Risk and Burden of Preterm Birth Associated with Prenatal Exposure to Ambient PM_2.5: 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 (PM_2.5) 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 PM_2.5 concentrations during pregnancy were estimated using validated spatiotemporal models. Associations between prenatal PM_2.5 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 PM_2.5 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 PM_2.5 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 PM_2.5 exposure exceeding the WHO first-stage interim target (IT1, 35 μg/m³). Our results suggested improved ambient PM_2.5 quality may substantially reduce PTB burden in Iran.