Heat and Pregnancy: The Numbers Your Doctor Should Be Sharing

Rising temperatures are not just an environmental issue. They are an obstetric issue, and the evidence is stronger than most clinicians realize.

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).

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.

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