While a normal placenta has connections to the inside wall of the uterus during pregnancy, it is usually easily separated and shed after the baby is delivered. But in some women, the placenta actually grows into and attaches itself more strongly to the wall of the uterus. This is called "placenta accreta," and it happens in about 1 out of every 2,000 pregnancies.
When this occurs, parts of the placenta can stay firmly attached to the uterine wall after the baby is born. Then, when the placenta should be delivered, it tears or can't be removed because it stays attached to the inside of the uterus. This usually causes serious bleeding. Surgery may be required to stop the bleeding, and in some cases, a hysterectomy may be necessary.