Heat Exposure During Pregnancy Triggers Decline in Male Births—What's the Cause?

Heat waves, increasingly frequent across the globe, are not only affecting the environment but also beginning to shift human population balance. A large-scale study published in the Proceedings of the National Academy of Sciences reveals that hot days during pregnancy correlate with declining numbers of male infants born across vast regions of Africa and India.

The research, led by Jasmin Abdel Ghany from the University of Oxford, analysed nearly five million birth records. This data was cross-referenced with daily temperature records to examine how heat exposure influences sex ratios at birth.

In sub-Saharan Africa, declining male births were clearly evident when mothers were exposed to heat during the first trimester of pregnancy. Biologists linked this to the “frail male hypothesis”. Biologically, male foetuses tend to grow faster and require more energy, oxygen, and nutrients from the mother. When maternal body temperature rises sharply due to environmental heat, blood flow is diverted to the skin for cooling, potentially reducing oxygen supply through the placenta.

This stress condition makes male pregnancies more vulnerable to miscarriage compared to female foetuses. The strongest effects were found among mothers in rural areas with limited access to shade, clean water, and air conditioning.

In contrast to African findings, declining male births in India result from heat exposure during mid-pregnancy (second trimester). Researchers found that extreme heat in India tends to restrict population mobility to visit health clinics. Intense heat can halt travel activity, cut working hours, and increase living costs. This indirectly makes clinic visits more difficult during crucial weeks of pregnancy. This effect is most visible in northern Indian states, particularly among mothers with multiple children.

Notably, this decline does not require record-breaking global temperatures. The pattern begins emerging once daily temperatures exceed a moderate threshold of approximately 20 degrees Celsius (68°F) and consistently remain above this level across various regions.

Despite local adaptations, absolute heat remains the primary determinant. Although this decline may appear small, it could mean the loss of thousands of prospective male infants at population scale if hot weather persists for months.

“Understanding these processes is crucial to anticipating how the environment affects populations in a warming climate,” said Ghany.

This study provides a demographic warning for the medical world. In regions where cooling facilities and medical access remain scarce, the burden of climate change will fall most heavily on reproductive health. Efforts to reduce heat exposure for pregnant women and ensure medical service access during hot weather are key to preventing undetected pregnancy loss.