Recent research led by scientists-physicians at the Beth Israel Deaconess Medical Centre (BIDMC) suggests that low-dose aspirin initiated early in pregnancy may mitigate the effects of heat exposure on preterm birth.
Rising temperatures are contributing to worse health outcomes for mothers and newborns, with mounting evidence linking extreme heat to preterm (or premature) birth and other complications.
This is already a leading cause of infant illness and death worldwide, with an estimated 12m to 15m babies born too early each year, meaning even modest increases in heat-related risk could have substantial global consequences.
The research by a team at BIDMC points to a practical way to help reduce that risk. In a secondary analysis published in JAMA Network Open, the scientists found that humid heat exposure 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 corresponding and senior author Blair Wylie, MD, MPH, Chair of Obstetrics and Gynaecology at BIDMC.
“Scalable interventions are urgently needed to mitigate the adverse effects of heat on pregnancy and newborn health.”
The analysis drew on data from a large, NIH-funded randomised clinical trial known as the Global Network for Women’s and Children’s Health Research ASPIRIN trial.
More than 11 500 first-time pregnant women 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 woman’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 – while this is a modest effect, 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.
Low-dose aspirin is already routinely used in some pregnancies to reduce the risk of conditions such as pre-eclampsia, and the findings suggest it could offer a practical, low-cost way to reduce heat-related risk.
At the same time, the study underscores how rising temperatures are reshaping risks for pregnant women and their babies. The 10 warmest years on record have all occurred in the past decade, and extreme heat has been linked to preterm birth, low birth weight, and stillbirth.
The findings come with important caveats. The study also observed that heat exposure was associated with perinatal mortality among participants who did take aspirin, particularly in areas where malaria is endemic, highlighting the need for further studies.
“The increasing global prevalence of heat stress warrants testing the effects of aspirin more broadly among pregnant women as well as its safety with respect to perinatal mortality,” Wylie said.
Study details
Aspirin and Preterm Birth Among Pregnant People With Increased Heat Exposure: Secondary Analysis of a Randomised Clinical Trial
Gabriella Meltzer, Luke Duttweiler, Sarah Saleem et al.
Published in JAMA Network on 6 May 2026
Abstract
Importance
Scalable interventions are urgently needed to mitigate the adverse effects of heat on pregnancy and newborn health.
Objective
To evaluate whether low-dose aspirin modifies the association between heat exposure and preterm birth.
Design, Setting, and Participants
This secondary analysis of the Global Network for Women’s and Children’s Health Research Aspirin Supplementation for Pregnancy Indicated Risk Reduction in Nulliparas (ASPIRIN) randomised, double-blinded, placebo-controlled clinical trial was conducted from March 2016 to June 2018. Statistical analyses were performed from June 2024 to June 2025. The study settings included the Democratic Republic of Congo, Zambia, Kenya, Guatemala, Pakistan, and Belagavi and Nagpur, India. Participants included nulliparous individuals between 6 and 13 weeks’ gestation recruited through local clinics and communities, with delivery at 20 or more weeks’ gestation.
Exposures
Prenatal care site–specific daily maximum humid heat averaged across gestation and by gestational week, and randomisation to aspirin or placebo.
Main Outcome and Measure
The main outcome was preterm birth (delivery between 20 and <37 weeks’ gestation) with gestational age confirmed by enrolment ultrasonography.
Results
Of 11 558 participants (mean [SD] age, 20.9 [3.3] years), 5787 were randomized to receive aspirin and 5771 to receive placebo. Preterm birth occurred among 754 placebo recipients (13.1%) and 668 aspirin recipients (11.6%). In mixed-effects pooled logistic regression, each 1 °C increase in mean daily maximum shaded wet-bulb globe temperature across gestation was associated with a 5% increased odds of preterm birth (adjusted odds ratio, 1.05; 95% CI, 1.01-1.10). In stratified analyses, this increased risk was observed only among placebo recipients (adjusted odds ratio [AOR], 1.07; 95% CI, 1.02-1.13), not among aspirin recipients (AOR, 1.03; 95% CI, 0.97-1.10). In pooled mixed-effects logistic distributed lag models, increased odds of preterm birth were observed 17 to 19 weeks before delivery among individuals whose daily maximum shaded wet-bulb globe temperature exceeded the site-specific 75th percentile compared with the lowest 3 quartiles. This vulnerability was not observed among aspirin recipients. In contrast, the association of heat with perinatal mortality was observed only among those receiving aspirin (AOR, 1.15; 95% CI, 1.05-1.26) and not among those receiving placebo (AOR, 1.03; 95% CI, 0.96-1.11).
Conclusions and Relevance
The findings of this secondary analysis of the Global Network ASPIRIN trial suggest that low-dose aspirin initiated early in pregnancy among nulliparous individuals may mitigate the effects of heat exposure on preterm birth. The increasing global prevalence of heat stress warrants testing its efficacy more broadly among pregnant people as well as its safety with respect to perinatal mortality.
See more from MedicalBrief archives:
Higher still birth risks from climate change – Wits research
Extreme heat in pregnancy can affect future life of baby – SA review
Extreme heat exacerbating global health risks — UN scientific report
Heat in sub-Saharan Africa tied to miscarriage risk – UKZN study