As temperatures rise due to climate change, heat above 20°C leads to fewer male births, according to a new study.
When temperatures rise above 20°C, fewer baby boys are born compared to girls, a new study has found.
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Researchers at the University of Oxford found that heat exposure can increase prenatal mortality in early pregnancy, particularly among males.
For many years, human sex ratios at birth – the ratio of male to female offspring – were thought to be constant, genetically determined, and invariant to social or environmental shocks, the authors wrote.
However, the results published in Proceedings of the National Academy of Sciences of the United States of America (PNAS) show that extreme heat may play a role in the number of boys and girls that are born worldwide.
The researchers analysed data from more than five million births in 33 sub-Saharan countries and India. They found that ambient heat can increase prenatal mortality in early pregnancy, particularly among males, in both world regions.
“We show that temperature fundamentally shapes human reproduction by influencing who is born and who is not born,” said Abdel Ghany, co-author of the study.
He noted that the findings indicate that temperature has measurable consequences for foetal survival and family planning behaviour, with implications for population composition and gender balance.
“Understanding these processes is essential for anticipating how the environment affects societies in a warming climate,” he added.
The 20°C threshold
The study identified 20°C as the temperature at which the sight in ratios occurs, although hotter days do not proportionally amplify the effect.
Previous research has found that gestational heat exposure threatens the maternal body’s ability to thermoregulate, increasing the risk of pregnancy loss. If the mother is dehydrated, the baby may not get enough blood, oxygen, or nutrients.
Not only a biological response
Heat exposure not only harms maternal health but also influences family planning behaviour.
High temperatures could impact abortion access through mobility disruptions or by increasing financial uncertainty and reducing income generation, the authors noted.
The study highlights that the effects of heat are not evenly distributed, the authors wrote.
Women with fewer resources and those living in more vulnerable settings are more strongly affected, raising concerns about widening health inequalities under climate change.
In Europe, countries are experiencing rising temperatures, with more than 100 days of heat season in regions such as Albania, Greece, Portugal, and Spain.
2024 saw the second-highest number of heat stress days and tropical nights, when the temperature didn't fall from 20°C on record, according to the World Meteorological Organization.
As global temperatures are expected to rise, the researchers call for further studies on environmental factors to protect maternal health and improve access to healthcare, to reduce long-term impacts on reproduction and population dynamics.
Climate change impact on fertility
Studies have documented the impact of climate health on both male and female fertility.
In 2024, many European countries reported their lowest birth rates in several decades. While experts say 2.1 children per woman are needed to keep the population's size stable, several countries present numbers consistently below 1.5.
A recent systematic review by researchers at the Catholic University of Chile found that increased climate-related events and natural disasters are severely disrupting reproductive processes ranging from conception to care, including reproductive intentions, pregnancy, birth, fertility, and parenting.
