Risk and Burden of Preterm Birth Associated with Prenatal Exposure to Ambient PM2.5: National Birth Cohort Analysis in the Iranian Population.

Preterm birth (PTB) is a major global public health concern with substantial impacts on neonatal morbidity and mortality. There is a growing body of evidence linking maternal exposure to fine particulate matter (PM2.5) with PTB, and national birth cohort data from the Middle East remains sparse. We analyzed 3,839,531 singleton live births in Iran from 2013 to 2018. Monthly PM2.5 concentrations during pregnancy were estimated using validated spatiotemporal models. Associations between prenatal PM2.5 exposure and multiple PTB subtypes, moderate to late (MPTB), very (VPTB), and extremely preterm birth (EPTB), were assessed using multivariable logistic regression. A 10 μg/m3 increase in PM2.5 was associated with increased odds of PTB (odds ratio [OR] = 1.048, 95% confidence interval [CI]: 1.044-1.051), MPTB (OR = 1.046, 95% CI: 1.042-1.049), VPTB (OR = 1.059, 95% CI: 1.048-1.070), and EPTB (OR = 1.064, 95% CI: 1.047-1.081), respectively. Age- and trimester-stratified analyses showed greater exposure-related risks among mothers aged 25-34 and during mid-pregnancy. We observed consistent evidence for a J-shaped exposure-risk pattern in overall and subgroup populations, suggesting a PM2.5 threshold near 40 μg/m3. From 2013 to 2018, 6716 (95% CI: 5336-8678) PTB cases, representing 2.7% (95% CI: 2.2-3.5%) of total PTB, were attributable to PM2.5 exposure exceeding the WHO first-stage interim target (IT1, 35 μg/m3). Our results suggested improved ambient PM2.5 quality may substantially reduce PTB burden in Iran.