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:
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)
Older maternal age
For patients with placenta previa or a low-lying placenta, risks include fetal malpresentation Fetal Dystocia Fetal dystocia is abnormal fetal size or position resulting in difficult delivery. Diagnosis is by examination, ultrasonography, or response to augmentation of labor. Treatment is with physical... read more , preterm premature rupture of the membranes Prelabor Rupture of Membranes (PROM) Prelabor rupture of membranes is leakage of amniotic fluid before onset of labor. Diagnosis is clinical. Delivery is recommended when gestational age is ≥ 34 weeks and is generally indicated... read more , fetal growth restriction Small-for-Gestational-Age (SGA) Infant Infants whose weight is the 10th percentile for gestational age are classified as small for gestational age. Complications include perinatal asphyxia, meconium aspiration, polycythemia, and... read more , vasa previa Vasa Previa Vasa previa occurs when membranes that contain fetal blood vessels connecting the umbilical cord and placenta overlie or are within 2 cm of the internal cervical os. Vasa previa can occur on... read more , 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 Placenta Accreta Placenta accreta is an abnormally adherent placenta, resulting in delayed delivery of the placenta. Placental function is normal, but trophoblastic invasion extends beyond the normal boundary... read more ; 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).
Symptoms and Signs of Placenta Previa
Symptoms usually begin during late pregnancy. Then, sudden, painless vaginal bleeding often begins; the blood may be bright red, and bleeding may be heavy, sometimes resulting in hemorrhagic shock. In some patients, uterine contractions accompany bleeding.
Diagnosis of Placenta Previa
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 Abruptio Placentae Abruptio placentae is premature separation of a normally implanted placenta from the uterus, usually after 20 weeks gestation. It can be an obstetric emergency. Manifestations may include vaginal... read more , 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
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, delivery at 36 weeks/0 days to 37 weeks/6 days
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 Treatment references 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 weeks gestation. Diagnosis... read more ).
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 Treatment references 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 weeks gestation. Diagnosis... read more ).
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 the clinician is comfortable with this method.
Hemorrhagic shock Treatment of hemorrhagic shock Shock is a state of organ hypoperfusion with resultant cellular dysfunction and death. Mechanisms may involve decreased circulating volume, decreased cardiac output, and vasodilation, sometimes... read more is treated. Prophylactic Rho(D) immune globulin Prevention Erythroblastosis fetalis is hemolytic anemia in the fetus (or neonate, as erythroblastosis neonatorum) caused by transplacental transmission of maternal antibodies to fetal red blood cells.... read more should be given if the mother has Rh-negative blood.
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.