The most common cause of postpartum hemorrhage is:

Uterine atony accounts for nearly 70–80% of postpartum hemorrhage cases. After delivery, the uterus must contract to compress the maternal blood vessels at the placental site. Failure of adequate contraction leads to excessive bleeding. Risk factors include prolonged labor, overdistended uterus, multiple pregnancy, and polyhydramnios. Management includes uterine massage and uterotonic drugs such as oxytocin.