What Is Placenta Previa?

Placenta previa, a rare complication of pregnancy in which the placenta hangs low in the uterus and covers all or part of the cervix, can cause serious bleeding.

The placenta grows inside your uterus when you are pregnant. Normally, it attaches to the wall of your uterus — also called your womb — at the top or the side. Its purpose is to supply the baby with food and oxygen through the umbilical cord. 

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What is placenta previa?

In about 1 in 200 pregnancies, the placenta lies very low in the uterus and covers all or part of the cervix, the opening at the top of the vagina that leads to the uterus. This is called placenta previa.

Early in pregnancy, it usually isn’t a problem. Over time, as the uterus grows, the distance between the cervix and the placenta shrinks. By the third trimester, the placenta should be near the top of the womb.

During labor, the cervix begins to dilate so the baby can pass through. If you have placenta previa, blood vessels connecting the placenta to the uterus may tear. Severe bleeding could put you and the baby in danger, so your doctors will deliver your baby through a C-section if the placenta still lies low near your due date.

What are the symptoms of placenta previa?

The most common symptom is painless bright red bleeding from the vagina during the second half of pregnancy. It may come and go or come after intercourse. Some women also have cramps or sharp pains.

Call your healthcare provider right away if you are bleeding. If it is severe, go to the hospital.

What causes placenta previa?

Researchers don’t know exactly what causes placenta previa, but there are risk factors. Placenta previa is more common if:

• You smoke cigarettes.
• You use cocaine.
• You’re 35 or older.
• You’ve been pregnant before.
• You’re pregnant with twins, triplets, or more.
• You’ve had placenta previa before.
• The baby is in an unusual position (breech is buttocks first, transverse is lying horizontally).
• You have scars on your uterus, often from removing fibroids, or have had a previous C-section or a D&C (dilation and curettage), which may have been necessary after a miscarriage.

How is placenta previa diagnosed?

About a third of women with placenta previa don’t bleed. During a routine, second trimester ultrasound, your doctors will see the location of the placenta. At this point, the chances are the problem will go away as the uterus expands.

The Royal College of Obstetricians and Gynaecologists reports that only 10 percent of women with a low-lying placenta at 20 weeks will still have one at their next ultrasound. By the end of their pregnancy, that number drops to less than half a percent.

How is placenta previa treated?

If you have minimal or no bleeding, your doctor may advise you to follow the practice called pelvic rest, which means you abstain from sexual intercourse and avoid exercises that could strain your pelvic floor.

The goal will be to avoid a premature birth. Any bleeding will need to be closely monitored in the hospital with a transvaginal ultrasound (a probe inside the vagina), transabdominal ultrasound (a handheld unit placed over your abdomen that sends sound waves to make a picture on a screen), or magnetic resonance imagining (or MRI).

If you have dangerously heavy bleeding, your doctor will schedule a C-section as soon as it’s safe — ideally after 36 weeks. You may need a transfusion of new blood. Your provider may suggest checking the fluid around your baby to see if his or her lungs are fully developed.

You may also receive medicines that help speed up the development of your baby.

If your bleeding can’t be stopped, you will receive an emergency C-section, even if the baby is premature, as you could bleed to death. A vaginal birth in such cases is too dangerous.

How can you avoid placenta previa?

You typically cannot prevent placenta previa, but avoiding the risk factors listed in the “causes” section above and following your doctor’s advice may help. It’s best to have a C-section only if it is medically necessary, since the procedure can increase your chances of having pregnancy complications on another birth, including placenta previa.

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