Placenta Previa

Topic Overview

What is placenta previa?

The placenta is an organ that forms on the inner wall of the uterus shortly after conception. Nutrients and oxygen pass from the mother to her fetus through the placenta, which also carries waste products away from the fetus.

Normally, the placenta is attached to the uterus above the cervix. In rare cases, the placenta forms low in the uterus and is partially to completely covering the cervix. When the placenta is blocking the cervix, it is called placenta previa. See illustrations of a normal placenta and placenta previa.

What are the symptoms?

Some women with placenta previa do not have any symptoms; others have sudden, painless vaginal bleeding that ranges from slight to heavy. The blood is often bright red.

What causes placenta previa?

The specific cause of placenta previa is not known. However, there are several risk factors that can increase your chances of developing placenta previa, including:¹

• Cigarette smoking, which is strongly linked to 1 of every 4 previas.¹ Smoking decreases the amount of oxygen transferred to the fetus, thereby stimulating the growth of a larger placenta, which is more likely to grow low into the uterus.
• History of medical procedures that affect the uterine lining, such as dilation and curettage (D&C) done with sharp curettage (rare) after a miscarriage (spontaneous abortion) or a medical abortion.^{2, 3}
• Previous caesarean delivery (C-section). Of women who have had a caesarean delivery in the past, as many as 4 in 100 develop placenta previa; of women who have had four or more C-sections, 10 in 100 develop placenta previa.⁴
• History of several previous pregnancies. Placenta previa occurs in 1 in 1,500 first-time pregnancies. In women who have had five or more pregnancies, this increases to about 5 in 100.⁵
• Advancing maternal age. Among women 19 and younger, only 1 in 1,500 develops placenta previa. Of women 35 and older, 1 in 100 develops placenta previa.⁶
• Cocaine or crack cocaine use during pregnancy.⁷
• History of a previous placenta previa.

If your health professional has identified a placenta previa or low-lying placenta before your 20th week of pregnancy, chances are good that it will resolve on its own. About 90% of placenta previa cases diagnosed before the 20th week resolve on their own by the end of the pregnancy.⁴ As the lower uterus grows, the position of the placenta can change in relation to the cervix so that by the end of the pregnancy, the placenta no longer blocks the cervix.

How is placenta previa diagnosed?

Placenta previa is diagnosed with ultrasound. Most previas are identified during the second trimester, with routine ultrasound, or when assessing the cause of vaginal bleeding or when bleeding begins at the onset of labour.

How is it treated?

Treatment for placenta previa depends on how much you are bleeding and whether your fetus is mature enough to survive early delivery. Providing that you and your fetus are stable, you can be closely monitored until the baby can be safely delivered.

With a bleeding previa, it is important that you avoid sexual intercourse, office vaginal examinations, or putting anything else in your vagina. (You may, however, have a carefully done vaginal examination at the hospital.)

When your fetus is mature enough, or if too much bleeding is endangering you or your fetus, your baby will be delivered. Because disturbing the placenta with a vaginal delivery can cause severe bleeding, a caesarean section is always used when placenta previa is present.

What are the potential complications of placenta previa?

Placenta previa can cause complications for both mother and fetus, including:

• Early separation of the placenta from the uterine wall (placenta abruptio).
• Severe maternal bleeding (hemorrhage) before or during delivery, which can be life-threatening for both a mother and her fetus. When the placenta has abnormally attached or grown into the uterine wall (placenta accreta, placenta increta, or placenta percreta), bleeding can be severe enough to require a hysterectomy.⁵
• Premature, or preterm, delivery (before the 37th week of pregnancy), which typically poses the greatest risk to the fetus.
• Birth defects. Birth defects occur 2.5 times more often in pregnancies affected by placenta previa than in unaffected pregnancies. The cause is currently unknown.⁶ It may just be that placenta previa is slightly more common among older women, as are babies with birth defects.

Frequently Asked Questions

Learning about placenta previa:	What is placenta previa? What are symptoms of placenta previa? What causes placenta previa? What are the risks of placenta previa?
Being diagnosed:	When do I call my health professional about possible placenta previa? How is placenta previa diagnosed?
Getting treatment:	How is placenta previa treated?
Ongoing concerns:	How will having placenta previa affect me in the future?

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Author:	Kathe Gallagher, MSW Carrie Henley	Last Updated: May 15, 2006
Medical Review:	Joy Melnikow, MD, MPH - Family Medicine Tom Bailey, MD - Family Medicine William Gilbert, MD - Perinatology
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