200 Obstetrics and Gynecology Board Review •••
❍ What is the incidence of placenta previa in nulliparas at the time of delivery?
1 in 1500.
❍ What is the incidence of placenta previa in grand multiparas at the time of delivery?
Up to 1 in 20.
❍ What is the relative risk of previa for history of one cesarean delivery?
4.5×.
❍ What is the relative risk of previa for history of four prior cesarean deliveries?
45 ×.
❍ How are placental previas classified?
Complete: The internal os is entirely covered by placenta.
Partial: The os is partially covered.
Marginal: The placenta edge just reaches the internal os.
Low-Lying: <2 cm from the internal os.
❍ What percentage of women with second trimester bleeding have a low-lying placenta diagnosed by
ultrasound at that time?
Up to 45%.
❍ What percentage of placenta previas diagnosed in the second trimester resolve by the time of delivery?
90%.
❍ What is the classic presentation of placenta previa?
Sudden onset of painless vaginal bleeding in the second or third trimester.
❍ What is the most accurate method for diagnosing placenta previa?
Transvaginal ultrasound.
❍ How has maternal mortality from placenta previa changed since the 1950s?
From 25% to <1%.
❍ How has perinatal mortality from placenta previa changed since the 1950s?
From 60% to <5%.
❍ What is the method of delivery for placenta previa?
Cesarean delivery.
❍ What is the recommended timing of delivery for placenta previa?
By 38 weeks (36–37 if fetal lung maturity is documented by amniocentesis).