A sweeping new study analysing nearly five million births across sub-Saharan Africa and India has found that high temperatures during pregnancy are linked to fewer male births — and the reasons differ dramatically between the two regions.

Researchers from the University of Oxford, the Max Planck Institute, the Barcelona Institute for Global Health, and the University of Bologna combined birth records from 104 Demographic and Health Surveys with daily temperature data to track how heat exposure during each trimester of pregnancy affected whether a baby was male or female.

Heat and miscarriage in Africa

In sub-Saharan Africa, the effect showed up in the first trimester. Each additional day with temperatures above 20°C was associated with a measurable drop in male births. When researchers calculated what a one-standard-deviation increase in extreme heat days (above 30°C) would mean, they found the sex ratio shifted from about 103.5 boys per 100 girls down to 101 — a reduction of roughly 2.5 fewer boys per 100 girls.

The pattern was strongest among women in rural areas, those with little or no formal education, and mothers giving birth to their fourth child or later. Women in urban areas and those with secondary education showed no effect at all.

The researchers say the most likely explanation is that heat stress during early pregnancy triggers miscarriages, and that male foetuses are disproportionately vulnerable. This aligns with what biologists call the “frail male” hypothesis — the idea that male pregnancies are more resource-intensive and more likely to be lost under environmental stress.

A different story in India

In India, the temperature effect appeared not in the first trimester but in the second — the window when foetal sex can be determined by ultrasound and when sex-selective abortions typically take place. The overall effect was smaller than in Africa: a one-standard-deviation increase in heat days between 25°C and 30°C shifted the sex ratio from about 110 boys per 100 girls down to roughly 109.

But among specific groups, the numbers were far more striking. For mothers over 30 who were having their fourth or later child, the reduction was up to 1.9 percentage points per standard-deviation increase in heat. And for sonless mothers in northern Indian states — where son preference and sex-selective abortion are most concentrated — one additional day between 25°C and 30°C was associated with a reduction of nearly 2.8 fewer boys per 100 girls.

By contrast, mothers in southern India (where son preference is weaker) and mothers who already had sons showed no effect whatsoever.

Heat disrupts access, not biology

The researchers argue that in India, high temperatures likely disrupt women’s ability to access sex-selective abortion services rather than directly causing miscarriages. The timing of the effect — centred on weeks 13 to 20 of gestation, when sex determination and selective termination would occur — supports this interpretation.

They also note that the affected groups closely match the demographic profile of women most likely to seek sex-selective abortions: older mothers, those with multiple daughters but no sons, and those in regions with the strongest cultural preference for boys.

As a check, the researchers ran the same analysis on second-trimester births in sub-Saharan Africa, where sex-selective abortion is extremely rare. They found no effect, reinforcing the idea that the Indian pattern is behavioural rather than biological.

Climate change unlikely to shift sex ratios further

Despite the clear link between heat and birth sex, the authors caution that climate change is unlikely to push sex ratios further in these regions. The effect kicks in above 20°C — a threshold that most of sub-Saharan Africa and India already exceed for the majority of the year. Future warming will mainly push already-hot days even hotter, rather than converting cooler days into the 20°C-plus range where the effect begins.

The study does, however, highlight that even moderate heat — not just extreme heatwaves — can affect pregnancy outcomes differently for male and female foetuses, with implications for maternal health policy in the Global South.

The research was published in the Proceedings of the National Academy of Sciences on February 19, 2026.

Source: Abdel Ghany, J., Wilde, J., Dimitrova, A., Kashyap, R. & Muttarak, R. (2026). Temperature and sex ratios at birth. Proceedings of the National Academy of Sciences, 123(8), e2422625123.