Above 20°C, pregnancies yield fewer boys, according to a large University of Oxford study.

An Oxford University study analyzing more than five million births reveals a troubling phenomenon: when temperatures rise above 20°C, pregnancies are less likely to result in boys. A finding that suggests global warming could alter our future.

Millions of births analyzed reveal a direct impact of extreme heat

At first glance, a baby’s sex might seem like a well-tuned biological lottery. In most populations, slightly more boys are born than girls. Yet by delving into more than five million births, Oxford University researchers observed a variation too pronounced to ignore.

Their study, published in the journal PNAS, draws on data from 33 countries in Sub-Saharan Africa and India. When temperatures rise beyond 20°C, the male-to-birth ratio declines. In other words, pregnancies exposed to significant heat are less likely to result in the birth of a boy.

This isn’t science fiction, nor a climate prophecy dressed up to scare. It’s a subtle statistical signal, buried in birth records and weather observations. A detail almost invisible, but one that becomes more worrying as heat waves spread.

Male fetuses appear more vulnerable to thermal stress during pregnancy

The most troubling aspect of the study concerns the vulnerability of male fetuses. The researchers suspect that maternal thermal stress weighs more heavily on their survival before birth. The pregnant body, already mobilized by a complex physiology, must then manage an additional constraint.

In Sub-Saharan Africa, the effect is most marked during the first trimester. It is a pivotal period, barely visible from the outside, when decisive developmental steps are set in motion. Excessive heat could raise prenatal losses, with a stronger impact on male embryos.

In India, extreme temperatures also influence family behaviors

The Indian case adds an unexpected layer to the story. The country has long shown a gender imbalance, linked notably to a cultural preference for sons and to selective abortion practices. In this context, heat does not produce only a biological effect.

According to the researchers, episodes of extreme temperatures could also disrupt access to clinics, travel, or certain family decisions. During the second trimester, this temporary disruption appears to coincide with a drop in male births. Climate then enters a more intimate territory: that of human behavior.

This detour through India serves as a reminder that birth statistics are never merely columns in a table. They carry habits, economic constraints, and sometimes deeply rooted family pressures. When extreme heat joins this mix, it can shift the balance in ways that are hard to predict.

Climate change could permanently reshape the demographic balance

Climate change is often framed by fires, droughts, or rising seas. This study forces a shift in focus toward something quieter: life before birth. The climate no longer acts only around us; it could already influence who comes into the world.

The implications are substantial. A lasting change in the girl-to-boy birth ratio could affect demographics, families, and health systems. Nothing guarantees these effects will be uniform, but the study shows that each additional degree can yield unforeseen consequences.

The question now extends beyond mere scientific curiosity. Protecting pregnant women from heat, improving access to care, and adapting housing becomes a public health issue. In a warmer world, demographics could be shaped even in the shadow of thermometers.