| Summary: | Air pollution exposure during pregnancy has been associated with impaired fetal growth and postnatal weight gain, but few studies have examined the effect on weight growth trajectories. We examine the association between validated 1 km<sup>2</sup> resolution particulate matter (PM<sub>2.5</sub>) concentrations, averaged over pregnancy, and sex-specific growth trajectories from birth to age six of participants in the Boston-based Children’s HealthWatch cohort (4797 participants, 84,283 measures). We compared weight trajectories, predicted using polynomial splines in mixed models, between prenatal PM<sub>2.5</sub> above or below the median (9.5 µg/m<sup>3</sup>), and examined birth weight as an effect modifier. Females exposed to average prenatal PM<sub>2.5</sub> ≥ 9.5 µg/m<sup>3</sup> had higher weights compared to females exposed to < 9.5 µg/m<sup>3</sup> throughout the study period (0.16 kg at 24 months, 0.61 kg at 60 months). In males, higher prenatal PM<sub>2.5</sub> exposure was associated with significantly lower weights after 24 months of age, with differences increasing with time (−0.17 at 24 months, −0.72 kg at 60 months). Associations were more pronounced among low birth weight (<2500 g) females, but did not differ by birth weight status in males. Our findings demonstrate the complex association between air pollution exposures and childhood weight trajectories and emphasize the importance of sex-stratified analyses.
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