A 28-year-old primigravida delivers a healthy 3.2 kg male infant vaginally after an 8-hour labor. The third stage of labor lasts 35 minutes. Immediately after expulsion of the placenta, the uterus is soft and boggy on abdominal examination. Vaginal bleeding is brisk, estimated at 800 mL in the first 10 minutes postpartum. Vital signs: BP 100/62 mmHg, HR 110/min, RR 22/min. Hemoglobin at admission was 11.2 g/dL. What is the most appropriate immediate next step in management?