A 19-year-old primigravida with known factor XII deficiency (prepregnant factor XII level of 21%) presented with placental abruption and preterm labor at 26 weeks' gestation. A healthy 925-g female infant was born by spontaneous vaginal delivery. The mother had no postpartum hemorrhage or further complications, and the infant demonstrated no intracranial or other forms of hemorrhage up to 70 days of age. The infant's factor XII level was 34% (normal for her age). There are only two previous reports of factor XII deficiency in pregnancy cited in the English literature, and both were uncomplicated. In view of the risk of thromboembolic complications in nonpregnant individuals with factor XII deficiency, pregnant women with a prolonged activated partial thromboplastin time and no lupus anticoagulant or anticardiolipin antibody syndrome should also be investigated for deficiencies of factors VIII, IX, and XII. These patients should be given the appropriate counseling and should be monitored for features of thromboembolism if factor XII deficiency is confirmed.