Placenta previa Health Article

Definition

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.

Causes, incidence, and risk factors

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)

Symptoms

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

Signs and tests

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.