Covariates
Child’s sex was obtained from delivery records. Mother’s age at delivery was calculated by subtracting child’s birthdate from mother’s birthdate collected at enrollment. Caregiver’s report of ever having asthma was collected through questionnaires at enrollment (second or third trimester). Maternal education was categorized into three levels: less than high school, high school, or greater than high school. Breastfeeding was assessed at 1 month postpartum and categorizes as, never/ no attempt to breastfeed and attempted to breastfeed up to 1 month. Socioeconomic status (SES) was assessed utilizing 13 variables derived from prenatal questionnaire results which were used to classify study participant families into six levels based on the SES index created by the Asociación Mexicana de Agencias de Investigación de Mercados y Opinión Pública (AMAI)22. These levels were then collapsed into lower, medium, and higher socioeconomic status. Exposure to particulate matter <2.5 microns in diameter (PM2.5) was estimated at each participant’s residenceduring pregnancy and over the first two postnatal years using a novel spatio- temporal model incorporating Moderate Resolution Imaging Spectroradiometer (MODIS) satellite- derived Aerosol Optical Depth (AOD) measurements at a 1x1km spatial resolution23. These remote sensing data are calibrated with municipal ground level monitors of PM2.5, land use and meteorological data to yield estimates of daily residential PM2.5 levels for each study participant23. Daily particulate matter during the first year of life was averaged for the entire gestational period and over the first and second year postpartum13.