poor environmental conditions. After birth, males have lower survival probabilities than females and thus require greater maternal investment, the authors write.