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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 opening to the cervix.

The placenta is the organ that nourishes the developing fetus.

During pregnancy, the placenta moves as the uterus stretches and grows. In early pregnancy, a low-lying placenta is very common. But as the pregnancy progresses, the growing uterus should "pull" the placenta toward the top of the womb. By the third trimester, the placenta should be near the top of the 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. This is called a previa.

There are different forms of placenta previa:

• Marginal: The placenta is against the cervix but does not cover the opening.
• 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:

• Abnormally developed uterus
• Many previous pregnancies
• 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 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. Most cases of placenta previa are identified by routine ultrasound during pregnancy.

Placenta previa is most often diagnosed before bleeding occurs. Careful monitoring of the mother and unborn baby can prevent many of the significant dangers.

The biggest risk is that severe bleeding will require your baby to be delivered early, before major organs, such as the lungs, have developed.

Most complications can be avoided by 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.

Francois KE, Foley MR. Antepartum and postpartum hemorrhage. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics - Normal and Problem Pregnancies. 5th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2007:chap 18.

Houry DE, Abbott JT. Acute complications of pregnancy. In: Marx J, ed. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 6th ed. St Philadelphia, Pa: Mosby Elsevier; 2006:chap 177.

Cunnigham FG, Leveno KL, Bloom SL, et al . Obstetrical hemorrhage. In: Cunnigham FG, Leveno KL, Bloom SL, et al, eds. Williams Obstetrics. 22nd ed. New York, NY; McGraw-Hill; 2005:chap 35.