During pregnancy, the body prepares for an expected blood loss of 500 cubic centimeters (cc), about two cups, by doubling its volume of blood. The term postpartum hemorrhage applies when a mother loses more than 500 cc during a vaginal birth and more than 1,000 cc during a cesarean. Despite amazing advances in medical and obstetrical knowledge and technology, excessive blood loss at birth continues to be a significant cause of maternal death.
About 700 to 1,000 cc per minute flows to the uterus at 40 weeks' gestation, so blood loss can be very rapid if steps are not taken to stem the flow. In most cases, the uterus contracts down following the birth to expel the placenta. The site of placental implantation then becomes smaller and bleeding vessels are compressed. However, in some cases the uterus may not contract as well and bleeding continues. This situation is known as uterine atony and can occur when the uterine muscle is fatigued after a very long or very rapid labor, or when labor is induced with oxytocin. Atony also occurs if there are clots in the uterus or if pieces of the placenta remain. Additionally, it occurs when the uterus has been overdistended, which can happen with a twin birth or after the birth of a very large baby. Other causes of postpartum hemorrhage include lacerations of the vagina or cervix and lacerations from the site of an episiotomy.
Following the birth, a gush of blood and the lengthening of the cord signals that the placenta may have fully separated. Oxytocin and prostaglandins, the hormones responsible for the uterine contractions that limit blood loss, are released. Under most circumstances, the placenta is expelled after a few pushes or by gentle traction applied to the cord by the birth attendant during a contraction.