Low-dose aspirin may offset premature birth risk linked to extreme heat

by Jacqueline Mitchell, Harvard Medical School

edited by Lisa Lock, reviewed by Robert Egan

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Mounting evidence links extreme heat to preterm (often called premature) birth, low birth weight, and stillbirth, indicating that rising temperatures are contributing to worse health outcomes for pregnant people and newborns. Preterm birth is already a leading cause of infant illness and death worldwide, with an estimated 12 to 15 million babies born too early each year, meaning even modest increases in heat-related risk could have substantial global consequences.

Research led by Harvard Medical School researchers at Beth Israel Deaconess Medical Center points to a practical way to help reduce that risk.

In a secondary analysis published May 6 in JAMA Network Open, the team found that exposure to humid heat during pregnancy was associated with higher odds of preterm birth—but not among those who took low-dose aspirin starting early in pregnancy.

"Extreme heat events are intensifying in frequency and severity, with the 10 warmest years on record all occurring in the past decade," said senior author Blair Wylie, HMS professor of obstetrics, gynecology, and reproductive biology at Beth Israel Deaconess. "Scalable interventions are urgently needed to mitigate the adverse effects of heat on pregnancy and newborn health."

The study suggests that low-dose aspirin—which is routinely used in some pregnancies to reduce the risk of conditions such as preeclampsia—could also be valuable for reducing heat-related preterm births.

Linking pregnancy outcomes to climate

The analysis drew on data from a large randomized clinical trial known as the Global Network for Women's and Children's Health Research ASPIRIN trial. More than 11,500 first-time pregnant people across seven countries in Africa, South Asia, and Latin America were enrolled and randomly assigned to receive either daily low-dose aspirin or a placebo beginning early in pregnancy to assess whether aspirin could reduce the risk of preterm birth and other adverse outcomes.

Wylie and colleagues linked each participant's location to high-resolution climate data to estimate heat exposure, using a measure that combines temperature, humidity, sunlight, and wind to reflect how hot it actually feels. They then examined how this humid heat exposure was associated with preterm birth, defined as delivery before 37 weeks.

Among participants who did not take low-dose aspirin, each 1°C increase in average daily humid heat during pregnancy was associated with a 5% increase in the odds of preterm birth. On a population level, this translates to many more preterm births.

This increased risk was not observed among those who took low-dose aspirin early in pregnancy.

More research needed

The work underscores how rising temperatures are reshaping risks for pregnant people and their babies and suggests that low-dose aspirin could offer a practical, low-cost way to reduce heat-related risk.

The study comes with important caveats, however. The researchers found that, among participants who took aspirin, heat exposure was associated with higher perinatal mortality—defined as stillbirth or death within a week of birth. It's unclear if this finding is a statistical error (there were only a small number of perinatal deaths in the data) or if it represents real effects of aspirin.

This result did not appear in the original analysis of ASPIRIN trial participants, which did not examine heat-related effects and found that participants taking aspirin were marginally less likely to experience perinatal mortality (increased risk of perinatal mortality with aspirin was only noted in participants with malaria).

More studies with larger cohorts are needed to determine the effects of aspirin on perinatal mortality, especially in places where malaria is endemic, the researchers said.

"The increasing global prevalence of heat stress warrants testing the effects of aspirin more broadly among pregnant people, as well as its safety with respect to perinatal mortality," Wylie said.

Publication details

Gabriella Y. Meltzer et al, Aspirin and Preterm Birth Among Pregnant People With Increased Heat Exposure, JAMA Network Open (2026). DOI: 10.1001/jamanetworkopen.2026.11402

Journal information: JAMA Network Open

Key medical concepts

AspirinPremature BirthHeat Stress DisordersPerinatal MortalityPreeclampsia

Clinical categories

Obstetrics & gynecologyPregnancyWomen's healthCommon illnesses & Prevention Provided by Harvard Medical School Who's behind this story?

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