Furthermore, to examine the effect of trimester-specific air pollution exposure on the risk of term LBW,we calculated air pollution exposures from 7 to 9 months (first trimester), from 4 to 6 months (second trimester), and from 1 to 3 months (third trimester) prior to the birth in each municipality and assigned the exposures to the participants. We used the sampling stations that measured the corresponding pollutants during the entire 3 months. Then, we repeated the analyses adjusting for both individual and ecological variables.