New Delhi, March 31 Exposure to high temperatures during pregnancy may be linked to a decline in male births, according to an analysis of five million births across India and 33 countries in sub-Saharan Africa.
Researchers, led by those at the University of Oxford in the UK, linked data from large-scale surveys with temperature records to examine how exposure to heat during pregnancy can impact the sex ratio at birth.
In sub-Saharan Africa, exposure to high temperatures during the first trimester of pregnancy was linked to a decline in male births.
However, in India, the effects of heat exposure appear later in pregnancy, with higher temperatures during the second trimester associated with fewer male births – especially among older mothers, women with multiple births, and women without sons in northern states.
Published in the journal Proceedings of the National Academy of Sciences (PNAS), the evidence from the study has important implications for population health and gender balance in a warming world, the researchers said.
Lead author Abdel Ghany, a postdoctoral research fellow in sociology at the University of Oxford, said, "Extreme heat is not only a major public health threat. We show that temperature fundamentally shapes human reproduction by influencing who is born and who is not born."
"Our findings indicate that temperature has measurable consequences for fetal survival and family planning behavior, with implications for population composition and gender balance. Understanding these processes is essential for anticipating how the environment affects societies in a warming climate," Ghany said.
Sex ratios at birth are a key demographic indicator and reflect underlying patterns of maternal health, prenatal survival, and, in some contexts, gender discrimination.
The team found that temperatures above 20 degrees Celsius are consistently associated with fewer male births in both the world regions – but through different mechanisms.
"We show that high temperatures in the nine months before birth are negatively associated with male births in sub-Saharan Africa and India," the authors wrote.
They added, "The timing of exposure demonstrates that ambient heat can increase prenatal mortality in early pregnancy, particularly among males, in both world regions."
The researchers also "demonstrate that in regions with high son preference, elevated temperatures during windows where sex-selective abortions could take place reduce these abortions."
The pattern suggests that exposure to heat may reduce access to, or use of, sex-selective abortion, temporarily narrowing gender imbalances, the team said.
The findings show that heat exposure could have complex behavioral and biological implications for maternal and fetal health and ramifications on social phenomena such as gender discriminatory practices, the researchers said.