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 or completely covering the cervix. When the placenta is blocking the cervix, it is called placenta previa. See pictures 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. But there are several risk factors that can increase your chances of developing placenta previa, including:

• 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
• Previous cesarean delivery (C-section). Of women who have had two cesarean deliveries in the past, about 2 out of 100 have placenta previa. For women with three or more cesarean deliveries, the chance of placenta previa increases to more than 4 out of 100.³
• 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 condition increases to about 5 in 100.⁴
• 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.³
• Cocaine or crack cocaine use during pregnancy.
• 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.⁵ 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 labor.

How is it treated?