Ninety percent of all postpartum hemorrhages are caused by uterine atony-that is, failure of the uterine muscles to contract normally after the baby and placenta are delivered. The blood vessels supplying the placenta during pregnancy are severed when the placenta separates from the wall of the uterus. The bleeding that results from these severed vessels normally stops when the uterus contracts, compressing the vessels. However, if the uterus doesn't contract enough, the bleeding can continue. Significant blood loss can result from a floppy, uncontracted uterus.
Factors that may prevent the muscles of the uterus from contracting include the following:
- prolonged labor;
- the use of oxytocin (Pitocin) during labor;
- general anesthesia;
- twin or multiple births;
- increased amounts of amniotic fluid (polyhydramnios);
- delivery of a large baby;
- history of more than five pregnancies;
- abnormal labor (dystocia); and
- infection (chorioamnionitis).
In addition, fragments of placenta remaining in the uterus after delivery or benign growths within the walls of the uterus (known as fibroids) can also prevent the uterus from contracting normally.
Many practitioners actively manage the third stage of labor, gently pulling the umbilical cord and administering oxytocin to help the uterus contract and promote delivery of the placenta. The uterus can also be massaged to help it contract firmly. Many studies show this technique reduces postpartum hemorrhage and the need for blood transfusions. For this reason, use of oxytocin after delivery, sometimes right after the baby is delivered and sometimes after the placenta has been delivered, is now commonplace. Oxytocin is given intravenously or injected into a muscle.
If heavy bleeding from atony occurs despite the use of oxytocin after delivery, then other drugs such as Prostaglandin F-2-alpha (Hemabate) and methylergonovine (Methergine) may be used to help control hemorrhage. In rare cases, other more drastic procedures may be required. Occasionally, surgery may be required to tie off large blood vessels that supply the uterus or to perform a hysterectomy for life-threatening circumstances. Newer techniques are also becoming available, including radiological procedures called uterine artery embolization. In this procedure, a radiologist injects small particles into the uterine artery to block blood flow to the uterus which may be able to control uterine hemorrhage.