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The placent implants directly into the myometrium and lies of the cervical os.

A hysterectomy specimen demonstrates a placenta previa accreta. In an accreta, the placenta implants directly on the myometrium.

In this bivalved specimen, the cervical os is hidden by the overlying placenta.


Placenta previa, as so termed, occurs when the placenta lies previous to the fetus. The incidence during midtrimester is approximately 5%, however, 90% of these placentas 'migrate' upward to a non-previa by term, a phenomenon known as 'dynamic placentation'.1

Placenta previa is now divided into central and partial categories. Central previa occurs when the os is completely covered by the placenta. Partial previa occurs when the placenta is only partially covering the os. Note that low-lying placentas lie very close to the os, but do not actually cover any portion of the os.


Classic presentation is painless bleeding during the third-trimester. Tends to occur in older women; and associated with multiparity, previous uterine curettage, or previous cesarean section.

Previa is often associated with accreta and is called placenta previa accreta. In previa, the placenta implants in the lower uterine segment where the endometrium is not as hormonally responsive and thus does not form decidua.1


Treatment is via cesarean delivery in the unstable bleeding patient. In the nonbleeding, stable patient, vaginal deliveries may be possible in cases of partial previas. With central previas cesarean deliveries are mandatory.


This condition carries a risk for life-threatening hemorrhage in both mother and infant which may mandate a preterm delivery. In addition, the underlying risk of accreta further puts these patients at risk for hemorrhage and an increased risk of hysterectomy.


1 Baergen RN. Manual of Benirschke and Kaufmann's Plathology of the Human Placenta. New York, NY: Springer; 2005: 219-221.

Last updated: 2010-10-26
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