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Placenta Previa - Topic Overview

What is placenta previa?

Placenta previa is a problem with the placenta during pregnancy. The placenta is a round, flat organ that forms during pregnancy to give the baby food and oxygen from the mother. The placenta forms on the inside wall of the uterus soon after conception.

During a normal pregnancy, the placenta is attached higher up in the uterus, away from the cervix. But in rare cases, the placenta forms low in the uterus. If this happens, it may cover all or part of the cervix. When the placenta blocks the cervix, it is called placenta previa.

See pictures of a normal placenta and placenta previa .

What causes placenta previa, and how can you lower your risk?

Doctors aren't sure what causes placenta previa. But there are things that raise a woman’s risk of it. These things are called risk factors. Some risk factors you can control to lower your risk. Others are things you can't control.

Risk factors for placenta previa that you can control include:

Risk factors that you can't control include:

  • Past surgeries or tests that affected the lining of the uterus, such as uterine surgery, dilation and curettage (D&C), or myomectomy.
  • Past cesarean delivery (C-section).
  • A history of five or more past pregnancies.
  • Being 35 or older.
  • A history of a past placenta previa.

If your doctor finds out before your 20th week of pregnancy that you have a placenta that is attached low in the uterus, chances are good that it will get better on its own. In fact, 9 out of 10 cases found before the 20th week go away on their own by the end of the pregnancy.1 This is because as the lower uterus grows, the position of the placenta can change. So by the end of the pregnancy, the placenta may no longer block the cervix.

What are the symptoms?

Some women with placenta previa do not have any symptoms. But there are a few warning signs. If you have placenta previa, you may notice one or more symptoms. These include:

  • Sudden, painless vaginal bleeding that is light to heavy. The blood is often bright red.
  • Symptoms of early labor, such as regular contractions and aches or pains in your lower back or belly.

Call your doctor or go to the nearest emergency room right away if you have:

  • Medium to severe vaginal bleeding during the first trimester.
  • Any vaginal bleeding in the second or third trimesters.

How is placenta previa diagnosed?

Most cases of placenta previa are found during the second trimester when a woman has a routine ultrasound. Or it may be found when a pregnant woman has vaginal bleeding and gets an ultrasound to find out what is causing it. Some women find out that they have placenta previa only when they have bleeding at the start of labor.

How is it treated?

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WebMD Medical Reference from Healthwise

Last Updated: February 23, 2010
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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