Placenta previa

Placenta previa is a condition in which the placenta implants in the lower portion of the uterus and covers all or part of the internal cervical os. The incidence at term is approximately 1 in 200 births. Although the etiology remains unclear, the risk factors listed in Box 15.1 implicate previous decidual damage and/or large placental surface area. Three categories are defined below. An additional form of abnormal placentation is the low-lying placenta, in which the placental edge extends to within 2 cm of the internal cervical os.

• Complete placenta previa: placenta completely covers internal cervical os
  
• Partial placenta previa: placenta partially covers internal cervical os
  
• Marginal placenta previa: placenta extends to the margin of the internal cervical os

During routine second-trimester ultrasound, the placenta is observed to cover the cervical os in 5–20% of pregnancies. However, differential growth of the uterus and placenta throughout gestation results in realignment of the placenta with respect to the internal cervical os. By term, more than 90% of early placenta previas convert to a normal location. Conversion to normal location is less common in centrally located complete placenta previa.

Clinical presentation

Placenta previa is characterized by painless vaginal bleeding in the late second or third trimester. However, uterine pain and/or contractions do not preclude the diagnosis in a woman who presents with vaginal bleeding. In many cases, placenta previa remains asymptomatic throughout pregnancy.

Historically, placenta previa has been associated with increased maternal and perinatal morbidity and mortality. Preterm delivery and complications of prematurity are the most common sources of perinatal morbidity, occurring in nearly two-thirds of cases. Among 590 women with placenta previa, Miller and colleagues reported an average gestational age at delivery of 34.9 weeks. Delivery occurred before 37 weeks in 63% and before 34 weeks in 32%. Increased maternal morbidity and mortality are attributable primarily to hemorrhage and complications of cesarean delivery. Blood product replacement is necessary is one-third to one-half of cases. Approximately 10% of cases of placenta previa are associated with placenta accreta, an abnormally firm adherence of the placenta to the uterine wall. Placenta accreta is discussed in detail in Chapter 16.

Diagnosis

Placenta previa is most often diagnosed by routine sonography. In other cases, the initial diagnosis is made at the time of presentation for vaginal bleeding during the second half of pregnancy. In such cases, sonographic confirmation of placental location is recommended prior to digital cervical examination. Transabdominal ultrasound may confirm the suspicion of placenta previa. When adequate visualization of the relationship between the placenta and the internal cervical os is not possible with transabdominal ultrasound, the transperineal or transvaginal approach may be beneficial. Careful transvaginal sonography does not appear to increase the risk of hemorrhage in placenta previa.

Management of placenta previa without hemorrhage

Placenta previa diagnosed by routine second-trimester sonography is managed expectantly. The patient can be reassured that the likelihood of spontaneous resolution is greater than 90%. It is reasonable to recommend avoidance of strenuous activity, but further limitations probably are not necessary early in pregnancy. Placental location should be re-evaluated at 28–30 weeks. If placenta previa persists, the patient should be cautioned that rigorous activity and/or intercourse might provoke bleeding. If complete placenta previa persists beyond 32–34 weeks, resolution by term is unlikely. Cesarean delivery should be scheduled at a gestational age that will maximize the likelihood of fetal maturity and minimize the risk of hemorrhage that may result from the normal onset of uterine contractions. In the asymptomatic patient, amniocentesis should be considered at 34–36 weeks to assess fetal pulmonary maturity. If the test result is consistent with pulmonary maturity, delivery is indicated. If the test suggests pulmonary immaturity, decisions regarding corticosteroid administration and delivery timing must be individualized, taking into account such factors as obstetric history, gestational age, L/S ratio, phosphatidylglycerol level, fetal status, amniotic fluid volume and uterine activity. Beyond 37 weeks, expectant management should not be expected to yield a substantial benefit for the fetus or mother.

Management of placenta previa with hemorrhage

The management of placenta previa complicated by acute hemorrhage is directed at optimizing the outcomes of the mother and the fetus. In many cases, bleeding resolves spontaneously and the patient may be managed expectantly. In other cases, severe hemorrhage may require intervention. Detailed management of hemorrhage is discussed below.

Expectant management: inpatient versus outpatient

If the initial episode of bleeding resolves, the mother and fetus remain stable, and the fetus is premature, a period of expectant management may be appropriate. Bedrest usually is prescribed, antenatal corticosteroids are administered to accelerate fetal maturation, Rh immune globulin is given if indicated, and blood product availability is confirmed. In women who remain stable for a period of days after an initial episode of bleeding, the benefit of continued hospitalization is controversial. Wing and colleagues randomized 53 such women to receive either inpatient or outpatient expectant management after ≥72 hours of observation. There were no differences between the groups with respect to gestational age at delivery, birthweight, transfusion requirements, neonatal morbidity or mortality. Although outcomes were similar with inpatient and outpatient management, 62.3% of patients had recurrent episodes of bleeding. Among these, more than three-quarters required expeditious delivery. The authors concluded that outpatient management of placenta previa following an initial episode of bleeding appears to be an acceptable approach in stable, carefully selected patients.

Delivery

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