The placenta attaches to the wall of the uterus and allows oxygen and nutrients from the mother's blood to reach the baby. This permits the baby to grow and develop during pregnancy.

After the baby is delivered, the placenta normally separates from the wall of the uterus and is itself delivered. (This is why it is sometimes called the afterbirth.)

Rarely a placenta may grow into the wall of the uterus and not detach after the baby's delivery. Such conditions are called, as a group, placenta accreta.

This diagnosis is often made after the uterus is removed and examined. It may then be given a more distinct label:

• Placenta accreta: the placenta is attached to just the surface of the uterus
• Placenta increta: the placenta can be seen growing into the uterine muscle
• Placenta percreta: the placenta grows entirely through the uterus, perhaps even invading surrounding organs such as bowel, bladder or blood vessels

Sometimes an ultrasound before delivery can suggest various degrees of placenta accreta, but ultrasound and even MRI images are not perfect. Sometimes things look concerning, but the placenta separates easily. Other times things appear fine, but the placenta does not separate.

Depending on how concerned your obstetrician is, he or she will talk with you about what preparations should be made in advance of delivery.