Placental implantation in the lower uterine segment, encroaching on the internal cervical os

Common cause of bleeding during the second half of pregnancy (in patients who develop placenta previa during the second trimester, less than 15% have persistent previa at term)

Occurs about 1 in every 200 pregnancies, more commonly after age 35; more common in multigravida than primigravidas

Carries good maternal prognosis if hemorrhage can be controlled

Usually necessitates pregnancy termination if bleeding heavy

Fetal prognosis dependent on gestational age and amount of blood lost; risk for death greatly reduced by frequent monitoring and prompt management (see Three types of placenta previa)