Heat Waves Raise the Risk of Early and Preterm Births

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May 31, 2024 – Pregnant people are more likely to deliver preterm and have early births after heat waves that last for 4 straight days, new research shows. 

The researchers looked at the health records of 53 million women who gave birth from 1993 to 2017 in 50 metro areas in the United States. They found that after 4 days of a heat wave, there were 2% more preterm births and 1% more early-term births. The findings were published in the journal JAMA Network Open.

The study said that each 1-degree Celsius increase in average temperature above the threshold was tied to a 1% rise in the rate of both preterm and early-term births.

The finding is timely because a very hot summer is expected this year, following a 2023 summer that had record-breaking temperatures, researchers said.

“We are forecasting a very warm summer this year, and because of climate change, we will experience more heat waves in the future," Howard Chang, PhD, a professor of biostatistics and bioinformatics at the Rollins School of Public Health at Emory University, and senior author of the paper, said in a news release. "Our study shows that this increase in temperature could mean worse outcomes for the babies, because babies born prematurely can have health issues and additional health care costs."

The study defined a heat wave as occurring when the average temperature was higher than 97.5% of typical days for 4 consecutive days. Premature births happen before 37 weeks of gestation, and early-term births happen before 39 weeks. Normal gestation is 40 weeks.

The study said heat waves revealed inequities in health care because mothers who were 29 or younger, had a high school education or less, and belonged to a minority ethnic or racial group were even more likely to have premature or early-term births after a heat wave.

Researchers said this could be due to lack of air conditioning and access to health care.