Placenta Previa

In this disorder, the placenta implants in the lower uterine segment, where it encroaches on the internal cervical os. Placenta previa, one of the most common causes of bleeding during the second half of pregnancy, occurs in about 1 in 200 pregnancies, more commonly in multigravidas than in primigravidas.

The placenta may cover all, part, or a fraction of the internal cervical os.

Among patients who develop placenta previa in the second trimester of pregnancy, less than 15% have a persistent previa at term. The elongation of the upper and lower uterine segments causes the placenta to be located higher on the uterine wall.

Generally, termination of pregnancy is necessary when placenta previa is diagnosed in the presence of heavy maternal bleeding. The maternal prognosis is good if hemorrhage can be controlled; the fetal prognosis depends on gestational age and the amount of blood lost.

Causes

Placenta previa occurs when the embryo implants in the lower part of the uterus and then grows to cover the exit. Doctors and researchers don't understand why this happens. They hypothesize that the condition may be related to:

• Scars in the lining of the uterus (endometrium)
• A large placenta, such as in multiple pregnancy
• An abnormally shaped uterus

Signs and Symptoms

Typically, a patient with placenta previa reports the onset of painless, bright red, vaginal bleeding after the 20th week of pregnancy. Such bleeding, beginning before the onset of labor, tends to be episodic; it starts without warning, stops spontaneously, and resumes later.

About 7% of all patients with placenta previa are asymptomatic. In these women, ultrasound examination reveals the disorder incidentally.

Palpation may reveal a soft, nontender uterus. Abdominal examination using Leopold's maneuvers reveals various malpresentations due to interference with the descent of the fetal head caused by the placenta's abnormal location. Minimal descent of the fetal presenting part may indicate placenta previa. The fetus remains active, however, with good heart tones audible on auscultation.

Diagnostic tests

Transvaginal ultrasound scanning is used to determine placental position.

Pelvic examination (under a double setup [preparations for an emergency cesarean] because of the likelihood of hemorrhage) performed immediately before delivery is used to confirm the diagnosis. In most cases, only the cervix is visualized.

Laboratory studies may reveal decreased maternal hemoglobin levels (due to blood loss).

Differential diagnosis excludes genital lacerations, excessive bloody show, abruptio placentae, and cervical lesions.

Treatment

Medical management of placenta previa is intended to assess, control, and restore blood loss; deliver a viable infant; and prevent coagulation disorders.

Immediate therapy includes starting an I. V. infusion using a large-bore catheter; drawing blood for hemoglobin and hematocrit levels and for typing and cross matching; 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 complete placenta previa, the patient is usually hospitalized due to 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 the bleeding is minimal and the placenta previa is marginal or when the labor is rapid.

Because of possible fetal 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.

Prevention

There are no guidelines for preventing placenta previa. However, if you have it, you need to do the following to prevent bleeding:

• Have the condition checked regularly
• Carefully follow any instructions you are given regarding bed rest and what to do if you have bleeding or contractions