Placenta accreta is an abnormally firm attachment of the placenta to the uterine wall. There are three variants of this condition, though, all may be called simply accreta:

  • accreta -the placenta attaches to the muscle of the uterine wall. This type of attachment occurs in approximately 75 to 78% of cases of placenta accreta;
  • increta -the placenta extends into the muscles of the uterus. This degree of attachment occurs in approximately 17% of cases; and
  • percreta -the placenta extends through the entire wall of the uterus. This deepest form of attachment occurs in approximately 5 to 7% of cases.

Though it may be possible to detect placenta accreta by vaginal ultrasound, accreta is usually not diagnosed before delivery-mainly because this condition does not cause symptoms until delivery.

Because the placenta is attached so firmly to the uterine wall, it cannot separate as it normally would after delivery. Further, the uterus cannot contract enough to control bleeding after delivery. The severe bleeding that results-and the operations performed to control it-are the main causes of injury and death due to placenta accreta. Blood transfusions are required in over half of the women with placenta accreta and a hysterectomy (removal of the uterus) is almost always necessary.

The most significant risk to the baby is prematurity. Nearly two-thirds of babies born to mothers with placenta accreta are born prematurely, usually between the 34th and 35th week of pregnancy.

The most significant risk factor for placenta accreta seems to be placenta previa. Ten percent of all placenta previa cases are complicated by placenta accreta. Many experts believe that placenta previa is associated with previous cesarean delivery. When a woman becomes pregnant again, the placenta may attach itself over the uterine scars and grow over the cervix.

In the absence of placenta previa, placenta accreta is rare. The reported incidence of placenta accreta ranges from 1 in 540 to 1 in 70,000 deliveries, with an average incidence of approximately 1 in 7,000. The incidence of placenta accreta may be increasing because of the increasing number of women with histories of cesarean sections.

Risk Factors for Placenta Accreta

You are at increased risk for placenta accreta if:

  • you have placenta previa (the placenta covers the cervix);
  • you have a history of cesarean section or other operations on the uterus;
  • your placenta is implanted over a scar in the uterus;
  • you are over age 35;
  • you have been pregnant before; or
  • you have had your uterine lining scraped during dilation and curettage procedure.