To evaluate whether the administration of prophylactic methylergonovine in addition to oxytocin in patients undergoing intrapartum cesarean birth reduces the need for additional uterotonic agents.This was a single-center, placebo-controlled, randomized trial of patients undergoing intrapartum cesarean birth. Patients were randomly allocated to receive intravenous oxytocin 300 mL/minute plus intramuscular methylergonovine 0.2 mg (1 mL) or intravenous oxytocin 300 mL/minute plus intramuscular normal saline (1 mL). The primary outcome was the receipt of additional uterotonic agents. Secondary outcomes included surgeon assessment of uterine tone, incidence of postpartum hemorrhage, quantitative blood loss, and blood transfusion. To detect a twofold decrease in the need for additional uterotonic agents (assuming a 42% baseline) with a two-sided type 1 error of 5% and power of 80%, a sample size of 76 patients per group was required.From June 2019 through February 2021, 80 patients were randomized to receive methylergonovine plus oxytocin and 80 were randomized to receive to oxytocin alone. Significantly fewer patients who were allocated to the methylergonovine group received additional uterotonic agents (20% vs 55%, relative risk [RR] 0.4, 95% CI 0.2-0.6). Participants receiving methylergonovine were more likely to have satisfactory uterine tone (80% vs 41%, RR 1.9, 95% CI 1.5-2.6), lower incidence of postpartum hemorrhage (35% vs 59%, RR 0.6, 95% CI 0.4-0.9), lower mean quantitative blood loss (967 mL vs 1,315 mL; mean difference 348, 95% CI 124-572), and a lower frequency of blood transfusion (5% vs 23%, RR 0.2, 95% CI 0.1-0.6).The administration of prophylactic methylergonovine in addition to oxytocin in patients undergoing intrapartum cesarean birth reduces the need for additional uterotonic agents.ClinicalTrials.gov, NCT03904446.