Description

• 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)

Epidemiology

• 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.

Incidence

• 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).

Prevalence

• 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.

Risk Factors

Some below are simple associations and not risks per se:

• Previous placenta previa
• Previous cesarean delivery
• Multiple gestation
• Multiparity
• 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)

General Prevention

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
• Malpresentation
• Intrauterine growth restriction
• Respiratory distress syndrome
• Anemia
• Vasa previa and velamentous umbilical cord insertion
• Congenital anomalies
• Amniotic fluid embolism