Placenta accreta

A 33-year-old woman, gravida 3 para 2, is admitted to the labor and delivery unit with active contractions every 3-5 minutes. Her previous births were via cesarean sections. One hour later, she gives birth to a healthy child. Thirty minutes after delivery, the placenta has still not been delivered. Manual extraction of the retained placenta is performed and leads to profuse bleeding. The patient’s temperature is 37.0°C (98.6°F), pulse is 120/min, and blood pressure is 85/60 mmHg. Oxygen saturation is 95% on room air. On physical examination, the uterus is firm and nontender. Hemoglobin is 9 g/dL and platelet count is 220,000/mm3. Which of the following is the most likely underlying mechanism for this patient’s bleeding?