Critical windows and risk thresholds of prenatal mixed air pollutant exposure for oligohydramnios: Evidence from a population‑based study

Background: Oligohydramnios is a clinically relevant but understudied pregnancy complication. This study evaluated the association between maternal exposure to mixed ambient air pollutants and the risk of oligohydramnios, focusing on identifying critical exposure windows and pollutant-specific concentration thresholds. Methods: We conducted a retrospective cohort study of 7,608 singleton live births from a tertiary hospital in northwestern China (2015–2019). Individual-level air pollution exposure was estimated by inverse distance weighting. Weighted quantile sum (WQS) and lagged WQS (lWQS) models were used to assess mixture effects and time-specific susceptibility. Restricted cubic spline models were applied to estimate concentration–response relationships and preventive thresholds of representative weeks and corresponding key pollutants. Results: The WQS index showed a significant joint effect for daily average exposure during whole pregnancy (odds ratio = 1.204, 95% confidene interval 1.049, 1.285), mainly driven by NO 2 and O 3 . The lWQS model identified the early and late pregnancy as critical exposure windows. As representative time points for early, mid, and late pregnancy, estimated O 3 thresholds were 49.28 μg/m 3 (week 4), 36.28 μg/m 3 (week 16), and 37.40 μg/m 3 (week 32); the NO 2 threshold at week 32 was 37.41 μg/m 3 . Conclusion: Maternal exposure to mixed air pollutants, particularly O 3 and NO 2 , increases the risk of oligohydramnios. Findings highlight gestational timing and pollutant-specific targets for prenatal environmental protection.