Abstract: This was a prospective cohort study including all vaginal births (N=11,323) between October and December 2003 and October and December 2005 from 24 maternity units in two South American countries (Argentina and Uruguay). Blood loss was measured in all births using a calibrated receptacle. Moderate postpartum hemorrhage and severe postpartum hemorrhage were defined as blood loss of at least 500 mL and at least 1,000 mL, respectively.