Published Mar 28, 2026 | 7:00 AM ⚊ Updated Mar 28, 2026 | 7:00 AM
Representational image. Credit: iStock
Synopsis: A new international study links rising temperatures during pregnancy to shifts in sex ratios at birth. Analysing five million births across India and Sub-Saharan Africa, researchers found heat exposure reduced male births, with timing differing by region. Biological vulnerability of male foetuses and India’s social context suggest climate change may subtly influence reproductive outcomes and maternal health.
As heatwaves become more common across the world, scientists are uncovering a surprising possibility: could rising temperatures during pregnancy influence whether a baby is born a boy or a girl?
A new international study titled “Temperature and Sex Ratio at Birth” suggests the answer may be yes. The researchers found that higher temperatures during pregnancy were associated with fewer male births.
In India, the effect appeared strongest when heat exposure occurred during the second trimester, suggesting that extreme heat may subtly influence the natural balance between boys and girls.
The pattern, however, appears more complex in the country, as researchers say heat exposure during pregnancy may also influence sex-selective abortion patterns historically linked to son preference.
“We used data on 5 million births from the Demographic and Health Survey and linked each birth with high-resolution temperature conditions during pregnancy,” said Dr Jasmin Abdel Ghany, researcher at the University of Oxford and co-author of the study
“Our regression models are seasonality-adjusted and take into account differences between geographic areas,” she told South First.
Together, the findings point to an unexpected intersection between climate change, biology and social behaviour, raising new questions about how extreme heat may shape pregnancy outcomes.
The researchers analysed birth records from India and 33 countries in Sub-Saharan Africa, covering births between 2000 and 2022. These were linked with detailed temperature data to understand how heat exposure during pregnancy may influence the sex ratio at birth — the number of boys born for every 100 girls.
Typically, the natural sex ratio ranges between 103 and 107 boys per 100 girls.
But the study found that higher temperature exposure during pregnancy was associated with fewer male births.
The authors noted that in Sub-Saharan Africa, increased exposure to days above 30°C during the first trimester was linked with a decline in male births by about 2.47 boys per 100 girls.
In India, the decline was smaller — about 1.15 fewer boys per 100 girls — but the timing of the effect was different.
While Africa showed the strongest impact in early pregnancy, in India the effect appeared primarily during the second trimester.
The researchers believe social factors may explain why India’s pattern differs. Unlike Sub-Saharan Africa, India has historically experienced distortions in sex ratios due to son preference and sex-selective abortions.
According to the study, this social context may mean that high temperatures could indirectly influence reproductive decisions or access to medical services.
Dr Abdel Ghany highlighted that comparing the two regions helped researchers separate biological effects from social ones.
“Sub-Saharan Africa is a setting where son preference is not known to cause sex ratio distortions, whereas in India sex-selective abortions have distorted sex ratios,” she said.
She added that this contrast allowed the researchers to treat Africa as a ‘placebo’ setting, helping them understand what might happen naturally under heat exposure while also identifying behavioural effects in India.
High temperatures during pregnancy may interfere with mobility and clinic visits, “potentially reducing sex-selective abortion procedures and leading to relatively more female births,” the study said.
Biology may also help explain the pattern. Scientists have long observed that male foetuses tend to be more vulnerable during pregnancy than female foetuses, particularly when the mother experiences stress.
The study draws on the “frail male” hypothesis, also known as the Trivers–Willard hypothesis — an evolutionary theory suggesting that male offspring are generally more sensitive to environmental shocks during pregnancy.
Dr Abdel Ghany explained that the sex of a baby is determined at the moment of conception and cannot be changed by temperature later in pregnancy.
“The chromosomes are determined at fertilisation of the egg,” she said. However, environmental conditions during pregnancy may affect whether the foetus survives.
In the study, researchers observed that in Sub-Saharan Africa, exposure to high temperatures during the first trimester was associated with fewer male births.
This suggests that heat stress may increase the likelihood of pregnancy loss among male foetuses.
“We observe an effect of heat exposure in Sub-Saharan Africa in the first trimester that indicates temperature may affect survival during pregnancy — with disproportionate impacts for boys,” Dr Abdel Ghany said.
According to the frail male hypothesis, this pattern may have evolutionary roots. Male foetuses tend to grow faster and require more energy and maternal resources during pregnancy and infancy than females.
Because of this higher biological investment, they may be more sensitive to stress experienced by the mother, such as illness, malnutrition or extreme heat.
“Males have higher mortality rates throughout the life course and require more maternal investment than girls during infancy,” she explained.
From an evolutionary perspective, this means that in stressful environments, frailer male pregnancies may be less likely to continue naturally. If conditions improve later, the mother may still be able to have another pregnancy with better chances of survival.
“Our findings support this hypothesis: in a stressful environment, males may be less likely than females to survive in-utero,” she said.
How the researchers analysed the data
To examine the relationship between temperature and birth outcomes, the study analysed about five million births recorded in the Demographic and Health Survey (DHS), a large international dataset covering reproductive-age populations in more than 90 countries.
Each birth record was matched with high-resolution daily temperature data corresponding to the location and estimated pregnancy period.
The researchers then analysed temperature exposure across three pregnancy stages, first second and third trimester.
Statistical models were used to control for factors such as seasonal variations, regional differences, and rainfall, ensuring that the results reflected genuine temperature effects rather than unrelated seasonal patterns.
Because DHS surveys usually record only the month of birth rather than the exact day, the researchers estimated pregnancy periods to approximate temperature exposure.
“More fine-resolution data would be needed,” Dr Abdel Ghany said, noting that the surveys do not always contain exact birth-day information.
The findings raise questions about how rising temperatures could shape reproductive health.
However, the researchers say climate change may not dramatically alter sex ratios in India or Africa, at least in the near future.
This is because the study found a threshold effect.
“Temperature above 20°C already reduces male births, and above this threshold we find no gradient with increasing heat intensity,” Dr Abdel Ghany said.
In other words, once temperatures exceed this level, further increases do not appear to produce a stronger effect on sex ratios.
Still, climate change is expected to bring more extremely hot days above 30°C or even 35°C, which could intensify heat stress during pregnancy.
That may have wider consequences for maternal health, foetal survival and pregnancy outcomes, particularly in regions where healthcare access and social conditions already place women at greater risk.
“These findings highlight potential implications for maternal and reproductive health in vulnerable settings,” the study noted.
The research highlights a little-explored dimension of climate change — its potential influence on human reproduction and demographic patterns.
Heat exposure during pregnancy could interact with both biological vulnerability and social behaviour, producing complex outcomes.
While the overall impact on population sex ratios may remain modest, the findings suggest that rising temperatures is not just an environmental issue, it may also affect pregnancy health and reproductive decisions, particularly in countries like India where social factors already shape birth outcomes.
