Placenta Previa

Basics

Description

  • 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 wk will migrate and have normal placental location at term
  • Causes 20% of all antepartum hemorrhage
  • Classification:
    • Placenta previa: The internal cervical os is covered completely or partially by the placenta (previously referred to as total or partial previa, respectively)
    • Low-lying placenta: The placenta is implanted in the lower uterine segment and the edge does not reach the internal os and remains outside a 2-cm wide perimeter

Etiology

  • Incidence: 3–5/1,000 births
  • 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 (risk in twin pregnancy increases 30–40%)
    • Prior C-section (up to 3× increase, increases with number of prior C-sections)
    • Increased maternal age (0.1% age 12–19 yr, 0.9% age ≥35 yr)
    • Previous placenta previa (4–8% recurrence)
    • Smoking (2–4 times increase)
    • Male fetus (14% increase)
    • Assisted fertilization
  • Associated conditions:
    • 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
    • Preterm premature rupture of membranes
    • Amniotic fluid embolism; associated with pathologies of the placenta
    • Vasa previa: Fetal vessels course through membranes and cover os
    • Congenital anomalies
    • Abnormal fetal presentation
  • Higher risk for massive bleeding
    • Short uterine cervical length in the third trimester
    • Sinus venosus at margin of placenta
    • Advanced maternal age at time of C-section
    • Prior C-section

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