Objective  To investigate whether there is a correlation between ambient air pollution and the incidence of birth defects.

  Methods  The data on 58 744 live births with birth defects were collected in Liaoning province among 3 198 348 puerperae having last menstrual period between January 1st, 2006 through December 2015. We also collected data on ambient air pollution during the same period from 71 monitoring sites in 14 cities across Liaoning province. The associations of exposure to various air pollutants of the puerperae before and after pregnancy with incidences of different types of birth defects were analyzed.

  Results  Heavier ambient air pollution in winter and spring seasons were observed during the period in the province. The total incidence of birth defects was 18.67/10 000 for all the live births. The top five defects were congenital heart disease, cleft lip and palate, polydactyly, hydrocephalus and external ear malformation, altogether accounting for 51% of all the birth defects observed. For all the live births of the puerperae with their last menstrual periods in winter and spring seasons, the incidence of congenital heart disease was associated with following exposures: concentration of nitrogen dioxide (NO₂) during the second month or whole duration of pregnancy, air pollution index (API) during the third or the fifth month or whole duration of pregnancy, sulfur dioxide (SO₂) during the first month of pregnancy, and particulate matter less than 10 μm in aerodynamic diameter (PM₁₀) during the third month of pregnancy; the incidence of polydactyly was related the exposure to SO₂ during the fourth month of pregnancy; the incidence of external ear malformation was correlated with exposures to SO₂ during the first month or the fourth month of pregnancy.

  Conclusion  Maternal exposure to air pollution during some months of pregnancy is associated with incidences of some birth defects.