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Background: Evidence has linked pre and postnatal air pollution exposure with children’s weight; however, studies assessing the effects on children’s growth trajectories are lacking and not consistent. We investigated the role of time-varying postnatal 90 days average spatio-temporal fine particulate matter (PM_2.5) as an effect modifier on sex-specific growth trajectories in early childhood and the magnitude of the impact at different ages. 
Methods: We included children recruited to the Boston-based Children’s HealthWatch cohort from 2009 through 2014 (0 to 6 years old). Weight values were obtained from electronic health records at each hospital visit, (number of boys=1859, girls=1601). We applied generalized additive mixed models adjusting for individual and maternal confounders. We examined effect modification by 1) comparing weight trajectories with factor-smooth interaction between age and PM_2.5 quartiles, and 2) using varying-coefficient models allowing for smooth non-linear interaction between age and PM_2.5. Additionally, we stratified by birthweight status.
Results: We found no differences in growth trajectories by different levels of PM_2.5.  Using varying-coefficient models, we found that PM_2.5 characterized as a continuous exposure significantly modified the association between growth trajectories and weight in boys, with a positive association between PM_2.5 and weight in children less than 3 years and a negative association afterwards. In boys, for each 10 μg/m³ increase in PM_2.5 we found a 2.6% increase (95% CI: 0.8, 4.6) in weight at 1 year of age and a -0.6% (95% CI: -3.9, 2.9) at 5 years. We found similar but smaller changes in girls. Stratification by birthweight status suggests that most of the effects were in low birthweight children.
Conclusions: The study shows that in young children the non-linear effect of continuous postnatal PM_2.5 modify weight trajectories. Birthweight status is an effect modifier of the air pollution-weight relationship.