Heat and pregnancy: risks of early birth and other problems rose from about 31 C, a review finds

Summers are getting hotter, and pregnancy is one of the times when heat seems to matter most. Studies have linked hot weather to more preterm births and stillbirths, but they have measured heat in so many different ways that it has been hard to say how hot is too hot.

A review in Nature Communications tried to put numbers on it. Pooling 21 studies that covered 5.6 million pregnancies, it estimated that the risk of problems in pregnancy and birth rose by 5% on days with a high of about 31 C (87 F), and kept rising as temperatures climbed.

What heat does to health

Heat is one of the deadliest kinds of weather. The World Health Organization (WHO) notes that heat stress is a leading cause of weather-related deaths and can worsen heart disease, diabetes, asthma and mental health conditions. It also notes that heatwaves and prolonged excess heat conditions are increasing in frequency, duration, intensity and magnitude as the climate changes.

When the body cannot shed heat fast enough, its temperature rises. MedlinePlus explains that during hot weather, especially when it is very humid, sweating just isn’t enough to cool you off, and describes heat stroke as a life-threatening illness in which body temperature can climb rapidly.

What earlier research found about heat in pregnancy

The strongest earlier evidence came from a 2020 review in the BMJ of 70 studies from 27 countries. In 40 of 47 studies, preterm births were more common at higher than lower temperatures. On average, the odds of a preterm birth rose about 5% for each 1 C increase in temperature and about 16% during heatwaves.

That review also found links with stillbirth and, less consistently, with lower birth weight. The effects were largest among women in lower socioeconomic groups and at the youngest and oldest ages. Its authors noted that although summary effect sizes are relatively small, heat exposures are common and the outcomes are important determinants of population health.

How the heat threshold review worked

The new review searched five databases for studies that reported a temperature threshold, the point at which risk starts to rise, rather than just a trend. It excluded heat from saunas, hot tubs, pools or medical devices, and studies on conception or fertility treatment.

The researchers then used a statistical method called meta-regression to estimate the daily maximum temperature linked to a 5%, 10% and 20% increase in risk of adverse pregnancy and birth outcomes. Those outcomes included preterm birth, stillbirth, low birth weight and complications for the mother. A wider narrative review of 103 studies covered nearly 42 million people.

What the heat thresholds showed

The meta-analyses determine Tmax thresholds 30.55 C, 34.11 C and 39.07 C for 5%, 10% and 20% risk increase, where Tmax means the day’s highest temperature. In more familiar terms, that is about 87 F, 93 F and 102 F.

The pattern points to a steady climb rather than a single cutoff: the hotter the day, the higher the risk. The narrative review found that studies used many different heat measures and that few used indexes combining temperature with humidity, which matters because humid heat is harder for the body to shed.

Why pinning down heat thresholds matters

Many heat warning systems are set for the general population or for older adults, not for pregnancy. A threshold tied to pregnancy outcomes gives public health agencies and hospitals a concrete number to plan around, for example when deciding when to issue alerts or adjust prenatal care during heatwaves.

The authors frame the findings as increasing risk of adverse pregnancy and birth outcomes with increasing heat, and stress that pregnant people are particularly vulnerable to heat as global temperatures rise.

What the pregnancy heat review cannot show

The review’s authors are direct about its limits. Studies were very different from each other, and the review had no independent dual data extraction and a single person performing quality assessment, which raises the risk of errors.

The thresholds pool different outcomes and settings together, so they cannot say which outcomes are most affected or how the numbers apply in a particular city. People used to hot climates may cope with temperatures that would harm people elsewhere. And like all heat studies, these measured outdoor temperatures, not the heat each person actually experienced indoors.

What this changes for heat and pregnancy

The review adds weight to a consistent picture: hotter days go with higher risks in pregnancy, beginning at temperatures that are now common in many places in summer. It gives planners a starting point for pregnancy-specific heat warnings.

It does not provide individual thresholds, and it does not test any protective measure. The general guidance from health agencies on recognizing and preventing heat illness applies in pregnancy as it does to everyone, and questions about a specific pregnancy are for the person’s own care team.