Placenta Previa

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  • Placental tissue overlying or proximate to the internal cervical os
  • Uterine enlargement and cervical dilation cause placental vessels near the cervix to tear, resulting in vaginal bleeding
  • >90% of placenta previa diagnosed before 20 weeks will migrate and have normal placental location at term
  • If placenta covers the internal os by >20 mm, then previa is expected at birth
  • Increased amount of placental overlap (>15–23 mm) predicts placenta previa present at birth
  • Causes 20% of all antepartum hemorrhage
  • Classifications:
    • Complete placenta previa: Cervical os is completely covered by placenta
    • Partial placenta previa: Cervical os is partially covered by placenta
    • Marginal placenta previa: Edge of placenta is at margin of cervical os
    • Low-lying placenta: Placenta edge is within 2 cm to cervical os


  • Unknown etiology
  • Incidence: 4/1,000 births = 0.4% of pregnancies at term
  • Maternal mortality: 0.03%
  • Perinatal morbidity and mortality: Triple, due to preterm delivery
  • Factors affecting location of implantation:
    • Increased number of curettages from spontaneous or induced abortions
    • Abnormal endometrial vascularization
    • Delayed ovulation
  • Risk factors:
    • Multiparity (5% grand multiparous patients vs. 0.2% nulliparous)
    • Multiple gestation
    • Prior C-section (up to 3× increase, increases with number or prior C-sections)
    • Increased maternal age (0.7% age <19 yr, 1% age ≥35 yr)
    • Previous placenta previa (4–8% recurrence)
    • Smoking (2–4 times increase)
    • Male fetus (14% increase)
    • Assisted fertilization
    • Residence at higher altitude
    • Asian maternal race
    • Unexplained elevated maternal serum alpha fetal protein (MSAFP)
  • Associated conditions:
    • Congenital anomalies
    • Abnormal fetal presentation
    • Preterm premature rupture of the membranes
    • Amniotic fluid embolism; associated with pathologies of the placenta
    • Vasa previa: Fetal vessels course through membranes and cover os
    • Placenta accreta, increta, percreta (growth of placenta into uterine wall) occur in 5–10% of patients with placenta previa; sustained bleeding may require C-section hysterectomy

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