Placenta Previa

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Basics

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

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