various types of placenta previa
The placenta normally terminates >2 cm above the internal cervical os. Extension of the placental edge within 2 cm of the internal os is referred to as low-lying placenta. Extension of the placenta up to or over the cervix is referred to as placenta previa.
Placenta previa can be marginal (at the edge of the internal os), partial (partial coverage of the internal os), or complete (complete coverage of the internal os). Some cases of placenta previa may resolve as the pregnancy progresses, and therefore follow-up imaging is performed in the third trimester. If a placenta previa persists, then plans can be made for a cesarean section.