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.
Next: What Causes a Placenta Previa? »
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