Placenta previa is a low-lying placenta that covers part or all of the opening of the cervix. This positioning of the placenta can block the baby's exit from the uterus. As the cervix begins to thin and dilate in preparation for labor, blood vessels that connect the abnormally placed placenta to the uterus may tear, resulting in bleeding. During labor and delivery, bleeding can be severe, endangering mother and baby.
As with placental abruption, placenta previa can result in the birth of a premature baby.
How common is placenta previa?
Placenta previa occurs in about 1 in 200 pregnancies (4).
What are the symptoms of placenta previa?
The most common symptom of placenta previa is painless uterine bleeding during the second half of pregnancy. Women who experience vaginal bleeding in pregnancy should contact their health care provider.
How is placenta previa diagnosed?
An ultrasound examination can diagnose placenta previa and pinpoint the placenta's location. The provider usually avoids doing a vaginal examination when placenta previa is suspected because the examination may trigger heavy bleeding.
Some women who have not experienced vaginal bleeding learn during a routine ultrasound examination that they have a low-lying placenta. A pregnant woman should not be too worried if this happens to her, especially if she is in the first half of pregnancy. More than 90 percent of the time, placenta previa diagnosed in the second trimester corrects itself by term (3, 4).
How is placenta previa treated?
How a woman with placenta previa is treated depends on her stage of pregnancy, the severity of the bleeding and the condition of mother and baby. The goal, whenever possible, is to prolong pregnancy until the baby is at or near full term. Cesarean delivery is recommended for nearly all women with placenta previa because c-sections usually can prevent severe bleeding.
When a woman develops significant bleeding due to placenta previa after about 34 weeks of pregnancy, her provider may recommend a prompt c-section. Babies born after this time usually do well, though some have mild prematurity-related health problems during the newborn period.
Women who develop bleeding as a result of placenta previa before about 34 weeks are generally admitted to the hospital, where they can be monitored closely. If tests show that mother and baby are doing well, the provider will probably attempt to prolong the pregnancy. In some cases, when there has been a significant amount of bleeding, the mother may be treated with blood transfusions. She also will be treated with corticosteroid drugs if she is likely to deliver before 34 weeks.
Some women are able to go home after bleeding stops, but others must remain in the hospital until delivery. At 36 to 37 weeks, if she hasn't delivered, the provider may suggest a test of the amniotic fluid (obtained by amniocentesis) to see if the baby's lungs are mature. If they are, the provider will likely recommend a c-section at that time to prevent risks associated with any future bleeding episodes.
At any stage of pregnancy, a prompt c-section may be necessary if the mother develops dangerously heavy bleeding, or if mother or baby is having difficulties.
What causes placenta previa?
The cause of placenta previa is unknown. However, certain factors can increase a woman's risk (3, 4):
- Cigarette smoking
- Cocaine use
- Being more than 35 years of age
- Second or later pregnancy
- Previous uterine surgery, including a c-section; a D&C (dilation and curettage, in which the lining of the uterus is scraped), which is often done following a miscarriage or during an abortion
- Pregnant with twins, triplets or more
What is the risk of placenta previa happening again in another pregnancy?
A woman who has had a placenta previa in a previous pregnancy has a 2 to 3 percent chance of a recurrence (3).
Can a woman reduce her risk for placenta previa?
There is no way to prevent placenta previa. However, a woman may be able to reduce her risk by avoiding using cigarettes and cocaine. She also may be able to reduce her risk in future pregnancies by avoiding having an elective c-section (i.e., a c-section scheduled for convenience), unless there is a medical reason.