Definition of Placenta previa:

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 baby in the womb.

Causes, incidence, and risk factors:

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)