A recent analysis of demographic and health surveys in sub-Saharan Africa and India showed that when pregnant women experience higher ambient temperatures during pregnancy, fewer males are born.
An article titled “Temperature and Sex Ratios at Birth” in the journal Demography by Jasmin Abdel Ghany et al. after a detailed analysis, they conclude that higher ambient temperatures during pregnancy are associated with changes in the natural sex ratio at birth in India and sub-Saharan Africa. The paper analyzes more than five million births obtained from more than 90 demographic and health surveys that include local temperature data to determine how exposure to heat during the trimester affects the sex ratio at birth. Sex ratios at birth shape population composition and are closely linked to maternal health and gender discrimination.
The researchers write: “We found days with a maximum temperature above 20°C to be negatively associated with male births in both regions. In sub-Saharan Africa, we observe fewer male births after first-trimester heat exposure, consistent with increased spontaneous abortions due to maternal heat stress… In contrast, in India, we found second-trimester heat exposure to be associated with fewer male births.” The study highlights that these reductions target older mothers in rural areas who have had multiple children.
The maximum daily temperature in the month of birth is 30.0 °C in sub-Saharan Africa and 30.3 °C in India. In India, in the second trimester, results suggest a negative relationship between heat exposure and sex at birth. The effect of 25 to 30 °C indicates a lower probability of a male birth by 0.014 percentage points.
To account for both biophysical health and behavioral mechanisms, the researchers selected two regions with markedly different experiences of son preference and sex-based abortion: India (where several regions have high son preference and sex-based abortion) and sub-Saharan Africa (where there is little evidence of son preference and sex-based abortion is minimal).
The hypothesis is that these heat-induced pregnancy losses are male biased, consistent with Trivers and Willard’s “fragile male” hypothesis. “According to this evolutionary argument, weak males may have a lower chance of surviving to birth in poor environmental conditions. After birth, males are less likely to survive than females and therefore require greater maternal investment,” the authors write.
Vidhya Venugopal, Country Director (NIHR GHRC NCD-EC), Faculty of Public Health, Sri Ramachandra Institute of Higher Education and Research, Chennai, says the results are not unexpected. Instead, they highlight the urgency to explore solutions to protect vulnerable groups as temperatures rise.
“When your body temperature rises one or two degrees above basal temperature, it’s a fever. Pregnant women already have a higher body temperature, and if there are situations of rising heat, there will definitely be a cascade of negative effects… Physiologically, the organs are affected. We can expect the same thing that happens after a fever attack – extreme tiredness, fatigue, reduced cognition, in pregnant women, in pregnant women, definitive hypertension. gestational diabetes, premature births, the risk of underweight children increases,” he explains.
While heat is considered “extremely natural in tropical countries” such as India and sub-Saharan Africa, the impact of heat stress, particularly on people living in resource-poor settings, is grossly underestimated, says Dr. Vidya.
Jane Hirst, Chair of Global Women’s Health, The George Institute for Global Health, Imperial College London, agrees. “Most of the evidence that the risk is not coming from the hottest or low- and middle-income countries, he adds. That is slowly changing,” he says. Evidence is now emerging from these countries as well, suggesting that the risks may actually be greater. “For example, combined global evidence suggests a 25% increase in preterm births with heat stress. However, our study in Tamil Nadu showed a threefold risk, almost 300%.”
Governments play a huge role here, agrees Dr. Vidhya and Professor Jane. Both see risk communication to vulnerable populations as a key step, as risk perception and thus the ability to intervene are very low. “Communication must be done in a way that reaches everyone,” says Dr. Vidya. Also important is the protection of disappearing traditional knowledge systems that enable cooling mechanisms suitable for resources, including food, he adds.
According to prof. Jane also needs to initiate structural changes – make sure the antenatal clinic is located inside a well-ventilated building or safe drinking water is available to help them cool down. “The Indian government has taken the lead in drawing up regional and sub-regional heat action plans. However, a third of them did not consider pregnant and lactating women and infants as a vulnerable group,” he points out.
Prof. Jane says it’s important to learn from best practices in securing adequate funding and resources for climate engineering. The SMART Health Pregnancy trial, currently underway in two Indian states, looked at providing early warning signals to AASHA workers during weather events: “This has shown acceptability and will also be expanded to 7 other countries.”
ramya.kannan@thehindu.co.in
Published – 06 March 2026 07:15 IST
