Placenta Previa

Placenta previa occurs when the placenta grows over the cervix. Learn more about this condition and how it's treated.
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The placenta has been hard at work throughout your pregnancy, transporting nutrients and oxygen from your blood to your baby via the umbilical cord. Normally this organ is attached high on the wall of your uterus; problems may occur if instead the placenta grows over the cervix, a condition called placenta previa.

What's normal. During childbirth, the baby arrives before the placenta, traveling through your cervix and vagina. Within half an hour of your baby's birth, you will deliver the placenta when it separates from the wall of your uterus. This sequence of events is necessary because your baby relies on the placenta to provide life-sustaining oxygen until he can breathe on his own.

What's abnormal. If the placenta grows low in the uterus, it may partly or completely block the opening in the cervix that leads to your vagina. This happens in about 1 in 200 pregnancies. You're more at risk if you're older, if you've had several babies, if you've had a prior birth by cesarean, or if you smoke cigarettes.

Concerns. Placenta previa can heighten the risk of life-threatening hemorrhage, either before or during labor. This severe, uncontrollable bleeding can happen because, as your cervix thins out and opens in preparation for delivery, the placenta's attachments to the uterus can more easily become damaged due to its lowered position. If this happens you might start bleeding without any pain. Your provider will do an ultrasound to determine the cause of the bleeding. An ultrasound at 18-20 weeks may show a low-lying placenta or a placenta previa. The ultrasound will be repeated in the 3rd trimester, when most low-lying placentas will no longer be low because the uterus has grown larger. If the placenta still completely covers the cervix at the time of the 3rd trimester ultrasound, most of the time it stays there. Placenta previa is usually diagnosed during the last 2 months of pregnancy.

Treatment. Treatment is determined by the length of your pregnancy, whether the placenta has started to detach from the wall of the uterus, and your baby's health. The treatment goals are to prevent bleeding and lessen the risk of premature delivery. If placenta previa is diagnosed but there is no bleeding, your provider will probably tell you to go on bed rest or limit your activities to lessen the danger of bleeding until your baby is big enough to safely deliver by cesarean. If you start to bleed, you will be admitted to the hospital; how long you remain in the hospital depends on several factors. Cesarean deliveries are almost always necessary because the placenta would be torn from its roots during a vaginal delivery, causing life-threatening bleeding for mom and lack of oxygen for the baby.

Originally published in You & Your Baby: Pregnancy.

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