Prenatal exposure to wildfire fine particulate matter and fetal growth outcomes in the United States: Findings from the ECHO Cohort

Background: Increasing exposure to wildfire fine particulate matter (PM 2.5 ) may pose unique risks to fetal development. We assessed associations between prenatal wildfire PM 2.5 exposure and fetal growth outcomes, including birth weight at term, birth weight for gestational age z -scores, small for gestational age, and large for gestational age in the U.S. Environmental Influences on Child Health Outcomes Cohort. Methods: The study included 20,034 births (2006–2020) from 30 cohort sites nationwide. We assigned daily wildfire PM 2.5 at census-tract resolution and summarized average concentrations and the number of smoke days exceeding multiple intensity thresholds across pregnancy. We used covariate-adjusted mixed-effects models to estimate overall associations and distributed lag models and trimester-specific analyses to evaluate critical exposure windows across gestation, and we examined potential effect modification by US region, fetal sex, maternal race, and neighborhood poverty. Results: Pregnancy wildfire exposure metrics, including mean prenatal PM 2.5 and the number of smoke days, were not associated with study outcomes. Distributed lag and trimester-specific models did not show consistent sensitive windows of exposure associated with birth weight at term, birth weight for gestational age z -scores, or small for gestational age. A positive association was observed between higher early-pregnancy wildfire exposure and large for gestational age (odds ratio = 1.015 per smoke day, 95% confidence interval: 1.001, 1.029), though effect estimates were small in magnitude. No consistent effect modification was identified. Conclusion: In this nationwide cohort, wildfire PM 2.5 showed limited associations with fetal growth outcomes.