When the placenta implants such that the placental tissue is located adjacent to or overlying the internal cervical os, it is called placenta previa. Placenta previa is the leading cause of third-trimester bleeding, complicating 4 in 1000 pregnancies over 20 weeks. The incidence is higher in early pregnancy prior to the development of the lower uterine segment, and most of these previas resolve as the pregnancy progresses.
There are 3 types of placenta previa (Fig. 18–1):
When a placenta implants in the lower part of the uterus, the pregnancy is at risk for placenta previa. There are several risk factors, including multiparity, increasing maternal age, history of prior caesarean section or uterine surgery, and multiple gestation.
Multiparas are at higher risk for placenta previa compared to nulliparas. The incidence of previa in nulliparas is 0.2%, whereas grand multiparas have an incidence of 5%. The theory behind this phenomenon is that once a placenta has implanted into a certain part of the uterine wall, it has permanently altered its constitution, making implantation at a different site more likely in subsequent pregnancies. Increasing maternal age has been a risk factor, and the cause of this association is unclear. The increased risk may be due to higher parity in older mothers, but it may also be an independent risk factor.
The risk of placenta previa increases directly with the number of uterine surgeries a patient has had in the past. This is mostly seen with increasing numbers of caesarean sections. The risk of placenta previa in second pregnancies after a first pregnancy delivered by caesarean section is 1–4%. The risk increases to nearly 10% in patients with 4 or more previous caesarean deliveries. Furthermore, it has been suggested that previas identified in the second trimester in patients with a previous caesarean delivery have a lower likelihood of resolving as the pregnancy progresses. Risk increases with previous curettage for spontaneous or induced abortion, thought to be due to a scarred active segment of the uterus.
Other risk factors for placenta previa include multiple gestation and smoking. This is due to the greater surface area of the placenta in these situations.
Placenta previa can be associated with several other conditions, including malpresentation, preterm premature rupture of membranes, and intrauterine growth restriction. There may also be an increased risk of congenital anomalies; however, there is no association with any specific anomaly.
Patients with placenta previa are at higher risk for developing placenta accreta, increta, or percreta.
- Placenta accreta: There is no decidua basalis, and the fibrinoid layer is incompletely developed.
- Placenta increta: The placenta invades the myometrium.
- Placenta percreta: The placenta penetrates the myometrium and may invade nearby viscera.
Previous uterine surgery is the risk factor most associated with placenta accreta. Patients with no prior uterine surgery and placenta previa will have accreta 4% of the time. Patients with 1 prior uterine surgery and placenta previa will have accreta 10–35% of the time. Multiple prior caesarean deliveries and placenta previa incurs a 60–65% risk of accreta. Two-thirds of patients with placenta accreta will require a caesarean-hysterectomy.
Placenta previa typically presents with painless vaginal bleeding, usually in the third trimester. Lack of pain with the presence of bleeding is what distinguishes placenta previa from placental abruption. The bleeding occurs in conjunction with the development of the lower uterine segment. As the myometrium becomes thinner, the placenta–decidua interface is disrupted, causing bleeding. The thinness of the lower uterine segment prevents it from contracting to minimize the bleeding from the uterine surface of the implantation site; however, sometimes the bleeding itself can irritate the myometrium and precipitate contractions.
The clinician should have a high index of suspicion for placenta previa in all patients who present with bleeding after 24 weeks. One-third of patients with placenta previa will present with bleeding before 30 weeks, one-third will present between 30 and 36 weeks, and one-third will present after 36 weeks. Ten percent of all women with previa will reach full term without an episode of bleeding. On average, a patient's first episode of bleeding will occur at 34 weeks, with delivery at 36 weeks. Risk of perinatal mortality and morbidity decreases linearly as gestational age increases.
Baseline admission labs including blood type and Rh status, hemoglobin, hematocrit, and platelet count should be sent. Coagulation studies and fibrinogen concentration are not as important in patients with previa as in patients with abruption; however, if there is any doubt of the diagnosis, these should also be sent. A Kleihauer-Betke test should be sent for all women who are Rh negative.
Prior to the advent of routine second-trimester ultrasound, patients with placenta previa were diagnosed at the onset of bleeding. Currently, most cases are diagnosed by ultrasound in the second trimester, although most of these will resolve. Five to fifteen percent of all patients will have placenta previa at 17 weeks. Ninety percent of these will resolve by 37 weeks. This occurs because as the lower uterine segment develops, more distance is created between the placenta and the cervix. Complete previa and marginal or partial previa diagnosed in the second trimester will persist in 26% and 2.5% of patients, respectively. All patients who have placenta previa diagnosed before 24 weeks should have a sonogram between 28 and 32 weeks to reassess the position of the placenta.
The initial management of a patient with a bleeding placenta previa is very similar to the initial management of a patient with placental abruption. Hemodynamic status of the mother should be immediately evaluated and stabilization performed if necessary. Large-bore intravenous lines should be placed, and the fetal heart rate should be monitored continuously.
Hemodynamic stabilization should be performed immediately. Crystalloid infusion should be started in order to rapidly correct a volume deficit, and packed red blood cells should be given if severe anemia is evident or if there is continued uterine bleeding. The goal hematocrit is at least 30% if the patient is bleeding. If no transfusion is required immediately, 4 units of packed red blood cells should be crossed and held nearby. The urine output should be maintained above 30 mL/h.
Subsequent management depends on gestational age, stability of the mother and fetus, the amount of bleeding, and presentation of the fetus. Delivery is always indicated if there is a nonreassuring fetal heart rate pattern despite resuscitation efforts, including maternal supplemental oxygen, left-side positioning, or intravascular volume replacement; if there is life-threatening maternal hemorrhage; or if the gestational age is >34 weeks and there is known fetal lung maturity. If the fetus is ≥37 weeks of gestational age and there is persistent bleeding or persistent uterine activity, delivery is also indicated. Digital cervical exams should be avoided.
Between 24 and 36 weeks, if maternal and fetal stability and well-being are assured, conservative expectant management may be indicated. About 75% of patients with symptomatic placenta previa are candidates for conservative management, and 50% of these patients can prolong their pregnancy by at least 4 weeks. Thirty percent of patients treated this way will progress to term without bleeding again. Seventy percent will have at least 1 more episode of bleeding, and 10% of these patients will have a third episode.
Conservative management, after stabilization has occurred and little or no uterine bleeding is noted, consists of several steps. Hydration and blood transfusion are given if necessary. Continuous fetal heart rate monitoring is required in cases where there is continued uterine bleeding, contractile activity, or intrauterine growth restriction. Tocolytic agents, if there is no suspicion for placental abruption, may be given if membranes have not been ruptured and there is contractile uterine activity. The patient should be restricted to bed rest with bathroom privileges. She should be given stool softeners, iron supplementation, and vitamin C. Steroids to promote fetal lung maturity should be administered if the gestational age is <34 weeks.
After steroids have been administered, if there is little or no uterine activity or bleeding, the patient may be a candidate for home therapy. To be considered for home therapy, the patient must be very reliable, have 24-hour contact via telephone, and have the ability to return quickly to the hospital at any time. She should remain on bed rest with bathroom privileges and should continue to take stool softeners and vitamin therapy. Strict instructions should be given regarding returning to the hospital if she experiences contractions or another episode of bleeding.
Fetal growth, amniotic fluid index, and placental location should be assessed by ultrasound every 3 weeks. Most experts agree that for the patient with complete placenta previa that is otherwise uncomplicated, delivery is recommended at 36–37 weeks.
If uterine bleeding is not excessive, patients with marginal previa may be delivered vaginally. Upon descent, the fetal head should tamponade bleeding. Abdominal delivery is indicated in most cases of placenta previa and in all cases of complete previa. If a caesarean section is performed, care should be taken not to disrupt the placenta upon fetal delivery. If possible, a uterine incision away from the placental bed should be used. For example, if there is an anterolateral placenta, a vertical incision in the lower uterine segment opposite the site of placental implantation should be used. A high transverse incision may be necessary for a low anterior placenta. In all cases, the operating room should be prepared for the possibility of the necessity to perform a hysterectomy.
As noted earlier, 66% of patients with placenta accreta will require a caesarean-hysterectomy. There are several surgical options if uterine preservation is important. The placenta can be removed and the uterine defect can be oversewn to abate the active focus of bleeding. The area of accreta can be resected and the uterus repaired. The last option includes leaving the placenta in situ. This is only acceptable in patients who are not actively bleeding. The cord should be ligated and cut close to its base. The patient should be treated with antibiotics and possibly methotrexate postpartum. In the rare case where bladder invasion is evident, the placenta should not be removed. These patients will likely require hysterectomy and partial cystectomy. (See Chapter 21.)
Neonatal outcomes have much improved in the last 20 years, secondary to conservative management, the liberal use of caesarean section, improved neonatal care, and earlier diagnosis. The perinatal mortality rate has fallen from 60% to 10% over the past several decades due to the ability to resuscitate and support infants who are increasingly more premature. Most mortality is due to prematurity. An earlier of episode of bleeding brings with it a higher risk of prematurity and thus a higher risk of mortality. The maternal mortality rate has decreased from 25% to <1% in patients with access to health care. Maternal mortality remains high in developing countries.
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