Beyond the Thermometer: How Rising Temperatures Are Reshaping the Global Male-to-Female Birth Ratio

Published: April 1, 2026

A landmark study involving more than five million births has uncovered a startling hidden consequence of climate change: as global temperatures rise, the number of baby boys being born is beginning to decline.

The research, published on February 19, 2026, in the Proceedings of the National Academy of Sciences (PNAS), suggests that exposure to temperatures above 20°C (68°F) during pregnancy is linked to a measurable shift in the “sex ratio at birth”—the natural balance of male to female newborns. While the biological fragility of male fetuses plays a role in some regions, the study also reveals how extreme heat may be inadvertently disrupting social behaviors, such as access to sex-selective healthcare in countries like India.

The Core Discovery: Heat as a Demographic Disruptor

Typically, the human sex ratio at birth is remarkably stable, with approximately 105 boys born for every 100 girls. However, a team of sociologists and demographers from the University of Oxford found that this balance shifts when expectant mothers are exposed to sustained heat.

The study analyzed high-resolution temperature data alongside demographic surveys from India and 33 sub-Saharan African nations. The results showed that even moderate warmth—anything consistently above 20°C—correlated with fewer male births. Interestingly, the timing of this impact varied by geography:

• Sub-Saharan Africa: The decline in male births was most closely linked to heat exposure during the first trimester.

• India: The decline was most pronounced when high temperatures occurred during the second trimester.

“Extreme heat is not only a major public health threat,” says lead author Dr. Jasmin Abdel Ghany, a Nuffield Postdoctoral Prize Research Fellow at the University of Oxford. “We show that temperature fundamentally shapes human reproduction by influencing who is born and who is not born.”

The “Fragile Male” Hypothesis

Why would heat affect boys more than girls? Scientists have long noted that male fetuses are often more vulnerable to environmental stressors.

Biologically, male fetuses tend to grow faster and have higher metabolic demands, which may make them more susceptible to maternal complications caused by heat, such as dehydration, systemic inflammation, or reduced uterine blood flow. In sub-Saharan Africa, where the effect is strongest in the first trimester, researchers believe the heat may lead to higher rates of early-stage pregnancy loss among male fetuses.

The data showed that the risk was not uniform across all populations. The decline in male births was 2 to 3 times more likely among:

1. Rural women with limited access to climate-controlled environments.

2. Women with lower education levels who may have fewer resources for heat adaptation.

3. Higher birth orders, where the physical toll of multiple pregnancies may increase vulnerability.

A Social Shift: The Case of India

In India, the findings took a different turn. Researchers found that the decline in male births in the second trimester was particularly prevalent among older mothers and women in northern states—regions historically known for a strong “son preference.”

In this context, the shift may be behavioral rather than purely biological. Experts suggest that extreme heatwaves may limit a woman’s mobility, preventing her from traveling to clinics for ultrasounds or sex-selective procedures.

“In India, this intersects with declining sex-selective practices due to laws, but climate adaptation must prioritize equitable healthcare,” notes Dr. Rajesh Kumar, a public health expert at the Indian Council of Medical Research. While the heat might “balance” the sex ratio by preventing the selective termination of female fetuses, it does so through a breakdown in healthcare access and increased maternal stress, which is not a positive public health outcome.

Expert Perspectives: Connection vs. Causation

While the scale of the study—covering five million births—is massive, some experts urge a nuanced interpretation of the data.

Dr. Sarah Johnson, a reproductive epidemiologist at Johns Hopkins Bloomberg School of Public Health, who was not involved in the research, describes the findings as “compelling but observational.”

“While the large sample strengthens associations, causation requires randomized controls, which are ethically impossible,” Dr. Johnson explains. She notes that “confounders like concurrent illnesses during heatwaves—such as malaria or water-borne diseases—warrant further scrutiny.”

Furthermore, the study found no “dose-response gradient,” meaning that a 35°C day didn’t necessarily cause a larger drop in male births than a 25°C day. Any temperature consistently above 20°C seemed to trigger the effect, suggesting that 20°C may be a critical physiological threshold for pregnant individuals.

Public Health Implications: Protecting the Future

As global temperatures are projected to rise by 1–2°C by 2050, the implications for maternal care are urgent. The study estimates a 1% to 2% drop in the sex ratio at birth for every month of high-temperature exposure.

Recommendations for Expectant Mothers

Public health officials suggest that those pregnant in hot climates take the following precautions:

• Prioritize Hydration: Heat-induced dehydration is a primary trigger for pregnancy complications.

• Seek “Cooling Centers”: In urban areas, use public air-conditioned spaces during peak heat.

• Monitor Symptoms: Be vigilant for signs of heat exhaustion, including dizziness, rapid pulse, or extreme fatigue.

Policy Transitions

The researchers argue that climate policy must be integrated into maternal health programs. This includes “heat-warning” systems specifically tailored for prenatal clinics and ensuring that rural healthcare workers are trained to identify heat-related fetal distress.

Conclusion

The Oxford study serves as a “sentinel” for the broader health impacts of a warming planet. It reminds us that climate change is not just an environmental issue, but a fundamental biological one that reaches into the womb. By understanding how temperature influences reproduction, societies can better prepare for a future where safeguarding the health of mothers is the first line of defense against a changing climate.

Medical Disclaimer

This article is for informational purposes only and should not be considered medical advice. Always consult with qualified healthcare professionals before making any health-related decisions or changes to your treatment plan. The information presented here is based on current research and expert opinions, which may evolve as new evidence emerges.

References

• https://www.ndtv.com/health/exposure-to-high-temperatures-linked-with-decline-in-male-births-study-11291291