Aspirin and Preterm Birth Among Pregnant People With Increased Heat Exposure

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) randomized, 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 randomization to aspirin or placebo. Main Outcome and Measure: The main outcome was preterm birth (delivery between 20 and