Placenta previa is a placenta that has grown low in the uterus, partially or fully over the opening to the birth canal (cervix). Placenta previa can be dangerous during labor and delivery, when it is most likely to cause severe placental bleeding that can be life-threatening to the mother or fetus.

The amount of the cervix that is covered by the placenta may be slight (marginal), moderate (partial), or complete (total). Unless a placenta previa is only marginal, a surgical delivery (cesarean section) is needed to prevent severe placental bleeding.

A woman's risk for placenta previa increases with each pregnancy, cesarean section, uterine procedure that scars the uterus, or previous placenta previa. Smoking, cocaine use, and advancing age also increase placenta previa risk.