The destruction of the World Trade Center (WTC) on 11 September 2001 was a source of enormous psychological trauma that may have consequences for the health of pregnant women and their fetuses. In this report, we describe the impact of extreme trauma on the birth outcomes of women highly exposed to the WTC. We enrolled 187 women who were pregnant and living or working within close proximity to the WTC on 11 September. Among women with singleton pregnancies, 52 completed at least one psychological assessment prior to delivery. In adjusted multivariable models, both post-traumatic stress symptomatology (PTSS) and moderate depression were associated with longer gestational durations, although only PTSS was associated with decrements in infant head circumference at birth (β = −0.07, SE = 0.03, P = 0.01). The impact of stress resulting from extreme trauma may be different from that which results from ordinary life experiences, particularly with respect to cortisol production. As prenatal PTSS was associated with decrements in head circumference, this may influence subsequent neurocognitive development. Long-term follow-up of infants exposed to extreme trauma in utero is needed to evaluate the persistence of these effects.