Merck Manual

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Quick Facts

Placenta Previa


The Manual's Editorial Staff

Last full review/revision Sep 2019
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What is the placenta?

  • The placenta is an organ that grows on the inside, upper part of your uterus (womb) when you're pregnant

  • It has many large blood vessels to carry oxygen and nutrients from you to your baby

  • The placenta's blood vessels form the umbilical cord to connect the placenta to your baby

  • About 15 minutes after you deliver your baby, the placenta comes off your uterus and goes out your vagina

  • That's why it's also called the "afterbirth"

What is placenta previa?

  • Placenta previa is when the placenta attaches too low in your uterus, over or near your cervix

Your cervix is the lower part of your uterus. It has an opening that normally stays closed while you're pregnant. When your baby is ready to be born, the cervix opens (dilates) to let your baby out. If you have placenta previa, the placenta gets in the way.

  • Placenta previa often gets better on its own before delivery

  • If it doesn’t get better before delivery, your baby could tear the placenta as your baby moves through the cervix into the birth canal (vagina), causing very bad bleeding

  • If you have placenta previa, wait to have sex until after your baby is born—having sex while you have placenta previa can cause bleeding

Problems With the Placenta

Normally, the placenta is located in the upper part of the uterus, firmly attached to the uterine wall until after delivery of the baby. The placenta carries oxygen and nutrients from the mother to the fetus.

In placental abruption (abruptio placentae), the placenta detaches from the uterine wall prematurely, causing the uterus to bleed and reducing the fetus’s supply of oxygen and nutrients. Women who have this complication are hospitalized, and the baby may be delivered early.

In placenta previa, the placenta is located over or near the cervix, in the lower part of the uterus. Placenta previa may cause painless bleeding that suddenly begins late in pregnancy. The bleeding may become profuse. The baby is usually delivered by cesarean.

Problems With the Placenta

What are the symptoms of placenta previa?

  • Sudden, painless, bright red bleeding late in pregnancy

  • Call your doctor right away if you're pregnant and have vaginal bleeding—your life and your baby’s life could be in danger

How can doctors tell if I have placenta previa?

  • Doctors suspect placenta previa if you have vaginal bleeding that starts after 20 weeks of pregnancy

  • They’ll do an ultrasound to tell for sure—this test uses sound waves to create moving pictures of the insides of your uterus

  • During the ultrasound, they’ll also check to make sure your placenta hasn’t pulled away from your uterus early (placental abruption)

How do doctors treat placenta previa?

If doctors think your baby's lungs are developed enough to be delivered safely (usually after 36 weeks of pregnancy), they'll do:

  • Surgery to deliver your baby, called a cesarean section (C-section)

Doctors try to do the C-section before you go into labor. Going into labor may trigger bleeding.

If you’re bleeding before 36 weeks of pregnancy:

  • Doctors will usually have you stay in the hospital on bed rest to see if the bleeding stops

  • Doctors will monitor your baby's heart rate

  • If the bleeding stops, you may be able to go home, but you’ll have to be ready to go back to the hospital quickly if you start bleeding again

  • If the bleeding doesn't stop or your baby's heart rate is abnormal, they'll do a C-section

If doctors think you need to deliver early, they may take a sample of the fluid around your baby (amniotic fluid). Doctors can run tests on the fluid to tell if your baby's lungs have grown enough for you to deliver.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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