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- Complete/total previa: Placenta covers the entire os.
- Partial previa: Placenta covers part of internal cervical os.
- Marginal previa: Placental edge is adjacent to cervical os by ultrasound (US) but not overlapping, usually within 2 cm of os.
- Low-lying placenta: placental edge located in the lower uterine segment but does not encroach on/cover the os; has been defined as within 2 to 3 cm of cervical os by US (1)
- Most common cause of painless vaginal bleeding in the 2nd and 3rd trimester
- 10% of low-lying placentas at 10 to 20 weeks persist to term.
- 1–6% of pregnancies have sonographic evidence of previa between 10 and 20 weeks of gestation.
- Recurrence rate of 4–8% with prior placenta previa
- Approximately 90% of women with placenta previa in the 2nd trimester will have resolution by term.
- Persistent placenta previa complicate 1 in 200 pregnancies.
- Extension over the cervical os is the best predictor of placenta previa at time of delivery.
- The earlier the previa is detected, the less chance it will persist at the time of delivery.
- Increased incidence parallels the increasing rate of cesarean deliveries in the United States.
- 1/3 present before 30 weeks, 1/3 between 30 and 36 weeks, and 1/3 after 36 weeks (1).
- Ranges from 3.5 to 4.6 per 1,000 births
- Probability of resolution increases as gestational age at time of diagnosis and distance over the cervical os decreases.
Etiology and Pathophysiology
- Uterine scarring may predispose due to interference with the natural growth of the placenta.
- Placental bleeding is thought to occur when changes in the cervix and lower uterine segment apply shearing forces to the placental attachment site leading to partial detachment.
- Bleeding is primarily maternal, although fetal bleeding may occur if a fetal vessel is disrupted.
Some below are simple associations and not risks per se:
- Previous placenta previa
- Previous cesarean delivery
- Multiple gestation
- Advanced maternal age
- Infertility treatment
- Previous abortion
- Previous intrauterine surgical procedure
- Maternal smoking
- Maternal cocaine use
- Male fetus
- Nonwhite race
- Preterm labor (1)
- Intrauterine growth restriction (1)
- Preterm premature rupture of membranes (1)
- Congenital abnormalities (1)
No means of preventing placenta previa except reducing primary cesarean section rate; may be able to decrease chances of vaginal bleeding due to activity modification
Commonly Associated Conditions
- Placenta accreta complicated 1–5% of pregnancies with placenta previa.
- Presence of placenta previa in the setting of prior cesarean delivery increases risk of an accreta being present:
- 11–25% with one previous cesarean delivery
- 35–47% with two cesarean deliveries
- 40% with three cesarean deliveries
- 50–67% with four or more cesarean deliveries
- High risk for need for hysterectomy at the time of cesarean delivery with accreta
- Preterm labor and preterm rupture of membranes
- Intrauterine growth restriction
- Respiratory distress syndrome
- Vasa previa and velamentous umbilical cord insertion
- Congenital anomalies
- Amniotic fluid embolism