Placenta previa

Definition
Placenta previa is a condition that may occur during pregnancy when the placenta implants in the lower part of the uterus and obstructs the cervical opening to the vagina (birth canal).

Causes, incidence, and risk factors

Possible causes of placenta previa include a scarred endometrium (lining of the uterus), a large placenta, an abnormal uterus, or abnormal formation of the placenta. The incidence of placenta previa is approximately 1 out of 200 births.

The incidence increases with each pregnancy, and it is estimated that women who have had 6 or more previous deliveries may be at risk in as many as 1 in 20 births. The incidence of placenta previa is doubled in multiple pregnancy (more than one baby). Risk factors include multiparity (previous deliveries), multiple pregnancy, previous myomectomy (removal of uterine fibroids through an incision in the uterus), and a previous C-section (if the scar is low and close to the vaginal cervix region).

Symptoms

     
  • Spotting during the first and second trimesters  
  • Sudden, painless, and profuse vaginal bleeding in pregnancy during the third trimester (usually after 28 weeks)  
  • Uterine cramping occuring with onset of bleeding

Note: Bleeding may not occur until after labor starts in some cases. Labor sometimes starts within several days after initial heavy, vaginal bleeding.

Signs and tests

The uterus is usually soft and relaxed. The infant position is oblique ( // ) or transverse ( == ) in about 15% of cases. Fetal distress is not usually present unless a cord accident occurs, or vaginal blood loss has been heavy enough to induce maternal shock or placenta abruptio.

An Abdominal ultrasound performed during the second trimester indicates low positioning of the placenta. Transvaginal or transperineal ultrasound can help physicians determine the position of a low-lying placenta.

Treatment

The course of treatment depends on the amount of abnormal uterine bleeding, whether the fetus is developed enough to survive outside the uterus, the amount of placenta over the cervix, the position of the fetus, the parity (number of previous births) for the mother, and the presence or absence of labor.

Early in pregnancy, transfusions may be given to replace maternal blood loss. Medications may be given to prevent premature labor, prolonging pregnancy to at least 36 weeks. Beyond 36 weeks, the benefits of additional infant maturity have to be weighed against the potential for major hemorrhage.

Cesarean section is the method for delivery. It has proven to be the most important factor in reducing maternal and infant death rates.

Expectations (prognosis)

The maternal prognosis (probable outcome) is excellent when managed appropriately. This is done by hospitalizing those at risk who are exhibiting signs and symptoms, and by performing C-section delivery.

Complications

Maternal complications include major hemorrhage, shock, and death. The risk of infection and formation of Blood clots or thromboembolism also increases, as does the likelihood of the need for a blood transfusion.

Prematurity (infant is less than 36 weeks gestation) is responsible for about 60% of infant deaths secondary to placenta previa. Fetal blood loss or hemorrhage may occur because of the placenta separating from the uterine wall during labor. It may also occur with surgical entry into the uterus during a C-section delivery.

Calling your health care provider

Call your health care provider if vaginal bleeding occurs at any point in the pregnancy. Placenta previa can endanger both the mother and the baby.

Prevention

This condition is not preventable.

Johns Hopkins patient information

Last revised: December 6, 2012
by Dave R. Roger, M.D.

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