Objective  To evaluate the association between maternal exposure to particulate matter ≤ 2.5 μm in aerodynamic diameter (PM_2.5) and the risk of preterm birth, and to provide a reference for the prevention of adverse pregnancy outcomes.

  Methods  We collected data on 91 756 puerperas having their singleton deliveries during 2015 – 2016 at 22 medical institutions covered by a critical maternal surveillance system in 10 cities of Hebei province; the puerperas′ exposures to PM_2.5 were estimated based on average daily PM_2.5 concentration from ambient air quality monitoring of the 10 cities. Multivariate logistic regression model was used to explore the association between maternal PM_2.5 exposure and the risk of preterm birth in different gestational weeks.

  Results  The estimated PM_2.5 exposure during whole and various pregnancy periods were all significantly higher for the puerperas with preterm birth than for those with full-term birth. A 10 μg/m³ increase in PM_2.5 exposure during entire pregnancy, first trimester (< 14 gestation weeks), the third trimester (≥ 28 gestation weeks), the first month of pregnancy, the first month before delivery, and the second month before delivery were associated with a 9.9%, 4.2%, 7.4%, 2.9%, 3.4%, and 3.6% increment in preterm birth risk, respectively. Other risk factors for preterm birth included at an elder age, more pregnancies, pregnancy induced hypertension (PIH), gestational diabetes mellitus (GDM), less antenatal cares, with a higher education, and having a male fetus.

  Conclusion  Maternal exposure to high level of PM_2.5 during pregnancy, especially during the first and the third trimester, can significantly increase preterm birth risk and protective measures against the exposure should be adopted among pregnant women.