DescriptionSurgical Cesarean Section with CC

N ormally, the placenta implants in the body (upper portion) of the uterus. Implantation allows for delivery of the infant before the delivery of the placenta. With placenta previa, the placenta is implanted in the lower uterine segment over or near the internal os of the cervix. As the uterus contracts and the cervix begins to efface and dilate, the villi of the placenta begin to tear away from the uterine wall and bright red, painless, vaginal bleeding occurs. The bleeding is facilitated by the poor ability of the myometrial fibers of the lower uterine segment to contract and constrict the torn vessels. Bleeding can occur antepartally or intrapartally. Hemorrhage from the placental site may continue into the postpartum period because the lower uterine segment contracts poorly, contrasted with the fundus and body of the uterus. Placenta previa is classified in four ways, depending on the degree of placental encroachment on the cervical os (Box 1). The degree of the previa depends largely on the cervical dilation. For example, a marginal previa at 2 cm may become a partial previa at 8 cm because the dilating cervix uncovers the placenta. Sometimes, a placenta may correct itself, especially if it low-lying; as the uterus enlarges, the placenta moves cephalad. Depending on the amount of blood loss and gestational age of the fetus, placenta previa may be life-threatening to both the mother and the fetus.

• BOX 1 Classification of Placenta Previa

Low-lying

Partial

The placenta implants in the lower uterine

The placenta partially covers the cervical os;

segment but does not reach the cervical os;

as the pregnancy progresses and the cervix

often this type of placenta previa moves

begins to efface and dilate, the bleeding

upward as the pregnancy progresses, elimi-

occurs.

nating bleeding complications later.

Total

Marginal

The placenta covers the entire cervical os;

The edge of the placenta is at the edge of the

usually requires an emergency cesarean

internal os; the mother may be able to

section.

deliver vaginally.

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