The placenta is the organ that joins the mother and the baby. The placenta is attached to the uterus, providing oxygen and nutrients from the mother to the fetus. Abnormalities in the placenta, which is rich in blood vessels, can cause excessive maternal bleeding after vaginal delivery. One such abnormality is a retained placenta, in which the placenta is normal but for an unknown reason has not detached from the uterus. When it is retained within the uterus, the uterus will have difficulty contracting which can lead to heavy bleeding. Another set of abnormalities involves a placenta that is incorrectly attached to the uterus. The placenta can be attached too low in the uterus, called placenta previa, in which case the placenta is blocking the cervix and the baby cannot delivery normally without causing excessive bleeding. The placenta can also be attached too deeply into the uterine wall, termed placenta accrete, in which case it can be very difficult to completely remove the placenta after delivery.