In pregnancy, how hot is too hot?

As the planet warms, heat is becoming more and more of a public health concern, particularly for pregnant people, who have long been known to be especially vulnerable to high temperatures. But until now, researchers didn’t have a good way to measure the biological effects of heat exposure in pregnancy, or to identify those most at risk of adverse pregnancy outcomes. 

A study by researchers at Washington University’s Andrew M. and Jane M. Bursky School of Public Health and the University of Southern California identifies two proteins — heat shock proteins (HSP) 27 and 70 — as promising biomarkers of heat exposure, and provides preliminary data suggesting that one of them (HSP70) may predict the risk of preterm birth.

The findings are a first step toward developing guidelines and practices to help keep pregnant people and their fetuses and babies healthy even as global temperatures continue to rise. The study is published in Environmental Science and Technology. 

“There’s a growing recognition that, when we think about climate-related and extreme weather-related impacts on health, we need to really pay attention to extreme heat days and what that does to our physiological systems,” said senior author Carrie Breton, ScD, a professor at the Bursky School of Public Health and a co-director of the school’s Solutions through Planetary Health Research (SPHERE) network. 

Average global temperatures have been rising since the 1970s, and many places are seeing higher peak temperatures and longer, more frequent and more intense heat waves. Pregnant people tend to run hot, so they have more trouble staying cool and hydrated on hot days, putting them at risk of heat exhaustion and heat stroke, and worsening conditions such as high blood pressure that are already more common during pregnancy. Heat waves are associated with adverse birth outcomes including preterm delivery, stillbirths and low-birthweight infants. 

But heat waves don’t have the same impact on everyone. Someone who spends the day doing desk work in an air-conditioned office doesn’t feel the heat the way a person who spends the day delivering mail or packing boxes in a warehouse without air conditioning does. And not everyone can cool off at night in an air-conditioned home. To better understand the health effects of heat, scientists needed an objective measure of individual heat exposures.  

For that, Breton and colleagues — including first author Xiaoran Yang, PhD, then a doctoral student under Breton’s supervision at USC — turned to heat shock proteins, which are produced by cells in response to heat or other stressors to help restore and maintain normal cellular function. HSP27 and HSP70 are known to play a role in fetal development, and HSP70 has been linked to pregnancy complications. 

Breton and colleagues studied 227 pregnant participants in the larger Maternal and Developmental Risks from Environmental and Social Stressors (MADRES) cohort study. The MADRES cohort was recruited from November 2015 through April 2023 in urban Los Angeles and consists primarily of low-income Hispanic women.

The researchers measured levels of the heat shock proteins in the blood during early (15 weeks gestation) and late (33 weeks) pregnancy. They gauged each participant’s heat exposure in the 30 days prior to the blood draw using their home addresses and two measures of heat: the daily maximum heat index, which includes temperature and humidity; and the wet bulb globe temperature, which includes temperature and humidity plus shortwave radiation, windspeed and other weather measures.

Participants were surveyed — in English or Spanish, as preferred — to determine how much time they spend outdoors on a typical weekday, whether they had air-conditioning in their homes, and other factors. 

Both proteins responded to heat. HSP70 levels rose one to five days after a hot day, and HSP27 levels followed another week or two later, suggesting that both proteins may work as indicators of acute heat stress during pregnancy. 

Overall, both HSP27 and HSP70 levels decreased over the course of pregnancy. For a subset of women, however, HSP70 levels rose as the pregnancy progressed — and that group seemed to be at higher risk of preterm delivery. Mothers who had a large increase in HSP70 levels from early to late pregnancy were 5.2 times more likely to deliver prematurely, and mothers with low to moderate increases were 2.7 times more likely to deliver their babies early compared with mothers whose HSP70 levels remained stable or decreased. As there were only 18 preterm births in the study group, however, these results are preliminary, and further research needs to occur, the researchers said.  

These heat exposures — and potential adverse outcomes — occurred at a time when Los Angeles was experiencing fairly typical weather for the city.

“We find that heat stress exposures are associated with elevated HSP27 and HSP70 levels during pregnancy, even under heat conditions that are classified as minimal risk for normal outdoor activities in the general population,” Breton said. “This is important, because it implies that how we classify heat stress and the thresholds we apply may need to change for pregnant people.”

Not only that, heat-exposure thresholds may need to be tailored to each location and its unique weather patterns, Breton continued. 

“People always think about California as being hot, and it is, but because it’s a dry heat, when we incorporate humidity and these other meteorological components, it isn’t really all that extreme compared to other places in the country,” Breton said. “How we define an extreme heat event in LA may not work for St. Louis or Atlanta or other places.”

Breton and colleagues are looking into expanding their research to St. Louis and further investigating the preterm birth findings in a larger cohort.

Yang X, Hu Y, Pardo N, Xu Y, Khalili R, Wolf-Jacobs A, Eckel SP, Niu Z, Yang T, Chen X, Foley H, Guardado D, Grubbs B, Al-Marayati L, Toledo-Corral CM, Johnston J, Dunton GF, Farzan SF, Bastain TM, Habre R, Breton CV. Maternal heat shock protein as a potential biomarker of adaptation to heat stress during pregnancy and its relation to preterm birth. Environmental Science and Technology. May 1, 2026. DOI: 10.1021/acs.est.5c14632

This work was supported by a National Institute of Health (NIH) Climate Change and Health Administrative Supplement, grant number 3P50MD015705-09S1; the National Institute on Minority Health and Health Disparities, grant numbers P50MD015705 and UH3OD023287; and the National Institute of Environmental Health Sciences, grant numbers P50ES026086, 83615801-0 and P30ES007048. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Tamara Schneider, MPH, PhD, is the assistant director of communications and senior science writer at Bursky Public Health. She holds a bachelor’s degree in molecular biophysics & biochemistry and in sociology from Yale University, a master’s in public health from the University of California, Berkeley, and a PhD in biomedical science from the University of California, San Diego. She joined WashU as a science writer in 2016.