The objectives of this study were to estimate the prevalence of postpartum posttraumatic stress disorder (PTSD) in a maternity hospital for fetal high-risk pregnancies and to identify vulnerable subgroups. This was a cross-sectional study at a fetal high-risk maternity hospital in Rio de Janeiro, Brazil, with a sample of 456 women who had given birth at this hospital. The Trauma History Questionnaire and Post-Traumatic Stress Disorder Checklist were used to screen for lifetime traumatic events and PTSD symptoms, respectively. Overall prevalence of PTSD was 9.4%. Higher PTSD prevalence was associated with three or more births, a newborn with a 1-minute Apgar score of seven or less, history of mental disorder prior to or during the index pregnancy, postpartum depression, physical or psychological intimate partner violence during the pregnancy, a history of unwanted sexual experience, and lifetime exposure to five or more traumas. Rapid diagnosis and treatment of PTSD are essential to improve the mother's quality of life and the infant's health.