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. Placenta previa occurs in 1 of 200 deliveries. As many as 15% of pregnant women have placenta previa during the 2nd trimester. The placenta previa may be visible on ultrasonography. However, it resolves on its own in more than 90% of women before they deliver. 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.
When labor starts, the placenta tends to become detached very early, depriving the baby of oxygen. The lack of oxygen may result in brain damage or other problems in the baby.
Ultrasonography helps doctors identify placenta previa and distinguish it from a placenta that has detached too early (placental abruption).
When bleeding is minor and occurs before about 34 weeks of pregnancy, doctors typically advise bed rest in the hospital until bleeding resolves. If the bleeding stops, the woman is usually encouraged to walk. If bleeding does not recur, she is usually sent home, provided that she can return to the hospital easily.
Delivery, typically cesarean, is usually done if bleeding is profuse or does not stop or if the fetus's lungs are mature enough for delivery (usually after 36 weeks). Doctors can determine whether the fetus's lungs are mature enough by taking a sample of amniotic fluid, usually from the vagina, and analyzing it. A cesarean delivery is done before labor starts. Women who bleed profusely may need repeated blood transfusions.
Last full review/revision December 2008 by Edmund F. Funai, MD