Extreme Heat During Pregnancy Skews Birth Sex Ratios by Two Different Mechanisms

Days when temperatures climb above 20 degrees Celsius are statistically linked to fewer male births - and a study analyzing five million live births across 33 sub-Saharan African countries and India has now established why that relationship exists. The answer turns out to depend almost entirely on where you look.

The research, published in the Proceedings of the National Academy of Sciences and involving Portland State University demographer Joshua Wilde, deployed high-resolution temperature data matched to birth records to ask a pointed question: when extreme heat reduces male births, what is actually happening? The answer split cleanly along geographic lines - biology in one region, human behavior in the other.

Sub-Saharan Africa: the frail male hypothesis at population scale

In sub-Saharan Africa, the reduction in male births appears to be biological. First-trimester heat exposure increases maternal heat stress and raises miscarriage risk. Male fetuses are known to be biologically more vulnerable than female fetuses - they require greater maternal investment to survive and are disproportionately lost when the mother's body is under stress. This pattern, sometimes called the "frail male" hypothesis, has been observed in other contexts but rarely demonstrated at this scale across 33 countries simultaneously.

The effect concentrated among the most vulnerable groups: mothers in rural areas and those with less formal education - populations less likely to have air-conditioned living spaces or access to cooling during heat events.

India: a behavioral disruption with an unintended consequence

The mechanism in India was different. India, particularly in its northern regions, has a historically elevated male birth ratio reflecting the documented practice of sex-selective abortion targeting female fetuses. Against that backdrop, a drop in male births during extreme heat events carries a different meaning.

The researchers found the heat effect in India was concentrated in the second trimester - the period when fetal sex can be reliably determined by ultrasound. During heat waves, daily life is disrupted: mobility falls, income generation drops, and access to medical clinics becomes harder. Families who would otherwise seek sex-selective procedures cannot easily do so. More female fetuses therefore survive to term, and the sex ratio moves closer to parity. This is not a straightforward public health benefit - it reflects heat's capacity to disrupt deeply embedded social practices, with consequences that cut in multiple directions simultaneously.

A broader demographic signal

The study adds to growing evidence that rising global temperatures affect human health and demography through pathways unrelated to direct heat illness. Chronic reshaping of who is born - and how many of each sex - has downstream effects on population structure, labor markets, and social organization that compound over decades.

Limitations include reliance on population-level birth records that cannot capture individual-level data on pregnancy outcomes, maternal health, or heat exposure with precision. The behavioral pathway in India is inferred from temporal and regional patterns rather than directly observed clinical or mobility data. The two mechanisms identified here are not interchangeable, and interventions appropriate for one region may be irrelevant in the other.