The placenta (or "afterbirth") develops early in pregnancy and forms the connection between a mother and her baby. One side of the placenta is attached to the wall of the mother's uterus. The other side is connected to the baby via the umbilical cord. While in the womb, the developing baby receives oxygen and nutrients from the mother's bloodstream through the placenta and umbilical cord. When the baby is delivered, the doctor cuts the umbilical cord and the placenta then peels away from the wall of the uterus and is expelled from the mother's body.

There are several problems that can develop with the placenta, both before and after the baby is born. The placenta may:

  • attach to the uterine wall near the opening of the uterus (placenta previa). If the placenta covers or partially covers the cervix, it may block the baby's passage through the birth canal and may cause serious bleeding during pregnancy;
  • attach too firmly to the uterine wall (placenta accreta);
  • pull away from the uterine wall too early (placental abruption). If the placenta starts to peel away from the uterus before the baby is born or during the delivery, the supply of blood and nutrients can be reduced or cut off; or
  • stay in the uterus too long after delivery (retained placenta).

Because the placenta is rich in blood vessels, these conditions can cause heavy bleeding (hemorrhage) and can be obstetrical emergencies.