Placenta Previa

In placenta previa, the placenta is implanted in the lower uterine segment, where it encroaches on the internal cervical os. This disorder, one of the most common causes of bleeding during the second half of pregnancy, occurs in approximately 1 in 200 pregnancies, more commonly in multigravidas than in primigravidas. Generally, termination of pregnancy is necessary when placenta previa is diagnosed in the presence of heavy maternal bleeding. Maternal prognosis is good if hemorrhage can be controlled; fetal prognosis depends on gestational age and amount of blood lost.

Causes

The cause of placenta previa is unknown, but it is associated with certain conditions including the following:

• women who have scarring of the uterine wall from previous pregnancies
• women who have fibroids or other abnormalities of the uterus
• women who have had previous uterine surgeries or cesarean deliveries
• older mothers (over age 35)
• African-American or other minority race mothers
• cigarette smoking
• placenta previa in a previous pregnancy

Signs and symptoms

Placenta previa usually produces painless third-trimester bleeding (often the first complaint). Various malpresentations occur because of the placenta's location and interfere with proper descent of the fetal head. (The fetus remains active, however, with good heart tones.) Complications of placenta previa include shock or maternal and fetal death.

Types of placenta previa

There are three types of placenta previa, but they all cause the same general signs and symptoms. An ultrasound is necessary to tell which form of the condition you have:

• Total placenta previa. In total placenta previa, the placenta completely covers the cervical opening.
• Partial placenta previa. In partial placenta previa, the placenta partly covers the cervical opening.
• Marginal placenta previa. In marginal placenta previa, the edge of the placenta is at the margin of the cervical opening. This form of placenta previa may not cause much bleeding, and it may be possible for your baby to make its way into the birth canal without any difficulty.

"Low-lying placenta" is another term related to placenta previa. This term is usually used to describe a placenta that lies low in the uterus but isn't quite close enough to the cervical opening to qualify as marginal placenta previa. This condition usually doesn't require treatment during pregnancy, but it may cause bleeding after delivery.

Diagnosis

Special diagnostic measures that confirm placenta previa include:

• transvaginal ultrasound scanning for placental position.
• pelvic examination (under a double setup because of the likelihood of hemorrhage), performed only immediately before delivery to confirm the diagnosis. In most cases, only the cervix is visualized.

Treatment

Treatment of placenta previa is designed to assess, control, and restore blood loss; to deliver a viable infant; and to prevent coagulation disorders. Immediate therapy includes starting an I. V. line using a largebore catheter; drawing blood for hemoglobin and hematocrit as well as type and crossmatching; initiating external electronic fetal monitoring; monitoring maternal blood pressure, pulse rate, and respirations; and assessing the amount of vaginal bleeding.

If the fetus is premature, following determination of the degree of placenta previa and necessary fluid and blood replacement, treatment consists of careful observation to allow the fetus more time to mature. If clinical evaluation confirms total placenta previa, the patient is usually hospitalized because of the increased risk of hemorrhage. As soon as the fetus is sufficiently mature, or in case of intervening severe hemorrhage, immediate delivery by cesarean section may be necessary. Vaginal delivery is considered only when bleeding is minimal and the placenta previa is marginal, or when labor is rapid. Because of the possibility offetal blood loss through the placenta, a pediatric team should be on hand during such delivery to immediately assess and treat neonatal shock, blood loss, and hypoxia.

Complications of placenta previa necessitate appropriate and immediate intervention.

Prevention

This condition is not preventable.