Background: Exposure to particulate matter, particularly with aerodynamic diameter <2.5 ÎŒm (PM 2.5), may increase inflammation and oxidative stress in pregnant women and affect fetal growth. We examined trimester specific PM 2.5 exposure levels and small for gestational age (SGA) using the statewide birth registry of Ohio from 2007 to 2010. Methods: Exposure to PM 2.5 in each trimester and for each gestational week was determined using data from 57 Environmental Protection Agency network monitoring stations across the state of Ohio. We restricted the data to 224,921 singleton live births, with a gestational age of 20-42 weeks, no genetic disorders or congenital abnormalities, and who had home addresses within a 10 km radius of any PM 2.5 monitoring station. We estimated odds ratios of SGA using Generalized Linear Models (GLMs) and Distributed Lag Models (DLMs), and adjustment for maternal age, race, education, parity, body mass index, insurance type, tobacco use, prenatal care initiation, birth year, season of birth, and sex of the baby. Results: Mean PM 2.5 levels during the entire pregnancy were 13.03 ÎŒg/m 3 with a standard deviation of 1.57 ÎŒg/m 3. Covariates adjusted odds ratios and 95% confidence intervals of a 10 ÎŒg/m 3 increase in PM 2.5 levels with a 10 km buffer radius for SGA and trimesters modeled separately were 0.94 (0.88, 1.00) for the first trimester, 0.93 (0.86, 1.00) for the second trimester, 1.07 (1.00, 1.15) for the third trimester, and 0.92 (0.81, 1.06) for the entire pregnancy. When a 5