Prenatal exposure to wildfire-related PM_2.5 and birth outcomes: A population-based California statewide study

Wildfire frequency and intensity are increasing, yet impacts on birth outcomes remain unclear, particularly regarding patterns of exposure-response associations and developmental periods of susceptibility. We examined associations between wildfire-specific PM_2.5 exposure during pregnancy and adverse birth outcomes, aiming to identify potential nonlinear relationships and susceptible gestational periods in a large population-based cohort. We linked 4.75 million California live birth records (2011-2020) to daily wildfire-specific PM_2.5 exposure using geocoded maternal residential addresses to estimate weekly gestational exposure. Logistic regression estimated odds for preterm birth (PTB), very preterm (VPT), extremely preterm (EPT), low birth weight (LBW), term LBW, small for gestational age (SGA), and large for gestational age (LGA) adjusted for maternal, temporal and delivery characteristics; generalized additive models (GAMs) and discrete-time survival analysis assessed nonlinear patterns and cumulative exposure effects, respectively. Wildfire smoke PM_2.5 was ubiquitous, affecting about 98% of pregnancies. Per 1 μg/m³ increase in pregnancy-average exposure, odds increased for PTB (OR=1.03, 95%CI: 1.02-1.03), VPT (OR=1.07, 95%CI: 1.06-1.08), and EPT (OR=1.10, 95%CI: 1.09-1.11). LBW showed modest positive associations that were amplified at high exposures (≥10 µg/m³; OR=2.21, 95%CI: 1.96-2.50), and odds for LGA were consistently elevated while SGA was inversely associated. Second-trimester exposure consistently related to greater odds of the adverse birth outcomes. GAMs indicated steeper risk increases at lower-to-moderate exposures that plateaued at higher exposure. These findings indicate that prenatal wildfire-specific PM_2.5 exposure is associated with increased risks for adverse birth outcomes, underscoring the need to protect pregnant individuals during fire events.