The purpose of this study was to determine the effect of posttraumatic stress disorder (PTSD), diagnosed prospectively during pregnancy, on the occurrence of low birthweight (<2500 grams) and preterm delivery (<37 weeks gestational age). A cohort of 1362 women was recruited from prenatal care visits and screened for depression, panic disorder, posttraumatic stress disorder, and substance use. Current episodes of PTSD were assessed using the MINI International Neuropsychiatric Interview. Pregnancy outcomes were abstracted from hospital records after delivery, and the data were analyzed using logistic regression. Two hundred sixty two women (33%) were lost to follow-up due to unavailable medical records, leaving 1100 women in the final analyses. Among these 1100 women, 31 (3%) were found to have PTSD during pregnancy. Substance use in pregnancy, panic disorder, major and minor depressive disorders, and prior preterm delivery were significantly associated with PTSD in the sample, while age, language spoken, and race were not. Low birthweight (LBW) was present in 6.5% of sampled women and was not significantly associated with a diagnosis of PTSD in pregnancy when adjusting for potential confounders. However, LBW was significantly associated with minor depressive disorder OR= 1.82 (CI=1.01, 3.29). Preterm delivery occurred in 7.0% of those without and 16.1% of those with PTSD (p=0.055). Because prior preterm delivery data were not available for 33% of women with PTSD, this variable was included only in secondary analyses. However, the association between PTSD and preterm delivery depended on this variable, with OR= 2.82 (0.95, 8.38) before controlling for prior preterm delivery and OR=3.35 (1.04, 10.85) after controlling for prior preterm delivery. These data suggest that a possible association of PTSD and preterm delivery was limited by the low rates of PTSD in this cohort and the inability to control for all confounders. Taken together, these findings provide limited support for the hypothesized association between PTSD and preterm delivery and no support for an association of PTSD with LBW.