Heatwaves linked to pregnancy risks

Pregnancy changes how the body handles heat, and a February 25 JAMA Insights article says hotter heat waves are raising risks for pregnant patients. (jamanetwork.com) The article says ambient heat includes hot weather, high indoor temperatures, and job-related exposure, and it can push heat stress into heat exhaustion or heatstroke above about 104 degrees Fahrenheit, or 40 degrees Celsius. (jamanetwork.com) The Centers for Disease Control and Prevention updated clinical guidance on September 18, 2025, saying heat can harm pregnant women in any trimester and that even one day above the local 95th percentile of mean temperature may increase risk. (cdc.gov) One large United States study in JAMA Network Open analyzed 53,154,816 births in 50 metropolitan areas from 1993 to 2017 and found higher daily rates of preterm and early-term birth after heat waves. (jamanetwork.com) In that study, four consecutive days above a city’s local 97.5th percentile were linked to a rate ratio of 1.02 for preterm birth and 1.01 for early-term birth, with stronger effects after longer and hotter heat waves. (jamanetwork.com) Another cohort study, covering 403,602 pregnancies in Southern California from 2008 to 2018, found both long-term and short-term heat exposure during pregnancy were associated with higher risk of severe maternal morbidity during delivery hospitalization. (health.oregonstate.edu) That study counted 3,446 severe maternal morbidity cases, or 0.9% of pregnancies, and found significant associations with heat exposure across pregnancy and in the third trimester. (health.oregonstate.edu) A 2024 Nature Medicine study of 197,080 singleton live births in eight provinces in China found heat exposure from conception to 20 weeks was associated with a 15% to 21% increase in preterm birth and a 20% to 22% increase in medically indicated preterm birth. (nature.com) That paper also found maternal hypertension explained part of the link: 15.7% of the effect on overall preterm birth and 33.9% of the effect on medically indicated preterm birth at temperatures above the local 90th percentile. (nature.com) The climate backdrop is moving in the same direction. The World Meteorological Organization said 2024 was the warmest year in the 175-year observational record, at about 1.55 degrees Celsius above the 1850 to 1900 average. (wmo.int) For clinicians, the JAMA article and the Centers for Disease Control and Prevention guidance point to practical steps: review medicines that increase heat sensitivity, check the HeatRisk forecast and air quality index, and make a heat action plan before hot months begin. (jamanetwork.com) (cdc.gov)