A study co-authored by Associate Member Dr Jasmin Abdel Ghany, along with the Department’s Professor Ridhi Kashyap, provides new evidence that higher 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.
This research links these concerns to worries about increasing exposure to extreme heat worldwide, raising new questions about how environmental stress affects pregnancy outcomes and population composition.
Published in Proceedings of the National Academy of Sciences (PNAS), the study analyses more than five million births across 33 sub-Saharan African countries and India.
By linking large-scale survey data with high-resolution temperature records, the authors 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 – but through different mechanisms.
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 is 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 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, temporarily narrowing 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 foetuses, while also influencing family planning decisions in contexts where gender preferences shape reproductive behaviour.
Studying changes in the sex ratio at birth requires very large samples, and most surveys contain limited information on maternal health or behaviours during periods of heat exposure, making it difficult to identify the underlying mechanisms.
The authors addressed these challenges by pooling data from more than 90 Demographic and Health Surveys. To understand how temperature affects sex ratios, they examined whether different patterns emerged by timing of heat exposure, sociodemographic group, effect size, and cultural context.
Because biological and behavioural mechanisms would be expected to produce distinct patterns, these comparisons help identify the pathways through which heat influences birth outcomes.
Dr Abdel Ghany, lead author of the study, commented:
Extreme heat is not only a major public health threat. We show that temperature fundamentally shapes human reproduction by influencing who is born and who is not born.
Our findings indicate that temperature has measurable consequences for foetal 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 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.
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.
