ObjectivesAs current studies on the relationships between air pollutants exposure during the first trimester and birth defects were not fully elucidated, this study aimed to assess the association between selected air pollutants and birth defects.DesignAn observational study.ParticipantsWe obtained 70 854 singletons with gestational age <20 weeks who were delivered at a large maternal and child healthcare centre in Wuhan, China.Outcome measuresBirth defects data and daily average concentration of ambient particulate matter ≤10 µm diameter (PM10), PM ≤2.5 µm diameter (PM2.5), sulfur dioxide (SO2) and nitrogen dioxide (NO2) were obtained. Logistic regression analysis was applied to assess the association between maternal air pollutants exposure during first trimester and total birth defects, congenital heart defects (CHDs), limb defects and orofacial clefts with adjustments of potential covariates.ResultsThere were a total of 1352 birth defect cases included in this study, with a prevalence of 19.08‰. Maternal exposed to high concentrations of PM10, PM2.5, NO2and SO2in the first trimester were significantly associated with elevated ORs of birth defects (ORs ranged from 1.13 to 1.23). Additionally, for male fetuses, maternal exposed to high PM2.5concentration was associated with an elevated odd of CHDs (OR 1.27, 95% CI 1.06 to 1.52). In the cold season, the ORs of birth defects were significantly increased among women exposed to PM2.5(OR 1.64, 95% CI 1.41 to 1.91), NO2(OR 1.22, 95% CI 1.08 to 1.38) and SO2(OR 1.26, 95% CI 1.07 to 1.47).ConclusionsThis study showed unfavourable effects of air pollutants exposure during the first trimester on birth defects. Especially, the association between maternal PM2.5exposure and CHDs was only observed among male fetuses, and stronger effects of PM2.5, NO2and SO2exposure on birth defects were observed in the cold season.