Understanding Placenta Previa - the Basics

What Is Placenta Previa?

The placenta is the organ created during pregnancy to nourish the fetus, remove its waste, and produce hormones to sustain the pregnancy. The placenta is attached to the wall of the uterus by blood vessels that supply the fetus with oxygen and nutrition, and which remove waste from the fetus and transfer it to the mother.

The fetus is attached to the placenta by the umbilical cord. Through the cord, the fetus receives 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. The mother's blood and fetal blood usually don't mix in the placenta.

The placenta is usually attached to the upper part of the uterus, away from the cervix, the opening which the baby passes through during delivery. On rare occasions, the placenta lies low in the uterus, partly or completely blocking the cervix -- called a placenta previa.

As many as 1 in every 3 to 5 pregnancies has some form of placenta previa before the 20th week of pregnancy. As the uterus grows, the placenta usually moves higher in the uterus, away from the cervix. But if it remains near the cervix as your due date nears -- which happens in about 1 in 200 pregnancies -- you're at risk for bleeding, especially during labor as the cervix thins (effaces) and opens (dilates). This can cause major blood loss in the mother. For this reason, women with a placenta previa are usually delivered by cesarean delivery.

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 in the uterus as your due date approaches.
• A partial placenta previa covers part of the cervical opening.
• A total placenta previa covers and blocks the cervical opening.

What Causes a Placenta Previa?

The cause of placenta previa is usually unknown, although it occurs more commonly among women who are older, smoke, have had children before, have had a cesarean section or other surgery on the uterus, or have scars inside the uterus.

Women with placenta previa -- specifically if they have a placenta previa after having delivered a previous baby by cesarean section -- are at increased risk of placenta accreta, placenta increta, or placenta percreta.

In placenta accreta, the placenta is firmly attached to the uterus. In placenta increta, the placenta has grown into the uterus; and in placenta percreta, it has grown through the uterus. This condition is often first suspected because the woman has both a previa and a prior cesarean section. It can be confirmed by ultrasound, CT scan, or MRI, though those tests are not completely reliable. Women with one of these conditions usually require a hysterectomy after delivery of the baby, because the placenta does not separate from the uterus.