Placenta previa is attachment (implantation) of the placenta over or near the cervix, in the lower rather than the upper part of the uterus.
The placenta may completely or partially cover the opening of the cervix—the entrance to the birth canal. Placenta previa occurs in 1 of 200 deliveries. As many as 15% of pregnant women have placenta previa during the 2nd trimester. Placenta previa may be visible on ultrasonography. However, it resolves on its own in more than 90% of women before they deliver. If it does not resolve, the placenta may detach from the uterus, depriving the baby of its blood supply. Passage of the baby through the birth canal can also tear the placenta, causing severe bleeding.
Risk is increased by the following:
Placenta previa can cause painless bleeding from the vagina that suddenly begins late in pregnancy. The blood may be bright red. Bleeding may become profuse, endangering the life of the woman and the fetus.
Doctors suspect placenta previa in pregnant women with vaginal bleeding that starts after 20 weeks of pregnancy. Ultrasonography helps doctors identify placenta previa and distinguish it from a placenta that has detached too early (placental abruption).
If placenta previa is causing symptoms, doctors monitor the fetus's heart rate to determine whether the fetus is having problems, such as not getting enough oxygen (see Monitoring the Fetus). If doctors think an early delivery may be necessary, they may take a sample of the fluid around the fetus (amniotic fluid) to determine whether the fetus's lungs are mature. If the fetus's lungs are mature, the baby can be delivered.
When bleeding is minor and occurs before about 36 weeks of pregnancy, doctors typically advise bed rest in the hospital until bleeding resolves. If the bleeding stops, the woman may be allowed to gradually resume light activities. If bleeding does not recur, she is usually sent home, provided that she can return to the hospital easily. Doctors advise against sexual intercourse, which can trigger bleeding.
If bleeding recurs, the woman is readmitted to the hospital and kept there until delivery.
Delivery, typically cesarean, is usually done when the fetus’s lungs are mature enough for delivery (usually after 36 weeks) or when one of the following occur
Delivery is almost always cesarean, done before labor starts.
Women who bleed profusely may need repeated blood transfusions.