Pregnancy complications, such as placenta previa, are problems that occur only during pregnancy. They may affect the woman, the fetus, or both and may occur at different times during the pregnancy. However, most pregnancy complications can be effectively treated.
Normally, the placenta is located in the upper part of the uterus. In placenta previa, the placenta is located in the lower part. It covers part or all of the opening of the cervix—the entrance to the birth canal. Sometimes the placenta is located near the opening of the cervix, not over it (called a low-lying placenta).
Placenta previa occurs in about 1 of 250 deliveries. During the 2nd trimester, as many as 2% of pregnant women have placenta previa. 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 factors (conditions that increase the risk of a disorder) for placenta previa include 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.
Placenta previa can cause problems for the fetus, such as the following:
The fetus may be in an abnormal position.
The fetus may not grow as much as expected (intrauterine growth restriction).
The membranes around the fetus may rupture too soon (premature rupture of the membranes).
Blood vessels connecting the umbilical cord and placenta may block the fetus's exit through the opening of the cervix (vasa previa).
If women have had a cesarean delivery, placenta previa increases the risk that the placenta will be too firmly attached to the uterus (placenta accreta). Placenta accreta belongs to a group of disorders called the placenta accreta spectrum. These disorders differ in how firmly the placenta is attached to the uterus.
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.
Problems With the Placenta
When bleeding is minor and occurs before about 36 weeks of pregnancy, doctors typically advise that the woman be admitted to the hospital and told to limit her activity until the bleeding stops. Limiting her activity (called modified activity or modified bed rest) means that she should stay off her feet for most of the day. 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 usually readmitted to the hospital and may be kept there until delivery.
Some experts recommend using corticosteroids to help the fetus's lungs mature when early delivery—before about 34 weeks of pregnancy—may become necessary.
If the woman is not having contractions and if bleeding has stopped, doctors may deliver the baby at 36 to 37 weeks of pregnancy.
Delivery is usually done immediately when one of the following occur:
In women with placenta previa, delivery is cesarean, done before labor starts.
Women who bleed profusely may need blood transfusions.