Rationale: The influence of particulate air pollution on respiratory health starts in utero. Fetal lung growth and structural development occurs in stages; thus, effects on postnatal respiratory disorders may differ based on timing of exposure.

Objectives: We implemented an innovative method to identify sensitive windows for effects of prenatal exposure to particulate matter with a diameter less than or equal to 2.5 μm (PM_2.5) on children's asthma development in an urban pregnancy cohort.

Methods: Analyses included 736 full-term (≥37 wk) children. Each mother’s daily PM_2.5 exposure was estimated over gestation using a validated satellite-based spatiotemporal resolved model. Using distributed lag models, we examined associations between weekly averaged PM_2.5 levels over pregnancy and physician-diagnosed asthma in children by age 6 years. Effect modification by sex was also examined.

Measurements and Main Results: Most mothers were ethnic minorities (54% Hispanic, 30% black), had 12 or fewer years of education (66%), and did not smoke in pregnancy (80%). In the sample as a whole, distributed lag models adjusting for child age, sex, and maternal factors (education, race and ethnicity, smoking, stress, atopy, prepregnancy obesity) showed that increased PM_2.5 exposure levels at 16–25 weeks gestation were significantly associated with early childhood asthma development. An interaction between PM_2.5 and sex was significant (P = 0.01) with sex-stratified analyses showing that the association exists only for boys.

Conclusions: Higher prenatal PM_2.5 exposure at midgestation was associated with asthma development by age 6 years in boys. Methods to better characterize vulnerable windows may provide insight into underlying mechanisms.