A study led by Dr Jasmin Abdel Ghany, and including several LCDS associated researchers, provides new evidence that rising temperatures can influence the sex ratio at birth, with important implications for population health and gender balance in a warming world.
Sex ratios at birth — the number of boys born relative to girls — are a key demographic indicator. They reflect underlying patterns of maternal health, prenatal survival, and, in some contexts, gender discrimination. In recent decades, skewed sex ratios have raised concerns in several regions, particularly where son preference and sex-selective abortion are prevalent. The research links these concerns to worries about increasing exposure to extreme heat worldwide and raising new questions about how environmental stress affects pregnancy outcomes and population composition.
In this study, published in Proceedings of the National Academy of Sciences (PNAS), the authors analyse data on more than five million births from 33 sub-Saharan African countries and India. By linking large-scale survey data with high-resolution temperature records, they examine how exposure to heat during pregnancy affects the sex ratio at birth.
The results show that temperatures above 20°C are consistently associated with fewer male births in both regions. However, the underlying mechanisms differ across contexts.
In sub-Saharan Africa, exposure to high temperatures during the first trimester of pregnancy is linked to a decline in male births. This pattern is consistent with increased prenatal mortality driven by maternal heat stress, and particularly pronounced among women living in rural areas, those with lower levels of education, and those with higher birth orders.
In India, where sex ratios at birth have historically been distorted by son preference and sex-selective abortion, the effects appear later in pregnancy. Higher temperatures during the second trimester are associated with fewer male births, especially among older mothers, high-parity births, and women without sons in northern states. This pattern suggests that heat exposure may reduce access to, or use of, sex-selective abortion, leading to a temporary narrowing of gender imbalances.
Taken together, the findings show that heat affects sex ratios through both biological and behavioural pathways. Extreme temperatures can increase pregnancy losses, particularly among male fetuses, while also influencing family planning decisions in contexts where gender preferences shape reproductive behaviour.
A large sample size is needed to study variation in the sex ratio at birth and surveys typically contain little information of maternal health and behaviors under heat that could help explain underlying mechanisms. The authors addressed these challenges innovatively by pooling data from more than 90 Demographic and Health Surveys. To identify mechanisms, they exploited the idea that different mechanisms should produce different patterns during exposure: The authors use differences by sociodemographic groups, exposure timing, the magnitude of effects, and the cultural context to point to the mechanisms that could explain how temperature affects the sex ratio.
Dr Abdel Ghany, lead author of the study, commented: “Extreme heat is not only a major public health threat. We uncover that temperature fundamentally shapes human reproduction by influencing who is born and who is not born. Our findings show that temperature has measurable consequences for fetal survival and family planning behaviour, with implications for population composition and gender balance. Understanding these processes is essential for anticipating how the environment affects societies in a warming climate.”
The study also highlights that the effects of heat are not evenly distributed. Women with fewer resources and those living in more vulnerable settings are more strongly affected, raising concerns about widening health inequalities under climate change.
By combining large-scale demographic data with detailed climate records, this research demonstrates how environmental change can shape fundamental population processes. It contributes to growing evidence that climate change is not only an environmental and economic challenge, but also a major public health and demographic issue.
This research fundamentally relies on harmonized, cross-national, geo-referenced data alongside sociodemographic information from the Demographic and Health Surveys – the future of which remains uncertain due to historic USAID cuts.
As global temperatures continue to rise, the authors argue that protecting maternal health and improving access to healthcare will be central to reducing the long-term impacts of heat on reproduction and population dynamics – and that an ambitious research data infrastructure is needed to achieve these goals.
Reference
J. Abdel Ghany, J. Wilde, A. Dimitrova, R. Kashyap, & R. Muttarak, Temperature and sex ratios at birth, Proc. Natl. Acad. Sci. U.S.A. 123 (8) e2422625123, https://doi.org/10.1073/pnas.2422625123 (2026).
