Fine Particulate Air Pollution and Birthweight: Differences in Associations Along the Birthweight Distribution

Background: Maternal exposure to fine particulate air pollution (PM_2.5) during pregnancy is associated with lower newborn birthweight, which is a risk factor for chronic disease. Existing studies typically report the average association related with PM_2.5 increase, which does not offer information about potentially varying associations at different points of the birthweight distribution. Methods: We retrieved all birth records in Massachusetts between 2001 and 2013 then restricted our analysis to full-term live singletons (n = 775,768). Using the birthdate, gestational age, and residential address reported at time of birth, we estimated the average maternal PM_2.5 exposure during pregnancy of each birth. PM_2.5 predictions came from a model that incorporates satellite, land use, and meteorologic data. We applied quantile regression to quantify the association between PM_2.5 and birthweight at each decile of birthweight, adjusted for individual and neighborhood covariates. We considered effect modification by indicators of individual and neighborhood socioeconomic status (SES). Results: PM_2.5 was negatively associated with birthweight. An interquartile range increase in PM_2.5 was associated with a 16 g [95% confidence interval (CI) = 13, 19] lower birthweight on average, 19 g (95% CI = 15, 23) lower birthweight at the lowest decile of birthweight, and 14 g (95% CI = 9, 19) lower birthweight at the highest decile. In general, the magnitudes of negative associations were larger at lower deciles. We did not find evidence of effect modification by individual or neighborhood SES. Conclusions: In full-term live births, PM_2.5 and birthweight were negatively associated with more severe associations at lower quantiles of birthweight.