An increasing number of studies highlight an inverse association between air pollutant exposure and fetal growth estimated at birth [1, 2], but anthropometric birth data does not allow assessment of fetal growth patterns. Furthermore, birth weight does not clearly reflect fetal growth restriction [3]. The Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Fetal Growth Studies not only have longitudinal growth data on a large number of low-risk singleton, obese and dichorionic twin pregnancies, but also found significant differences in fetal growth among four US ethnic groups in the singleton low-risk group [4]. Both air pollution exposure [5] and hypertensive disorders of pregnancy [6] vary by maternal race/ethnicity in the US. This creates a unique opportunity to investigate potential variations in fetal growth related to environmental exposure by race/ethnicity as well as to explore potential biologic mechanisms for altered growth. Ultrasound measurements have been used to assess the effects of air pollutants on fetal development but rarely with longitudinal data, and prior studies have had 1-3 measures available compared to the six measures during pregnancy in our study. This detailed longitudinal data will allow us to highlight potential periods of sensitivity to environmental contaminants during fetal life. Our overall goal is to study the association of prenatal air pollution exposure on longitudinal fetal growth including estimated fetal weight, fetal body composition and organ volumes using NICHD Fetal Growth Studies ultrasound measurements. We will also investigate clinical maternal blood pressure and uterine artery Doppler results to provide insights into potential biologic mechanisms. To accomplish this, we propose to assemble the residential zip code data from all NICHD Fetal Growth Study cohorts (singleton, twin and obese), geo-code those zip code centroids in collaboration with the National Cancer Institute (NCI) Intramural Research Program and develop air pollution exposure scenarios using existing data and in collaboration with spatial modelers at NCI. Original study consent forms have been reviewed and the NICHD is not permitted to hold personally identifiable information, such as address, but zip codes are permitted. This has been determined to be Non-Human Subjects Research by the NIH Institutional Review Board (see Appendix 1).