Placenta previa is a complication of pregnancy in which the placenta grows in the lowest part of the womb (uterus) and covers all or part of the cervix.

The placenta is the organ that nourishes the developing fetus.

During pregnancy, your placenta repositions itself as your uterus stretches and grows. In early pregnancy, a low-lying placenta is very common. But as your pregnancy progresses, the enlarging uterus should "pull" the placenta toward the top of your uterus. By your third trimester, the placenta should be near the top of your uterus, leaving the opening of the cervix clear for the delivery.

Sometimes, though, the placenta remains in the lower portion of the uterus, partly or completely covering this opening. There are three types of placenta previa:

• Marginal: The placenta is located near the edge of the cervix but does not block it.
• Partial: The placenta covers part of the cervical opening.
• Complete: The placenta completely covers the cervical opening.

Placenta previa occurs in 1 out of 200 pregnancies. It is more common in women who have:

• Uterine fibroids or surgery to remove fibroids
• An abnormally developed uterus
• Multiple pregnancy (twins, triplets, etc.)
• Scarring of the uterine wall caused by previous pregnancies, cesareans, uterine surgery, or abortions

Women who smoke or have their children at an older age may also have an increased risk. Possible causes of placenta previa include:

• Abnormal formation of the placenta
• Abnormal uterus
• Large placenta
• Scarred lining of the uterus (endometrium)

The main symptom of placenta previa is sudden, painless, and often heavy vaginal bleeding that often occurs near the end of the second trimester or beginning of the third trimester. In some cases, there is severe bleeding or hemorrhage. The bleeding may stop on its own but can start again days or weeks later.

There may be uterine cramping with the bleeding. Labor sometimes starts within several days after heavy vaginal bleeding. However, in some cases, bleeding may not occur until after labor starts.

See: Vaginal bleeding in pregnancy

Your health care provider can diagnose placenta previa with an ultrasound exam.

The uterus is usually soft and relaxed (unless labor has started). In a small number of cases, the infant is in an unusual position.

Treatment depends on various factors:

• How much bleeding you had
• Where the fetus is developed enough to survive outside the uterus
• How much of the placenta is covering the cervix
• The position of the fetus
• The number of previous births you have had
• Whether you are in labor

If the placenta is near the cervix or is covering a portion of it, you may be ordered reduce activities and stay on bed rest. Your doctor will order pelvic rest, which means no intercourse and no douching. Nothing should be placed in the vagina.

If there is bleeding, however, you will most likely be admitted to a hospital for careful monitoring.

If you have lost a lot of blood, blood transfusions may be given. You may receive medicines to prevent premature labor and help the pregnancy continue to at least 36 weeks. Beyond 36 weeks, the benefits of the infant maturing have to be weighed against the possibility of a major hemorrhage.

Your health care providers will carefully weigh your risk of ongoing bleeding against the risk of an early delivery for your baby.

Women with placenta previa most likely need to deliver the baby by cesarean section. This helps reduce mother and infant deaths. An emergency cesarean may be performed if the placenta actually covers the cervix and the bleeding is heavy or life threatening.

Placenta previa is most often diagnosed before the mother or the fetus is in significant danger, so it no longer poses as much of a threat to babies and mothers as it once did.

The biggest risk is that severe bleeding will require your baby to be delivered preterm. Fetal distress doesn't usually occur unless there is a cord accident, or vaginal blood loss has been heavy enough to cause placenta abruptio or shock in the mother.

The outcome is excellent when the condition is managed properly. This means hospitalizing a mother who is having symptoms, and delivering by C-section.

Risks to the mother include:

• Death
• Major bleeding (hemorrhage)
• Shock

There is also an increased risk for infection, blood clots, and necessary blood transfusions.

Prematurity (infant is less than 36 weeks gestation) causes most infant deaths in cases of placenta previa. Fetal blood loss or hemorrhage may occur because the placenta separates from the wall of the uterus during labor. The fetus also can lose blood when the uterus is opened during a C-section delivery.

Call your health care provider if you have bleeding from the vagina at any point in your pregnancy. Placenta previa can be dangerous to both you and your baby.

This condition is not preventable.