The association between traffic-related air pollution and risk of non-reassuring fetal heart rate in newborns at birth.

Despite the biological plausibility linking traffic-related air pollution (TRAP) to adverse intrapartum outcomes, few studies have investigated its association with non-reassuring fetal heart rate (NRFHR), particularly in regions with high pollution levels. This study aimed to examine the relationship between prenatal exposure to multiple TRAP indicators and the risk of NRFHR in Sabzevar, Iran, a city characterized by elevated vehicular emissions. A cross-sectional study was conducted, including 4051 mother-infant pairs delivering at the sole maternity hospital in Sabzevar between May 2016 and December 2018. Residential exposure to ambient particulate matter (PM1, PM2.5, PM10) was estimated using land-use regression models, and traffic-related indicators were derived from municipal and OpenStreetMap data, including street length within buffers and proximity to roads and transportation infrastructure. NRFHR was defined based on continuous cardiotocography monitoring during labor. Multivariate logistic regression assessed associations between exposures and NRFHR, adjusting for maternal, neonatal, and neighborhood confounders. Stratified analyses were performed by fetal sex, and mixture effects of TRAP indicators were evaluated using g-computation (g-comp) and generalized weighted quantile sum (gWQS) regression. In adjusted models, higher PM10 (OR = 1.08, 95 % CI: 1.02-1.15) and PM2.5 (OR = 1.21, 95 % CI: 1.14-1.28) concentrations, as well as longer street length within 100 m buffers (OR = 1.20, 95 % CI: 1.11-1.30), were associated with increased NRFHR risk. Greater distances to gas stations, ring roads, major roads, and public parking were protective. Stratified analyses indicated stronger associations for PM10, PM2.5, and street length in male fetuses. Mixture analyses suggested non-significant associations overall, though certain exposures, particularly road proximity and fine PM, contributed more strongly to the mixture effect. Sensitivity analyses confirmed robustness of findings. Prenatal exposure to PM2.5, PM10, and dense traffic environments was associated with increased odds of NRFHR, with some evidence of stronger effects in male fetuses.