Placenta previa is implantation of the placenta over or near the internal os of the cervix. Typically, painless vaginal bleeding with bright red blood occurs after 20 wk gestation. Diagnosis is by transvaginal or abdominal ultrasonography. Treatment is bed rest for minor vaginal bleeding before 36 wk gestation, with cesarean delivery at 36 wk if fetal lung maturity is documented. If bleeding is severe or refractory or if fetal status is nonreassuring, immediate delivery, usually cesarean, is indicated.
Placenta previa may be total (covering the internal os completely), partial (covering part of the os), or marginal (at the edge of the os), or the placenta may be low-lying (within 2 cm of the internal os but not reaching it). Incidence of placenta previa is 1/200 deliveries. If placenta previa occurs during early pregnancy, it usually resolves by 28 wk as the uterus enlarges.
For patients with placenta previa or a low-lying placenta, risks include fetal malpresentation, preterm premature rupture of the membranes, fetal growth restriction, vasa previa, and velamentous insertion of the umbilical cord (in which the placental end of the cord consists of divergent umbilical vessels surrounded only by fetal membranes). In women who have had a prior cesarean delivery, placenta previa increases the risk of placenta accreta (see Placenta Accreta); risk increases significantly as the number of prior cesarean deliveries increases (from about 10% if they have had one cesarean delivery to > 60% if they have had > 4).
Placenta previa is considered in all women with vaginal bleeding after 20 wk. If placenta previa is present, digital pelvic examination may increase bleeding, sometimes causing sudden, massive bleeding; thus, if vaginal bleeding occurs after 20 wk, digital pelvic examination is contraindicated unless placenta previa is first ruled out by ultrasonography.
Although placenta previa is more likely to cause heavy, painless bleeding with bright red blood than abruptio placentae, clinical differentiation is still not possible. Thus, ultrasonography is frequently needed to distinguish the two. Transvaginal ultrasonography is an accurate, safe way to diagnose placenta previa.
In all women with suspected symptomatic placenta previa, fetal heart rate monitoring is indicated. Unless the case is an emergency (requiring immediate delivery), amniotic fluid is tested at 36 wk to assess fetal lung maturity and thus document whether delivery at this time is safe.
For a first (sentinel) episode of vaginal bleeding before 36 wk, management consists of hospitalization, modified rest, and avoidance of sexual intercourse, which can cause bleeding by initiating contractions or causing direct trauma. (Modified rest involves refraining from any activity that increases intra-abdominal pressure for a long period of time—eg, women should stay off their feet most of the day.) If bleeding stops, ambulation and usually hospital discharge are allowed.
Some experts recommend giving corticosteroids to accelerate fetal lung maturity when early delivery may become necessary and gestational age is < 34 wk. Typically for a 2nd bleeding episode, patients are readmitted and kept for observation until delivery.
Delivery is indicated for any of the following:
Delivery is almost always cesarean, but vaginal delivery may be possible for women with a low-lying placenta if the fetal head effectively compresses the placenta and labor is already advanced or if the pregnancy is < 23 wk and rapid delivery is expected.
Placenta previa is more likely to result in heavy, painless bleeding with bright red blood than abruptio placentae, but clinical differentiation is still not possible.
Determine how urgently delivery is needed by monitoring fetal heart rate (to detect fetal distress) and testing amniotic fluid (to assess fetal lung maturity).
For most first bleeding episodes before 36 wk, recommend hospitalization, modified rest, and abstinence from sexual intercourse.
Consider corticosteroids to accelerate fetal lung maturity if delivery may be needed before about 34 wk.
Delivery is indicated when bleeding is severe, the mother or fetus is unstable, or fetal lung maturity is confirmed.