A 19-year-old G1P0 Caucasian woman died 8 days postpartum because of the subinvolution of the uteroplacental arteries. Microscopic examination of the placental implantation site revealed large, dilated spiral arteries containing partially occluding thrombi in the superficial myometrium. The presence of cytotrophoblasts within and surrounding the spiral arteries was confirmed by low molecular weight cytokeratin immunohistochemistry. Infection of the Cesarean section incision site was demonstrated by the growth of Staphylococcus aureus and Streptococcus agalactiae Group B in the wound as well as the intrauterine blood clot. Although subinvolution of the placental site may be a cause of delayed postpartum hemorrhage and significant morbidity, the underlying pathophysiologic mechanism is unknown. (Published Abstract)