Placenta Previa

Last Editorial Review: 1/30/2005

What is Placenta Previa?

WebMD Feature

The placenta is the organ created during pregnancy to nourish the fetus, remove its waste, and produce hormones to sustain the pregnancy. It is attached to the wall of the uterus by arteries that supply it with maternal blood and oxygen. The fetus is attached to the placenta by the umbilical cord. Through this, the fetus receives blood, nourishment and oxygen, and expels waste. On one side of the placenta, the mother's blood circulates, and on the other side, fetal blood circulates. Mother's blood and fetal blood usually do not mix in the placenta.

Sometimes, rather than being attached to the upper wall of the uterus, the placenta lies low in the uterus, partly or completely blocking the cervix. This is called placenta previa.

It's very common to have some form of placenta previa before the 20th week of pregnancy -- in as many as one in 15 pregnancies. But often the placenta moves higher in the uterus later in pregnancy. If it remains near the cervix as your due date nears, which happens in about one in 200 pregnancies, you are at risk for bleeding as the cervix thins (effaces) and opens (dilates) during labor. This can cause major blood loss in the mother.

There are several types of placenta previa:

  • A low-lying placenta is near the cervical opening but not covering it. It will often move upward as your due date approaches.
  • Partial placenta previa covers part of the cervical opening
  • Total placenta previa covers and blocks the cervical opening


The main symptom of placenta previa, and the greatest risk to you and your baby, is vaginal bleeding. Other symptoms include anemia, pale skin, rapid and weak pulse, shortness of breath or low blood pressure. The bleeding that signals placenta previa may occur at the end of the second trimester or beginning of the third trimester. It may be heavy or light. It is usually painless and it may happen without labor or in labor. Mothers with placenta previa are also at increased risk of delivering prematurely, before 37 weeks of pregnancy.

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