When Weather Is Hotter, Fewer Boys May Be Born

Researchers analysed nearly five million births across 33 countries in sub-Saharan Africa and India.

From rising seas to record-breaking heatwaves, the impacts of climate change are well documented. But researchers are now pointing to a more personal consequence: hotter temperatures during pregnancy may influence whether a baby is born a boy or a girl.

A major new study suggests this effect is real—and that the reasons for it can vary dramatically depending on where you live.

According to a paper published in Proceedings of the National Academy of Sciences (PNAS), researchers analysed nearly five million births across 33 countries in sub-Saharan Africa and India. They linked each pregnancy to daily temperature records to see whether heat exposure during pregnancy affected the baby’s sex at birth.

Their central finding is striking: “We find that days with a maximum temperature above 20 °C are negatively associated with male births in both regions.”

In simple terms, when it is hotter during pregnancy, slightly fewer boys are born. But why this happens turns out to be very different in Africa and India.

A biological effect in Sub-Saharan Africa

In sub-Saharan Africa, the most sensitive period was early pregnancy — the first trimester. The researchers report: “In sub-Saharan Africa, we observe fewer male births after high first-trimester temperature exposure, consistent with increased spontaneous abortions from maternal heat stress.”

This points to a biological explanation. Pregnancy puts strain on the body, and high temperatures make it harder for the body to regulate heat. When the mother experiences heat stress, it may increase the risk of miscarriage.

Male fetuses are known to be more fragile early in pregnancy. That means under stressful conditions–such as extreme heat–male pregnancies may be more likely to be lost. The study found that this effect was strongest among women who may have fewer resources to cope with heat.

The declines in male births were driven by: “Births of mothers who reside in rural locations, have no or primary formal education, and are concentrated in birth orders four and higher.”

This suggests that vulnerability matters. Women in rural areas may work outdoors or lack cooling systems. Women with less education may have less access to healthcare or information. Larger families may also face greater financial strain.

A social effect in India

In India, however, the pattern looks different. The key window was not early pregnancy but the second trimester–around months four to six.

The researchers wrote: “In India, we find that second-trimester temperature exposure is associated with fewer male births, consistent with reductions in induced sex-selective abortions.”

India has long struggled with son preference in some regions. Because some families prefer boys, sex-selective abortion of female fetuses has skewed birth ratios toward males.

The strongest heat-related changes appeared among women who already had several children and no sons–especially in northern states, where son preference is stronger.

The researchers note: “We also find large reductions in male births by sonless mothers in northern Indian states, where son preference is greater.”

This study matters

There have been fewer large-scale studies like this focused on Africa. Future research may challenge or reinforce these findings. Nevertheless, this study is important because it shows that rising temperatures can influence pregnancy outcomes in more ways than we previously understood.

As the authors conclude:

“The findings demonstrate that heat exposure may have complex behavioral and biological implications for maternal and fetal health and ramifications on social phenomena such as gender discriminatory practices.”

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RESEARCH HIGHLIGHTS

The Plastic Divide: Tanzania’s Bag Ban and Its Unequal Impact: Tanzania’s 2019 plastic bag ban has had uneven effects, particularly in low-income communities. After nine months of fieldwork in Dar es Salaam, researchers found that while large carrier bags have largely disappeared from supermarkets, small plastic pouches remain essential in poorer neighborhoods. These thin bags are used to sell affordable portions of food like flour, oil and cooked snacks. Alternatives such as paper or cloth bags are often too expensive or impractical. The study introduces the idea of “The Plastic Divide,” highlighting how bans may disproportionately burden those who depend on plastics for daily survival and income, leaving the poorest communities with few workable alternatives. [Reference, Journal of Material Culture]

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