How does extreme heat impact pregnancy? Study links heat waves to early births

As the world braces for another summer of extreme heat, following the record-setting 2023 season, a recent study has acutely linked heat waves to the rate of early births among pregnant women.

Marking the largest, multi-institutional investigation into the correlation between unusually high temperatures and pregnancy to date, researchers analyzed 53 million births across the 50 most populous U.S. metropolitan areas over 25 years from 1993 to 2017.

Findings showed that rates of preterm and early-term births slightly increased after pregnant women experienced increased temperatures for more than four consecutive days.

"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, Ph.D., professor of biostatistics and bioinformatics and senior author of the paper said in a press 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," Chang said.

Preterm birth, which includes delivery at less than 37 weeks of gestation, is a leading cause of infant mortality and can cause lasting health challenges, according to the World Health Organization (WHO).

Early-term infants -- delivered between 37 and 38 weeks of gestation -- also experience increased mortality relative to those born at 39 to 40 weeks gestation, according to WHO.

Researchers found that after four or more consecutive days of extreme heat, there is a 2% higher chance of preterm births and a 3% higher chance for early-term births.

"Heat waves were positively associated with daily rates of preterm and early-term births, showing a dose-response association with heat wave duration and temperatures and stronger associations in the more acute 4-day window," researchers wrote in the study published by JAMA Network Open on May 24.

Of the births analyzed in the study, 30% of mothers were younger than 25, 53.8% were 25 to 34 years old, and 16.3% were 35 or older.

The study included researchers from Rollins School of Public Health, the University of Nevada Reno, Yale University, the University of Utah and the National Center for Atmospheric Research.

Researchers analyzed the data between October 2022 and March 2023 at the National Center for Health Statistics.

In December 2023, Copernicus, Europe's climate change service, announced that the world had experienced the hottest year in recorded history.

Summer 2023 brought unparalleled stretches of triple-digit temperatures throughout the southern U.S. and worldwide, the planet reached its hottest day ever recorded for four days in a row in July.

According to the National Weather Service, heat kills more people on average than any other weather disaster in the U.S.

When it comes to protecting your health in times of extremely high temperatures, hydrating, avoiding overexertion and seeking medical treatment if heat illness is suspected is advised, according to NWS.