Heat Is a Pregnancy Risk Factor. Almost No One Treats It Like One.

Only about half of the world’s heat-health plans even list pregnant women. The evidence says that is a serious miss.

A new publication highlights a growing gap between what we know about heat exposure in pregnancy and what public health systems actually tell pregnant women.

The authors review mounting evidence linking extreme heat to preterm birth, stillbirth, hypertensive disorders, placental complications, fetal growth restriction, and other adverse maternal and neonatal outcomes.

Then they deliver the uncomfortable part: most heat-health action plans still do not name pregnancy as a high-risk condition.

One review found that only 52% of heat-health action plans published between 2004 and 2024 listed pregnant individuals as a key population to protect, and none addressed heat risks for mothers, newborns, and children in any comprehensive way. When researchers ranked which vulnerable groups actually received targeted heat interventions, pregnant women came in last, behind older adults, people with chronic disease, children, and even athletes (9,10,11).

If public health agencies can warn elderly patients and people with chronic disease during heat waves, pregnant women should not remain an afterthought in climate-health planning.

A 5-degree Celsius rise in average temperature during early pregnancy was associated with a nearly seven-fold increase in the risk of preeclampsia in one study. In another, each 1°C increase in daily temperature added almost 4 additional preterm births per 1,000 live births.

These are not projections. These are findings from published epidemiological studies, and they point to a pregnancy risk factor that most clinicians never discuss with patients: heat.

Every summer, millions of pregnant people are exposed to temperatures that research links to serious complications. Yet heat exposure rarely appears on a prenatal checklist. It should.

What Happens to the Pregnant Body in Heat

Pregnancy changes how the body handles heat. Core body temperature drops slightly. The threshold for sweating lowers. Blood volume and skin blood flow increase. These are adaptive changes that help the body dissipate heat more effectively (1,2). The placenta itself generates heat as it transfers thermal energy from the fetus to the mother, increasing the mother’s overall heat load (3).

For healthy pregnant people doing moderate exercise, these adaptations work well. Research shows that pregnant individuals can safely exercise for up to 35 minutes at 80% to 90% of maximum heart rate in conditions of 25°C (77°F) and 45% relative humidity (2). There is no evidence that pregnancy itself increases the risk of heat injury compared to non-pregnant individuals.

But “no increased risk of heat injury” is not the same as “no increased risk from heat.” The real concern is not heatstroke. It is what sustained high temperatures do to the placenta, to blood flow, and to the developing fetus.

The Evidence: Preeclampsia, Preterm Birth, Stillbirth

A time-to-event study of 8,090 births in South Africa examined the relationship between ambient temperature and hypertensive disorders of pregnancy. The findings were striking. Exposure to a mean temperature of 23°C, compared with 18°C, between 2 and 5 weeks of gestation was associated with a significantly increased hazard of preeclampsia (hazard ratio 7.68; 95% CI, 1.72 to 34.28). The absolute numbers tell the story even more clearly: preeclampsia rates were 4.4% among those exposed to 18°C throughout pregnancy, compared to 29.2% among those exposed to 23°C in that critical early window. That translates to approximately 248 additional cases per 1,000 pregnancies (4).

A systematic review and meta-analysis of 47 observational studies, primarily from high-income countries, examined heat and preterm birth. Each 1°C increase in daily temperature was associated with a 1.05-fold increase in the odds of preterm birth (95% CI, 1.03 to 1.07). Against a baseline preterm birth rate of 7.9%, this corresponds to an absolute risk of 8.28%, or 3.8 additional preterm births per 1,000 live births per degree of warming (5).

The same review reported that each 1°C increase in temperature was associated with a 1.05-fold increase in stillbirth risk (95% CI, 1.01 to 1.08) (5). These are small relative increases that, applied across millions of pregnancies, translate to thousands of affected families.

The mechanisms are not fully understood, but the leading hypotheses center on dehydration reducing maternal blood volume, which decreases placental blood flow and raises oxytocin concentrations, potentially triggering preterm contractions (1,3). Heat exposure during pregnancy may also increase systemic inflammation (1).

Why the Same Temperature Is Not the Same Risk

The largest synthesis of this evidence to date reviewed 198 studies across 66 countries (12). It found that each 1°C rise in temperature increased the odds of preterm birth by about 4%, and that during heat waves the odds rose by 26%. The same review found higher odds of stillbirth, of congenital anomalies (48% higher odds), and of gestational diabetes (28% higher odds). These numbers line up with the smaller reviews and strengthen the case that heat is a real, measurable risk in pregnancy.

But the risk is not the same everywhere, and this is the part that often gets lost. The danger from heat depends not only on how hot it gets, but on whether the body is used to that heat. In the United States, a study of extreme heat and hospitalization during pregnancy found that the harm was driven mostly by women in historically cooler counties, not by women in places that are usually hot (13). A separate analysis found a clear link between temperature and both maternal death and stillbirth in the Democratic Republic of the Congo, where temperatures swing widely, but found no such link in Senegal, where heat is high and steady year-round (9,14). The body appears to adapt to heat it knows. A 30°C day in a cool climate may carry more risk than a 35°C day in a place that is hot every day.

For patients, this carries a practical lesson. The first heat wave of the season, and any sudden jump in temperature, deserves more caution than a steady hot spell your body has already adjusted to.

Mental Health and Heat in Pregnancy

A matched analysis from North Carolina covering 2011 to 2019 found that heat wave periods were associated with a higher risk of severe mental illness among pregnant individuals (relative risk 1.13; 95% CI, 1.08 to 1.19). No association was found between heat waves and perinatal mood or anxiety disorders specifically (6). This distinction matters: the risk appears to affect those with pre-existing severe mental illness, a population that already faces barriers to adequate prenatal care.

Early Pregnancy: Neural Tube Defects

Maternal hyperthermia in early pregnancy has long been recognized as a risk factor for congenital anomalies. During the August 2003 Paris heat wave, when mean daily temperatures exceeded 35°C for 14 consecutive days, researchers documented a 13% increase in neural tube defects among births conceived during that period, an estimated 6 additional cases (7). This is why pregnant people are advised against saunas: core body temperature above 39°C (102°F) is associated with higher risk of congenital anomalies.

Who Is Most at Risk

The populations most vulnerable to heat in pregnancy are those with the least ability to avoid it. Many pregnant people work in hot, humid environments performing physically demanding labor: agricultural work, factory work, jobs without adequate cooling. Some continue working in extreme heat late into pregnancy because they cannot afford to lose income. Workers may lack access to water or sanitation facilities, compounding dehydration risk.

People with pre-existing conditions including diabetes, cardiovascular disease, and mental illness face heightened risk. Low-income communities with limited access to air conditioning are disproportionately affected. This is not just a clinical issue. It is a health equity issue.

Pregnancy Keeps Getting Left Off the List

Here is the part that should bother every clinician. The biology is established. The epidemiology is published. And yet the systems built to protect people during heat waves still skip over pregnancy. Only about half of heat-health action plans even mention pregnant women, and none lay out a full plan to protect mothers and newborns from extreme heat (10). When you line up the groups that get targeted heat protection, pregnant women rank below athletes (11). Acknowledging the vulnerability of pregnancy carries its own risks, including more anxiety and, in some settings, fewer job opportunities (9). That concern is real. But silence is not protection. The answer is accurate information and concrete accommodations, not leaving pregnant women to figure it out alone.

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