A medical literature that provides biological pathways from maternal stress to adverse birth outcomes, coupled with a growing consensus that birth characteristics are predictive of later life wellbeing, suggest that events that cause psychological trauma during pregnancy may have dire consequences for the next generation. Due to the unexpected nature of the terrorist attacks of September 11th, 2001 a random cohort of in utero children where exogenously insulted by increased maternal psychological distress. The goal of this study is to rigorously examine the casual effect of acute maternal stress on birth outcomes. To explore this question, it is imperative to avoid two identification pitfalls common in natural experiment studies of this topic: non-stress related negative externalities and post-event endogenous fertility selection. With these issues in mind, this analysis excludes those individuals most at risk of health and resource shocks unrelated to stress, New York City and Washington D.C. residents, and does not rely on the endogenously selected post-event birth cohorts. Results suggest that children exposed while in utero were born significantly smaller and earlier than previous cohorts. The timing of the effect provides evidence that intrauterine growth is specifically restricted by first trimester exposure to stress, while gestational age is most reduced by increased maternal psychological distress in mid pregnancy.