Folic Acid Supplementation and the Association between Maternal Airborne Particulate Matter Exposure and Preterm Delivery: A National Birth Cohort Study in China

BACKGROUND: Potential modification of the association between maternal particulate matter (PM) exposure and preterm delivery (PTD) by folic acid (FA) supplementation has not been studied. OBJECTIVE: We examined whether FA supplementation could reduce the risk of PTD associated with maternal exposure to PM in ambient air during pregnancy. METHOD: In a cohort study covering 30 of the 31 provinces of mainland China in 2014, 1,229,556 primiparas of Han ethnicity were followed until labor. We collected information on their FA supplementation and pregnancy outcomes and estimated each participant’s exposure to PM with diameters of [Formula: see text] ([Formula: see text]), [Formula: see text] ([Formula: see text]), and [Formula: see text] ([Formula: see text]) using satellite remote-sensing based models. Cox proportional hazard regression models were used to examine interactions between FA supplementation and PM exposures, after controlling for individual characteristics. RESULTS: Participants who initiated FA [Formula: see text] prior to pregnancy (38.1%) had a 23% [hazard ratio [Formula: see text] (95% CI: 0.76, 0.78)] lower risk of PTD than women who did not use preconception FA. Participants with PM concentrations in the highest quartile had a higher risk of PTD [[Formula: see text] (95% CI: 1.26, 1.32) for [Formula: see text] , 1.52 (95% CI: 1.46, 1.58) for [Formula: see text] , and 1.22 (95% CI: 1.17, 1.27) for [Formula: see text]] than those with exposures in the lowest PM quartiles. Estimated associations with a [Formula: see text] increase in [Formula: see text] and [Formula: see text] were significantly lower among women who initiated FA [Formula: see text] prior to pregnancy [[Formula: see text] (95% CI: 1.08, 1.10) for both exposures] than among women who did not use preconception FA [[Formula: see text] (95% CI: 1.11, 1.13) for both exposures; [Formula: see text]]. The corresponding association was also significantly lower for a [Formula: see text] increase in [Formula: see text] [[Formula: see text] (95% CI: 1.02, 1.03) for FA [Formula: see text] before pregnancy vs. 1.04 (95% CI: 1.03, 1.04) for no preconception FA; [Formula: see text]]. CONCLUSION: Our findings require confirmation in other populations, but they suggest that initiating FA supplementation [Formula: see text] prior to pregnancy may lessen the risk of PTD associated with PM exposure during pregnancy among primiparas of Han ethnicity. https://doi.org/10.1289/EHP6386