Introduction: There is a growing body of literature that supports an association between prenatal air pollution exposure and increased risk of childhood obesity. However, relationships between prenatal air pollution and growth among preschool age children, a developmental stage that predates most child obesity onset, have not been well studied. Methods: In a pre-birth cohort of 495 children in Mexico City we measured body fat percentage at 7 years of age by bioimpedance. Our cohort was 53% male with mean body fat percentage of 23.3% (SD = 7.6); among females the mean body fat percentage was 26% (SD = 8.4). Average ambient fine particulate matter (PM2.5) exposure was estimated at the home address during pregnancy using a 1km resolution hybrid satellite land use regression model. Findings: We found that body fat percentage at 7 years of age was negatively associated with average PM2.5 exposure during pregnancy using a multivariable regression model (b = -0.074, 95% CI: -0.677, -0.048), when adjusted for child age, socioeconomic status, sex, and maternal pre-pregnancy weight (Adj R2 = 0.095). Average first trimester PM2.5 exposure was similarly negatively associated with body fat percentage at 7 years, PM2.5 during the second or third trimesters, or during the first year postpartum, was not associated with body fat at either timepoint. Conclusion: Although the magnitude of this finding may be small to be clinically relevant; further work including distributed lag modeling and longitudinal models of phenotypic trajectories may reveal specific exposure windows that are important. Our finding may also suggest that lower body fat in the preschool age children who were exposed to higher prenatal PM2.5 reflects an altered growth trajectory with consequences for later obesity in puberty. We will follow the cohort into puberty and continue to examine these associations.
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