Placenta previa refers to placental tissue that covers any portion of the internal cervical os. A placenta is termed low lying when the placental edge does not cover the internal os but is within 2 cm of it.
Incidence of placenta previa is 1/250 deliveries. If placenta previa occurs during early pregnancy, it usually resolves by 28 weeks as the uterus enlarges.
Risk factors for placenta previa include the following:
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; risk increases significantly as the number of prior cesarean deliveries increases (from about 6 to 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 weeks. If placenta previa is present, digital pelvic examination may increase bleeding, sometimes causing sudden, massive bleeding; thus, if vaginal bleeding occurs after 20 weeks, 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.
For a first (sentinel) episode of vaginal bleeding before 36 weeks, management consists of hospitalization, modified activity (modified rest), and avoidance of sexual intercourse, which can cause bleeding by initiating contractions or causing direct trauma. (Modified activity 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.
Typically for a 2nd bleeding episode, patients are readmitted and may be kept for observation until delivery.
Some experts recommend giving corticosteroids to accelerate fetal lung maturity when early delivery may become necessary and gestational age is < 34 weeks. Corticosteroids may be used if bleeding occurs after 34 weeks and before 36 weeks (late preterm period) in patients who have not required corticosteroids before 34 weeks (1).
Timing of delivery depends on the maternal and/or fetal condition. If the patient is stable, delivery can be done at 36 weeks/0 days to 37 weeks/6 days. Documentation of lung maturity is no longer necessary (2).
Delivery is indicated for any of the following:
Delivery is cesarean for placenta previa. Vaginal delivery may be possible for women with a low-lying placenta if the placental edge is within 1.5 to 2.0 cm of the cervical os and the clinician is comfortable with this method.
1. Gyamfi-Bannerman C, Thom EA, Blackwell SC, et al: Antenatal betamethasone for women at risk for late preterm delivery. N Engl J Med 374 (14):1311–1320, 2016. doi: 10.1056/NEJMoa1516783
2. Spong CY, Mercer BM, D'alton M, et al: Timing of indicated late-preterm and early-term birth. Obstet Gynecol 118 (2 Pt 1):323–333, 2011. doi: 10.1097/AOG.0b013e3182255999
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.
Consider placenta previa in all women who have vaginal bleeding after 20 weeks.
For most first bleeding episodes before 36 weeks, recommend hospitalization, modified activity, and abstinence from sexual intercourse.
Consider corticosteroids to accelerate fetal lung maturity if delivery may be needed before about 34 weeks or if bleeding occurs between 34 and 36 weeks in patients who have not required corticosteroids before 34 weeks.
Delivery is indicated when bleeding is severe or when the mother or fetus is unstable.
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