BACKGROUND In Mexico, during 2010, obstetric hemorrhage was second (19.6%) as a direct cause of maternal death. The aim of this paper is to evaluate the effect of oxytocin and carbetocin, in preventing postpartum hemorrhage in patients with risk factors for uterine atony. METHODS Study type clinical trial, patients experiencing at least one of the risk factors for uterine atony included. Randomly, they were divided into two groups: one was given Oxytocin and other received Carbetocin. The following variables were determined: risk factors for uterine atony, hemoglobin and hematocrit, vital signs, trans-surgical bleeding, whether or not presented uterine atony, uresis, need for additional tonics uterus and need for blood transfusion. RESULTS A total of 120 patients were studied in 6 months were excluded 3, 60 were treated with Carbetocin, and 57 with Oxytocin. It was determined that there is a greater number of events of uterine atony (p = 0.007, with RR 11.06) and therefore greater need for additional tonic uterus (p = 0.027, with RR 5.44) in the group of Oxytocin. There was no statistically significant difference in the other variables. CONCLUSIONS Carbetocin is recommended as prophylaxis of obstetric hemorrhage in patients with risk factors for uterine atony.