Exposure to fine particulate matter during pregnancy and risk of term low birth weight in Jinan, China, 2014-2016

Background: Existing studies exploring the association between low birth weight (LBW) and maternal fine particulate matter (aerodynamic diameter < 2.5 mu m, PM2.5) exposure have presented equivocal results, and one of the possible reasons for this finding might be due to relatively low maternal exposures. In addition, relatively narrow maternal exposure windows to PM2.5 have not been well established for LBW. Methods: We employed a nested matched case-control design among 43,855 term births in a large maternity and child care hospital in Jinan, China. A total of 369 cases were identified, and four controls per case matched by maternal age were randomly selected among those with normal birth weight (n = 1,476) from 2014 to 2016. Ambient air monitoring data on continuous measures of PM2.5, nitrogen dioxide (NO2), and sulfur dioxide (SO2) (24-h average concentrations) from 2013 to 2016 were collected from thirteen local monitoring stations. An inverse distance weighting method based on both home and work addresses was adopted to estimate the individual daily exposures to these air pollutants during pregnancy by weighting the average of the twelve nearest monitoring stations within 30 km of each 100 m x 100 m grid cell by an inverse squared distance, and then the average exposure concentrations for gestational months, trimesters and the entire pregnancy were calculated. Adjusted conditional logistic regression models were used to estimate the odds ratios (ORs) per 10 mu g/m(3) increment in PM2,5 and by PM2.5 quartiles during different gestational periods. Results: In this study, the estimated mean values of PM2.5, NO2, and SO2 exposure during the entire pregnancy were 88.0, 54.6, and 63.1 mu g/m(3), respectively. Term low birth weight (TLBW) increased in association with per 10 mu g/m(3) increment in PM2.5 for the 8th month [OR = 1.13, 95% confidence interval (CI): 1.04, 1.22], the 9th month (OR = 1.06, 95% CI: 0.99, 1.15), the third trimester (OR = 1.17, 95% CI: 1.05, 1.29), and the entire pregnancy (OR = 1.38, 95% CI: 1.07, 1.77) in models adjusted for one pollutant (PM2.5). In models categorizing the PM2.5 exposure by quartiles, comparing the second, third, and highest with the lowest PM2.5 exposure quartile, the PM2.5 was positively associated with TLBW during the 8th month (OR: 1.77, 95% CI: 1.09, 2.88; OR: 1.77, 95% CI: 1.03, 3.04; OR: 1.92, 95% CI: 1.04, 3.55, respectively) and for the 9th month, only association for exposure in the third versus the lowest quartile was significant (OR: 1.91, 95% CI: 1.02, 3.58). Conclusions: The study provides evidence that exposure to PM2.5 during pregnancy might be associated with the risk of TLBW in the context of very high pollution level of PM2.5, and the 8th and 9th months were identified as potentially relevant exposure windows.