Experiencing heat during pregnancy results in fewer male babies: study

When pregnant women experience higher ambient temperatures during gestation, fewer males are born, a recent analysis of demographic and health surveys in sub-Saharan Africa and India, showed.  

A paper titled ‘Temperature and sex ratios at birth’ in the journal Demography, by Jasmin Abdel Ghany et al., concludes after a detailed analysis that experiencing higher ambient temperatures during pregnancy is associated with changes in the natural sex ratio at birth across India and sub-Saharan Africa. The paper analyses over five million births drawn from more than 90 Demographic and Health Surveys, containing local temperature data, to examine how heat exposure across trimesters shapes 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 find that days with a maximum temperature above 20 °C are negatively associated with male births in both regions. In sub-Saharan Africa, we observe fewer male births after high first-trimester temperature exposure, consistent with increased spontaneous abortions from maternal heat stress… By contrast, in India, we find that second-trimester temperature exposure is associated with fewer male births.” These reductions are focused among older mothers in rural areas who have had a number of children, the study highlights.

The daily maximum 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, the results indicate a negative relationship between temperature exposure and birth sex. The effect of a 25 to 30 °C indicates a lower male birth probability by 0.014 percentage points.  

To account for both biophysical health and behavioral mechanisms, researchers chose two regions with vastly different experiences with son preference and sex-selective abortion: India (where several regions have high son preference and sex selective abortions) and sub-Saharan Africa (where there is little evidence of son preference and sex-selective abortions are minimal).  

The hypothesis is that these heat-induced pregnancy losses are male-biased, in line with Trivers and Willard’s “frail male” hypothesis. “According to this evolutionary argument, weak males may have a lower chance of surviving to birth under poor environmental conditions. After birth, males have lower survival probabilities than females and thus 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 underline the urgency to examine solutions to protect vulnerable groups when temperatures soar.   

“When your body temperature rises by one or two degrees above the basal temperature, it is a fever. Pregnant women already have a higher body temperature and if with rising heat situations, there will certainly be a cascade of negative impacts… Physiologically, the organss take a hit. We can expect the same things as happen after a bout of fever – extreme tiredness, fatigue, lower cognition, at a mimimum here. In pregnant women, there is a definite increase in adverse events including hypertension, gestational diabetes, preterm births, the risk of underweight babies increases,” she 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 who live in resource poor settings is grossly underestimated, Dr. Vidhya says.  

Jane Hirst, Chair in Global Women’s Health, The George Institute for Global Health, Imperial College London, agrees. “Most of the evidence associating that risk is not from the hottest, or the low and middle income countries, she adds. That is slowly changing,” she says. Evidence is now emerging from these nations also, indicating the risks might actually be greater. “For instance, the combined global evidence points to a 25 % increase in pre-term births with heat stress. Our study in Tamil Nadu however indicated that it was three times the risk, nearly 300 %.”  

There is a huge role for governments to play here, Dr. Vidhya and Prof. Jane agree. Both of them see communicating risk to the vulnerable populations as a key step, as risk perception is very low, and thereby, capacity to intervene as well. “The communication must be done in a manner that reaches all of them,” Dr. Vidhya says. Protecting vanishing traditional knowledge systems that allow for resource-appropriate cooling mechanisms, including the diet, is also important, she adds. 

Structural changes must also be initiated according to Prof. Jane – make sure the antenatal clinic is located inside a well-ventilated building, or safe drinking water is available to help them cool off. “The Indian government has taken the lead in putting together heat action plans, at the regional and sub regional levels. However, a third of them did not consider pregnant and lactating women, and infants as a vulnerable group,” she points out.

While ensuring that there is sufficient funding and resources for climate engineering, Prof. Jane says it is important to learn from the best practices. The SMART Health Pregnancy trial, currently underway in two Indian States has been looking at providing early warning signals to AASHA workers on weather events: “This has shown acceptability and is also to be expanded across 7 other countries.”

ramya.kannan@thehindu.co.in

Published – March 06, 2026 07:15 am IST