A condition of pregnancy in which the placenta is implanted abnormally in the uterus so that it impinges on or covers the internal os (opening) of the uterine cervix.

Placenta previa is the most common cause of painless bleeding in the third trimester of pregnancy. There are usually four degrees of this condition: total placenta previa (the placenta covers the entire opening to the cervix); partial placenta previa (the placenta partially encroaches to within six centimeters of the cervical os);...