It is known that temperature during pregnancy can have an effect on the number of male vs female babies born (the sex-ratio) among animals. An interesting study suggests that this is also true for humans.
The study found that exposure to days with a temperature 20 degrees Celsius (68 degrees Fahrenheit) or higher during the nine months of pregnancy leads to fewer male births. While at first glance that doesn't seem so warm, these areas also had days 30 degrees Celsius (86 degrees Fahrenheit) and higher.
The study was done by linking data on 5 million live births in 33 sub-Saharan countries and India with temperature data. The researchers observed fewer male births after high first-trimester temperature exposure. This was consistent with increased spontaneous abortions due to mother's heat stress. This is especially true for mothers in rural areas, with little education. (No air conditioning!)
From Euronews: As temperatures rise, the number of male births is decreasing, study finds
When temperatures rise above 20°C, fewer baby boys are born compared to girls, a new study has found.
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.
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.
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.