Normally, the placenta is attached to the inside wall of the uterus, up along the side or the top of it. "Placenta previa" occurs when the placenta is attached closer to the bottom, and either partially or completely blocks the opening in the cervix. This happens in about 1 out of every 200 pregnancies. The most common symptom is painless bleeding in the later stages of pregnancy.
When placenta previa is diagnosed earlier in the pregnancy, which is possible using ultrasound, it is monitored very closely. In cases of partial previa, where the placenta partially covers the cervix early in the pregnancy, it is possible for it to move away from the cervix as the pregnancy progresses. If a partial previa still exists at the time labor begins, a vaginal delivery may be possible.
But, in cases of complete placenta previa, where the placenta covers the entire cervix, a cesarean delivery is usually recommended. Why does placenta previa become serious if a normal vaginal delivery is attempted? As the cervix dilates, it usually causes the placenta to separate from the wall of the uterus too early, resulting in serious bleeding that can be fatal for both the mother and/or the baby. Because of this, cesarean delivery is usually recommended.