Placenta previa is implantation of the placenta over or near the internal os of the cervix. It typically manifests as painless vaginal bleeding after 20 weeks gestation; the source of bleeding in placenta previa is maternal. Diagnosis is by ultrasonography. Treatment is modified activity if minor vaginal bleeding occurs before 36 weeks gestation, with cesarean delivery at 36 to 37 6/7 weeks. If bleeding is severe or refractory or if fetal status is nonreassuring, immediate cesarean delivery is indicated.
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 approximately 5/1000 deliveries (1). If placenta previa occurs during early pregnancy, it usually resolves by 28 weeks as the uterus enlarges.
Risk factors
Risk factors for placenta previa include the following:
Multiparity
Prior cesarean delivery
Uterine abnormalities that inhibit normal implantation (eg, fibroids, prior curettage)
Prior uterine surgery (eg, myomectomy) or procedure (eg, multiple dilation and curettage [D and C] procedures)
Smoking
Multiple gestation
Older maternal age
Complications
For patients with placenta previa or a low-lying placenta, risks include fetal malpresentation, 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 and a placenta previa, the risk of placenta accreta spectrum or morbidly adherent placenta increases as the number of prior cesarean deliveries increases: 3%, 11%, 40%, 61%, and 67% for the 1, 2, 3, 4, and ≥ 5 previous cesarean deliveries, respectively (2).
General references
1. Cresswell JA, Ronsmans C, Calvert C, Filippi V: Prevalence of placenta praevia by world region: A systematic review and meta-analysis. Trop Med Int Health 18 (6):712–724, 2013. doi: 10.1111/tmi.12100
2. Silver RM, Landon MB, Rouse DJ, et al: Maternal morbidity associated with multiple repeat cesarean deliveries. Obstet Gynecol 107(6):1226-1232, 2006. doi:10.1097/01.AOG.0000219750.79480.84
Symptoms and Signs of Placenta Previa
Placenta previa is often asymptomatic and is discovered incidentally on routine second-trimester ultrasonography.
Symptoms of placenta previa typically manifests as sudden, painless vaginal bleeding; bleeding may be heavy, sometimes resulting in hemorrhagic shock. Bleeding may occur as early as 16 weeks of gestation. In some patients, uterine contractions accompany bleeding. The source of bleeding in placenta previa is maternal.
Diagnosis of Placenta Previa
Transvaginal ultrasonography
Placenta previa is considered in all women with vaginal bleeding, especially women in the second and third trimester. If placenta previa is present, pelvic examination by digital cervical examination may increase bleeding, sometimes causing sudden, massive bleeding. Thus, if vaginal bleeding occurs after 20 weeks, pelvic examination is contraindicated unless placenta previa is first ruled out by ultrasonography. Speculum examination is safe.
Although placenta previa is more likely to cause painless bleeding than placental abruption, 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.
Pearls & Pitfalls
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In all women with suspected symptomatic placenta previa, fetal heart rate monitoring is indicated.
Treatment of Placenta Previa
Hospitalization and modified activity for a first episode of bleeding before 36 weeks
Delivery if mother or fetus is unstable
If the woman is stable, cesarean delivery at 36 to 37 6/7weeks
For a first (sentinel) episode of vaginal bleeding before 36 weeks, management consists of hospitalization, modified activity (modified rest), and avoidance of sexual activity, which can cause bleeding by initiating contractions. 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 second bleeding episode, patients are readmitted and may be kept for observation, sometimes until delivery. Management should be individualized.
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 been given 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 to 37 6/7 weeks. Documentation of lung maturity is not necessary (2).
Delivery is indicated for any of the following:
Heavy or uncontrolled bleeding
Nonreassuring results of fetal heart monitoring
Maternal hemodynamic instability
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 if, after a shared decision-making process, the clinician and patient are comfortable attempting vaginal delivery.
Hemorrhagic shock is treated, if present. Prophylactic Rho(D) immune globulin should be given if the mother has Rh-negative blood.
Treatment references
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
Key Points
Placenta previa is implantation of the placenta over or near the internal os of the cervix.
Placenta previa typically manifests as painless vaginal bleeding during the second or third trimester, and placental abruption is usually associated with uterine pain and tenderness; however, clinical differentiation is often not possible.
Consider placenta previa in all women who have vaginal bleeding during the second or third trimester.
For most first bleeding episodes before 36 weeks, recommend hospitalization, modified activity, and abstinence from sexual activity.
Consider corticosteroids to accelerate fetal lung maturity if delivery may be required before 34 weeks or if bleeding occurs between 34 and 36 weeks in patients who have not been given corticosteroids before 34 weeks.
Cesarean delivery is indicated when the mother or fetus is unstable or, if mother and fetus are stable, at 36 to 37 6/7 weeks.
