Placenta Previa

Essentials of Diagnosis

• Spotting during first and second trimesters.
• Sudden, painless, profuse bleeding in third trimester.
• Initial cramping in 10% of cases.

General Considerations

In placenta previa, the placenta is implanted in the lower uterine segment within the zone of effacement and dilatation of the cervix, constituting an obstruction to descent of the presenting part

Etiology

The incidence of placenta previa is increased by multiparity, advancing age, and previous cesarean delivery. Thus, possible etiologic factors include scarred or poorly vascularized endometrium in the corpus, a large placenta, and abnormal forms of placentation such as succenturiate lobe. The incidence of placenta previa is slightly higher in multiple gestation. A cesarean section scar triples the incidence of placenta previa. Another contributory factor is an increased average surface area of a placenta implanted in the lower uterine segment, possibly because these tissues are less well suited for nidation.

Bleeding in placenta previa may be due to any of the following causes: (1) mechanical separation of the placenta from its implantation site, either during the formation of the lower uterine segment or during effacement and dilatation of the cervix in labor, or as a result of intravaginal manipulation; (2) placentitis; or (3) rupture of poorly supported venous lakes in the decidua basalis that have become engorged with venous blood.

Classification

1. Complete placenta previa: The placenta completely covers the internal cervical os.

2. Marginal placenta previa: The placenta is implanted at the margin of the internal cervical os, within 2 cm. If the placenta is seen to be more than 2 cm from the internal os, the rate of antepartum or intrapartum hemorrhage is not increased.

Diagnosis

Every patient suspected of placenta previa should be hospitalized, and cross-matched blood should be at hand. To avoid provoking hemorrhage, both vaginal and rectal examination should not be performed.

Symptoms and Signs

Painless hemorrhage is the cardinal sign of placenta previa. Although spotting may occur during the first and second trimesters of pregnancy, the first episode of hemorrhage usually begins after the 28th week and is characteristically described as sudden, painless, and profuse. With the initial bleeding episode, clothing or bedding may be soaked by an impressive amount of bright red, clotted blood, but the blood loss usually is not extensive, seldom produces shock, and almost never is fatal. In approximately 10% of cases there is some initial pain, probably because of coexisting placental separation and localized uterine contractions. Spontaneous labor can be expected over the next few days in 25% of patients. In a small minority of cases, bleeding is less dramatic or does not begin until after spontaneous rupture of the membranes or the onset of labor. A few nulliparous patients even reach term without bleeding, possibly because the placenta has been protected by an uneffaced cervix.

The uterus usually is soft, relaxed, and nontender. A high presenting part cannot be pressed into the pelvic inlet. The infant will present in an oblique or transverse lie in approximately 15% of cases. FHR abnormalities are unlikely unless there are complications such as hypovolemic shock, placental separation, or a cord accident.

Ultrasonography

 Bedside transabdominal ultrasonography can definitively identify 95% of placenta previas. Transvaginal or transperineal studies can make the diagnosis in virtually every case. This approach is particularly helpful with the posterior placenta previa.

During the middle of the second trimester, the placenta is observed by ultrasound to cover the internal cervical os in approximately 30% of cases. With development of the lower uterine segment, almost all of these low implantations will be carried to a higher station. An early ultrasonic diagnosis of placenta previa requires the confirmation of an additional study before definitive action is taken.

Differential Diagnosis

Placental causes of bleeding other than placenta previa include partial premature separation of the normally implanted placenta or circumvallate placenta.

Treatment

The treatment depends on the amount of uterine bleeding; the duration of pregnancy and viability of the fetus; the degree of placenta previa; the presentation, position, and station of the fetus; the gravidity and parity of the patient; the status of the cervix; and whether or not labor has begun. The patient must be admitted to the hospital to establish the diagnosis and ideally should remain in the hospital once the diagnosis is made. Blood should be readily available for transfusion.

Expectant Therapy

The initial hemorrhage of placenta previa may occur before pulmonary maturity is established. In such cases, fetal survival can often be enhanced by expectant therapy. Early in pregnancy, transfusions to replace blood loss and the use of tocolytic agents to prevent premature labor are indicated to prolong pregnancy to at least 32–34 weeks. After 34 weeks, the benefits of further maturation must be weighed against the risk of major hemorrhage. The possibility that repeated small hemorrhages may be accompanied by intrauterine growth retardation also must be considered. Approximately 75% of cases of placenta previa are now delivered between 36 and 40 weeks.

In selecting the optimum time for delivery, tests of fetal lung maturation, including assessment of amniotic fluid surfactants and ultrasonic growth measurements, are valuable adjuvants.

If the patient is between 24 and 34 weeks' gestational age, a single course of betamethasone (2 doses of 12 mg intramuscularly separated by 24 hours) or dexamethasone (4 doses of 6 mg intravenously or intramuscularly separated by 12 hours) should be given to promote fetal lung maturity. Repeat courses of steroids are not necessary and usually are considered only for patients who initially present and receive treatment with steroids at less than 24 weeks.

Because of the costs of hospitalization, patients with a presumptive diagnosis of placenta previa are sometimes sent home on strict bed rest after their condition has become stable under ideal, controlled circumstances. Such a policy is always a calculated risk in view of the unpredictability of further hemorrhage, but the practice has been studied and is an acceptable alternative.

Delivery

Cesarean Section

Cesarean section is the delivery method of choice with placenta previa. Cesarean section has proved to be the most important factor in lowering maternal and perinatal mortality rates (more so than blood transfusion or better neonatal care).

If possible, hypovolemic shock should be corrected by administration of intravenous fluids and blood before the operation is started. Not only will the mother be better protected, but an at-risk fetus will recover more quickly in utero than if born while the mother is still in shock.

The choice of anesthesia depends on current and anticipated blood loss. A combination of rapid induction, endotracheal intubation, succinylcholine, and nitrous oxide is a suitable method for proceeding in the presence of active bleeding.

The choice of operative technique is of importance because of the placental location and the development of the lower uterine segment. If the incision passes through the site of placental implantation, there is a strong possibility that the fetus will lose a significant amount of blood—even enough to require subsequent transfusion. With posterior implantation of the placenta, a low-transverse incision may be best if the lower uterine segment is well developed. Otherwise, a classic incision may be required to secure sufficient room and to avoid incision through the placenta. Preparations should be made for care and resuscitation of the infant if it becomes necessary. In addition, the possibility of blood loss should be monitored in the newborn if the placenta has been incised.

In a small percentage of cases, hemostasis in the placental bed is unsatisfactory because of the poor contractility of the lower uterine segment. Mattress sutures or packing may be required in addition to the usual oxytocin, prostaglandins, and methylergonovine. If placenta previa accreta is found, hemostasis may necessitate a total hysterectomy. Puerperal infection and anemia are the most likely postoperative complications.

Vaginal Delivery

Vaginal delivery usually is reserved for patients with a marginal implantation and a cephalic presentation. If vaginal delivery is elected, the membranes should be artificially ruptured prior to any attempt to stimulate labor (oxytocin given before amniotomy likely will cause further bleeding). Tamponade of the presenting part against the placental edge usually reduces bleeding as labor progresses.

Because of the possibility of fetal hypoxemia due to either placental separation or a cord accident (as a result of either prolapse or compression of low insertion of the cord by the descending presenting part), continuous fetal monitoring must be used. If FHR abnormalities develop, a rapid cesarean section should be performed unless vaginal delivery is imminent.

Deliver the patient in the easiest and most expeditious manner as soon as the cervix is fully dilated and the presenting part is on the perineum. For this purpose, a vacuum extractor is particularly valuable because it expedites delivery without risking rupture of the lower uterine segment.

Complications

Maternal

Maternal hemorrhage, shock, and death may follow severe antepartum bleeding resulting from placenta previa. Death may occur as a result of intrapartum and postpartum bleeding, operative trauma, infection, or embolism.

Premature separation of a portion of a placenta previa occurs in virtually every case and causes excessive external bleeding without pain; however, complete or wide separation of the placenta before full dilatation of the cervix is uncommon.

Placenta previa accreta is a serious abnormality in which the sparse endometrium and the myometrium of the lower uterine segment are penetrated by the trophoblast in a manner similar to placenta accreta higher in the uterus. In patients with 1 prior cesarean section, the rate of accreta in the presence of previa is 20–25% and rises to 50% with 2 or more prior cesarean sections.

Fetal

Prematurity (gestational age < 36 weeks) accounts for 60% of perinatal deaths due to placenta previa. The fetus may die as a result of decreased oxygen delivery intrapartum or birth injury. Fetal hemorrhage due to tearing of the placenta occurs with vaginal manipulation and especially upon entry into the uterine cavity as cesarean section is done for placenta previa. About half of these cesarean babies lose some blood. Fetal blood loss is directly proportional to the time that elapses between lacerating the cotyledon and clamping the cord.

Prognosis

Maternal

With rapid recourse to cesarean section, use of banked blood, and expertly administered anesthesia, the overall maternal mortality has fallen to less than 1 in 1000.

Fetal

The perinatal mortality rate associated with placenta previa has declined to approximately 1%. Although premature labor, placental separation, cord accidents, and uncontrollable hemorrhage cannot be avoided, the mortality rate can be greatly reduced if ideal obstetric and newborn care is given.