<p>Growing evidence suggests that maternal exposure to ambient fine particulate matter (PM) during pregnancy is associated with preterm birth; however, few studies have examined critical windows of exposure, which can help elucidate underlying biologic mechanisms and inform public health messaging for limiting exposure. Participants included 891 mother-newborn pairs enrolled in a U.S.-based pregnancy cohort study. Daily residential PM concentrations at a 1 × 1 km resolution were estimated using a satellite-based hybrid model. Gestational age at birth was abstracted from electronic medical records and preterm birth (PTB) was defined as &lt;37 completed weeks of gestation. We used Critical Window Variable Selection to examine weekly PM exposure in relation to the odds of PTB and examined sex-specific associations using stratified models. The mean ± standard deviation PM level averaged across pregnancy was 8.13 ± 1.10 µg/m. PM exposure was not associated with an increased odds of PTB during any gestational week. In sex-stratified models, we observed a marginal increase in the odds of PTB with exposure occurring during gestational week 16 among female infants only. This study does not provide strong evidence supporting an association between weekly exposure to PM and preterm birth.</p>