Hot days during pregnancy have been linked to a decrease in the number of boys born in large parts of Africa and India. The change shows that rising heat can silently shift the population balance, shaping families and futures long before anyone notices the change.
Almost five million of births recorded in sub-Saharan Africa and India reveal that the drop in male births followed periods of unusually hot days during pregnancy.
By analyzing these births in conjunction with daily temperature records, Jasmin Abdel Ghanya researcher at the University of Oxford, documented that male births decreased when pregnancy coincided with greater heat.
His results were presented in an article recently published in Proceedings of the National Academy of Sciences.
The decrease did not depend on rare peaks of temperature, but it manifested itself as soon as the maximum daytime temperatures exceeded a moderate thresholdremaining constant in different regions.
The timing of the drop in male births differs between Africa and India, a pattern that suggests that different forces may be at play. This divergence points to distinct biological and behavioral mechanisms that deserve further examination, explains Ghany at U.Oxford.
Without needing record heat, the pattern emerged as soon as maximum daily temperatures during pregnancy exceeded 20°C.
Demographers monitor the sex ratio at birth—the number of boys born per 100 girls—because it may vary in response to stress and selective pressures.
Most populations are closer to 103 to 107 boys for every 100 girls, but India has shown higher values in recent decades. A small variation in this ratio could mean thousands of children when the heat lasts for months.
In sub-Saharan Africa, the drop occurred after heat in the first quarterwhen many pregnancies are still fragile. High temperatures can trigger maternal heat stressincreasing the risk of miscarriage in the early stages, explain the study authors.
As rural mothers and those with less education formal births registered the most pronounced reductions, especially in fourth order births or higher. Also the limited access to shade, drinking water and cooling can end up turning an ordinary heatwave into a event that ends the pregnancy.
Male fetuses are more fragile
Biologists associate some early losses with the hypothesis of fragile male, the idea that male fetuses are biologically more vulnerable in the womb and require greater maternal investment to survive.
How do male embryos tend to grow faster and needing more energy, oxygen and nutrients, they may be less resilient when the mother’s body faces difficulties.
O heat directs blood to the skin for cooling and may reduce oxygen delivery across the placenta, increasing stress on the developing fetus.
Under these conditions, male pregnancies fail more frequently than females, and births tend to lean towards girls.
Due to the lack of detailed data on gestational length, the team was unable to clearly distinguish between fewer male conceptions and higher rates of male pregnancy loss.
In India, the decline in male births followed exposure to heat mid-pregnancy, and not in the early stages. During the second trimester, access to sex-selective abortion appeared become more limited during periods of intense heat.
The heat can make travel unfeasiblereducing working hours and increasing costs — making consultations difficult in decisive weeks.
Because this involuntary change was based on a temporary disruption, it did not constitute a lasting solution to male child preference or entrenched gender bias.
As older mothers and fourth order births or higher concentrated much of the variation, in line with the moment in which families tend to attempt selection.
Seen this way, the heat created a barrier at a specific momentand not a force that acted uniformly throughout the pregnancy. Beyond the 20 °C threshold, warmer days did not automatically produce greater drops in male births.
“Understanding these processes is essential to anticipate how the environment affects societies in a warming climate,” says Ghany.
According to the study authors, the heat changed birth patterns in two ways: by pressuring pregnancies in Africa and restricting access to clinics in parts of India.
Reduce heat exposure during pregnancy and ensure healthcare remains accessible during heatwaves may help prevent both invisible pregnancy losses and conditioned reproductive decisions.
