Comment on “Heat and Pregnancy”

Abstract

Climate change is increasing the frequency and intensity of heat waves, and in turn, causing greater exposure of pregnant patients to extreme heat. This article summarizes current evidence on maternal and pregnancy risks from ambient heat, including hot weather, high indoor temperatures, and occupational exposures, as well as strategies to reduce risk. Heat stress occurs when the body cannot adequately dissipate heat, which can lead to heat exhaustion or, in severe cases, heatstroke. During pregnancy, physiological adaptations, including a reduction in core temperature and a lower sweating threshold, help offset increased demand for heat dissipation due to placental heat transfer from the fetus. Pregnancy does not appear to impair thermoregulation in warm conditions or during moderate exercise, and there is no evidence of a greater risk of heat injury in pregnant patients. Epidemiologic studies suggest that high ambient temperatures are associated with adverse maternal and pregnancy outcomes, though mechanisms remain uncertain. Heat exposure around conception has been associated with increased risk of hypertensive disorders of pregnancy, while evidence for gestational diabetes is inconsistent. Heat waves have also been associated with greater hospitalization risk for severe mental illness during pregnancy. Heat exposure in the days before delivery is associated with increased risk of preterm birth and stillbirth, with one systematic review reporting that each 1 °C increase in daily temperature was associated with a 1.05-fold increased risk for preterm birth and stillbirth. Proposed mechanisms include dehydration, which could reduce placental blood flow and increase oxytocin concentration, and systemic inflammation. In early pregnancy, maternal hyperthermia has also been associated with increased risk of congenital anomalies. Pregnant patients should be counseled about heat-related risks, particularly those with risk factors like diabetes, cardiovascular disease, mental illness, or occupational heat exposure. Prophylactic measures include avoiding extreme heat, reducing physical activity during hot weather, maintaining hydration, lowering indoor temperatures when possible, and using personal cooling measures. Clinicians can help patients in low-income settings by directing them to social programs that assist with air conditioning costs during heat waves. Overall, the article emphasizes that patients and clinicians should recognize heat-related risks for adverse pregnancy outcomes and use practical interventions to reduce exposure. (Summarized from Kovats S, Lokubal P, Filippi V. Heat and pregnancy. JAMA. 2026;335:1263–1264. doi:10.1001/jama.2026.0001).