The incidence of placenta previa at term is approximately 1 in 200. The three categories of the disorder 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 cervix or is within reach of the examining finger introduced through the cervix.
Complete placenta previa: Placenta completely covers internal cervical os
Placenta previa is much more common in early pregnancy than at term. During routine second-trimester ultrasound, the placenta is observed to cover the cervix in 5 to 20% of pregnancies. However, because of the growth of the uterus throughout pregnancy, more than 90% of early placenta previas convert to a normal location by the time of delivery. Conversion to normal location is less common in centrally-located complete placenta previa.
Placenta previa classically is characterized by painless vaginal bleeding in the late second or third trimester. However, uterine pain and/or contractions do not exclude 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. Abnormal fetal presentation is observed in up to 30% of cases. Placental separation and bleeding may cause the newborn to be anemic. Hemorrhage and complications of cesarean delivery are the most common causes of maternal morbidity. Blood transfusions are necessary is one-third to one-half of cases.
In addition, 9 to 10% of cases of placenta previa are associated with placenta accreta, an abnormally firm attachment of the placenta to the wall of the uterus. Placenta accreta prevents the placenta from separating from the uterine wall at the time of delivery and can cause severe bleeding that often necessitates hysterectomy. Placenta accreta is particularly common in women with placenta previa and one or more previous cesarean sections and may complicate one-third to one-half of all such cases. More than 50% of patients with placenta accreta require blood transfusion.
Many cases of placenta previa are diagnosed by routine ultrasound. In other cases, the initial diagnosis is made when the patient comes to the hospital with vaginal bleeding during the second half of pregnancy. Ultrasound may confirm the suspicion of placenta previa. When adequate visualization of the relationship between the placenta and the cervix is not possible with abdominal ultrasound, a transvaginal ultrasound may be helpful. Careful transvaginal sonography does not appear to increase the risk of bleeding in placenta previa.
Placenta previa diagnosed by routine second-trimester ultrasound is managed expectantly. The likelihood of spontaneous resolution is greater than 90%. Strenuous activity may provoke bleeding and should be avoided. Placental location should be reevaluated at 28 to 30 weeks. If placenta previa is still present, the same precautions should be followed. If placenta previa persists beyond 32 to 34 weeks, resolution by term is uncommon. Cesarean section usually is 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 patients who are not experiencing bleeding, amniocentesis may be performed at 34 to 36 weeks to assess fetal lung maturity. If the baby's lungs are mature, delivery usually is indicated. Otherwise, management is individualized based on the condition of the mother and the baby. Waiting beyond 37 weeks is not likely to benefit for the fetus or mother.
In the case of bleeding placenta previa, the mother's interest is best served by immediate delivery. However, the decision also must take into account the interests of the fetus. In all cases of active hemorrhage, the primary consideration is to ensure the mother is stable and not in jeopardy. Large IV lines usually are established, fluids are administered and blood availability is confirmed. Blood transfusion often is necessary when active bleeding is present.
The condition of the baby usually is assessed with continuous electronic fetal heart rate (FHR) monitoring, and ultrasound may be ordered to estimate the gestational age and fetal weight. Medications such as magnesium sulfate, terbutaline, ritodrine, nifedipine or indomethacin may be used to stop uterine contractions.
If any of the following are present, immediate cesarean section usually is necessary:
- Deteriorating condition of the mother
- Persistent heavy bleeding
- Gestational age > 36 weeks
- Estimated fetal weight > 2500 gm
- Fetal distress in a viable fetus
- Contractions that do not respond to medication
Placenta accreta is a potential complication in all women with placenta previa, particularly those with previous cesarean sections. Other risk factors for placenta accreta include maternal age of 35 or more, multiple previous pregnancies, previous uterine surgery and previous D&C. It may be possible to diagnose placenta accreta with ultrasound.
If the initial episode of bleeding resolves, the mother and baby remain stable, and the fetus is premature, it is reasonable to delay delivery. The goal of this approach is to improve newborn outcome by allowing additional time for the baby to develop inside the uterus. Bed-rest usually is prescribed, steroids are given to hasten the development of the baby's lungs if needed. In women a negative blood type, an injection of Rh immune globulin or RhoGam is administered.
In patients who remain stable for a period of days after an initial episode of bleeding, the need for continued hospitalization is controversial. In selected patients, outpatient management in reasonable following the first episode of bleeding. If bleeding recurs, prolonged hospitalization may be necessary.
Cesarean section is the recommended method of delivery in nearly all cases of placenta previa. When possible, the procedure should be performed electively. Preparations should be made prior to delivery to ensure adequate venous access and ready availability of blood and necessary medications. If placenta accreta is anticipated, hysterectomy may be necessary and this should be discussed in advance. The management of placenta accreta encountered at cesarean section is discussed later in this chapter. Rarely, in the case of a low-lying or marginal placenta previa the descending fetal head may "tamponade" the bleeding placental edge and permit vaginal delivery. In the past, this possibility was assessed using a "double set-up" examination in which the patient was taken to the operating room and prepped for cesarean section. A careful examination then was undertaken to determine whether placental tissue could be seen or felt near the cervix, and the method of delivery was determined by the findings. Today, the "double set-up" examination largely has been replaced by ultrasound evaluation of placental location.
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