Amniocentesis

What is amniocentesis?

Amniocentesis (also called amnio) is a prenatal test that takes amniotic fluid from around your baby in the uterus (also called womb). The fluid is tested to see if your baby has certain health conditions. A prenatal test is a medical test you get during pregnancy. Talk to your health care provider to see if amnio is right for you.

Amnio is a diagnostic test. This means that it tells you for sure whether or not your baby has a health condition. This is different from a screening test that tells if your baby’s more likely than others to have a certain condition. If results of a screening test aren’t normal, you may want to have a diagnostic test like amnio to diagnose (find out) if your baby has a condition. Amnio results almost always are correct.

How do you know if you need an amniocentesis?

The American College of Obstetricians and Gynecologists (also called ACOG) recommends that all pregnant women have the choice of having prenatal tests, like an amnio. You may want to have an amnio if your baby is at risk for certain conditions like:

• Birth defects. These are health conditions that are present at birth. Birth defects change the shape or function of one or more parts of the body. They can cause problems in overall health, in how the body develops or in how the body works. Your provider can use amnio to diagnose certain birth defects, like birth defects of the brain and spine called neural tube defects (also called NTDs). Examples of NTDs are spina bifida and anencephaly. Amnio doesn’t check for every birth defect. For example, it can’t check for certain heart problems or birth defects in a baby’s lip or mouth called cleft lip and palate. Some birth defects are genetic (caused by changes in genes).
• Genetic and chromosomal conditions. These conditions are caused by changes in genes and chromosomes. A gene is part of your body’s cells that stores instructions for the way your body grows and works. Chromosomes are the structures in cells that hold genes. Genetic conditions include cystic fibrosis (also called CF), sickle cell disease and heart defects. A common chromosomal condition is Down syndrome. Sometimes these conditions are passed from parent to child, and sometimes they happen on their own.

If your baby’s at risk for having these conditions, you may have an amnio at 15 to 20 weeks of pregnancy. It’s not recommended before 15 weeks because it has a higher risk of miscarriage and other complications. Miscarriage is when a baby dies in the womb before 20 weeks of pregnancy.

Later in pregnancy, you may have an amnio to:

• Check your baby’s lung development (also called fetal lung maturity). Your provider may recommend amnio to check your baby’s amniotic fluid and find out if your baby’s lungs are developed for birth. This kind of amnio is only done if you need to give birth early to help prevent pregnancy complications. It’s usually done between 32 and 39 weeks of pregnancy.
• Check for infections or other health conditions in your baby. For example, if your baby’s at risk of Rh disease, your provider may use amnio to check your baby for anemia. Rh disease is a dangerous kind of anemia that’s preventable if it’s treated during pregnancy. Anemia is when a person doesn’t have enough healthy red blood cells to carry oxygen to the rest of the body.  
• Treat polyhydramnios. Polyhydramnios is when you have too much amniotic fluid. Amniotic fluid is the fluid that surrounds your baby in the womb. Polyhydramnios may increase your risk of having pregnancy complications, like premature birth (birth before 37 weeks of pregnancy). Your provider can use amnio to drain extra fluid from the womb.

Are you at risk of having a baby with a birth defect or a genetic or chromosomal condition?

You may be at increased risk of having a baby with one of these conditions if:

• You’re 35 or older. The risk of having a baby with a chromosomal condition, like Down syndrome, increases as you get older.
• You have a child with a birth defect or you had a previous pregnancy with a birth defect. Having a birth defect in a previous pregnancy makes you more likely to have a birth defect in another pregnancy.
• You have a family history of a genetic condition. If you, your partner or a member of either of your families has a genetic condition, like CF, fragile X syndrome, sickle cell disease, Tay-Sachs or thalassemia, you may want to have an amnio to find out if your baby also has the condition. You can get carrier screening for genetic conditions before or during early pregnancy. Carrier screening test checks your blood or saliva to see if you’re a carrier of certain genetic conditions that could affect your baby. If you’re a carrier, you don’t have the condition yourself, but you have a gene change for it that you can pass to your baby. If both you and your partner are carriers of the same condition, the risk that your baby has the condition increases.
• Your prenatal screening tests results are abnormal. There are no risks to you or your baby when you have a screening test, but it doesn’t tell you for sure if your baby has a health condition. A diagnostic test, like amnio, can diagnose a condition. If you have abnormal results from a screening test, like first-trimester screening or cell-free DNA testing, you may want to have a diagnostic test, like amnio.

Having an amnio is your choice, even if you’re at risk of having a baby with a birth defect or genetic condition. Talk with your partner, provider and a genetic counselor about your testing options. A genetic counselor is a person who is trained to help you understand about genes, birth defects and other medical conditions that run in families, and how they can affect your health and your baby's health. You also may want to talk to your religious and spiritual leaders to help you make decisions about testing for birth defects and genetic conditions during pregnancy.

Ask your provider about other prenatal tests and about seeing a provider like a maternal-fetal medicine specialist. This is an obstetrician with education and training to take care of women who have high-risk pregnancies. You can contact the Society for Maternal-Fetal Medicine to find a specialist in your area.

How is amnio done?

Here’s what happens when you have an amnio:

1. You lie on your back on an exam table, and your provider covers your belly with a thin layer of gel.
2. Your provider moves an ultrasound wand (also called transducer) across your belly to find your baby and the placenta. The placenta grows in your uterus and supplies your baby with food and oxygen through the umbilical cord. Ultrasound uses sound waves and a computer screen to show a picture of your baby inside the womb.
3. Your provider cleans your belly with an antibacterial liquid that kills germs on your skin.
4. Using ultrasound as a guide, your provider puts a thin needle through your belly and uterus into the amniotic sac. The amniotic sac (also called bag of waters) is the sac (bag) inside the uterus that holds your growing baby. It’s filled with amniotic fluid. 
5. Your provider uses the needle to remove a small amount of amniotic fluid (less than 1 ounce). The amniotic fluid contains cells from your baby.
6. Once the fluid sample is taken, your provider uses the ultrasound to check that your baby’s heartbeat is healthy.

Your provider sends the sample to a lab where your baby’s cells are separated from the amniotic fluid. The cells grow for about 10 to 12 days at the lab and then they're tested for birth defects and genetic conditions. The lab also can test the amniotic fluid for proteins like alpha-fetoprotein (also called AFP). AFP levels are often higher if your baby has a neural tube defect. Test results usually are available within 2 weeks.

If amnio shows that your baby has a health condition, talk to your provider about your options. For example, your baby may be able to be treated with medicines or surgery before or after birth. Knowing about a birth defect before birth may help you get ready to care for your baby. And you can make plans for your baby’s birth with your provider to make sure your baby gets special care or treatment he may need right after he’s born.

Are there any risks to having an amnio?

Serious complications from amnio are rare. Most women have only mild discomfort during an amnio. You may have a stinging feeling when the needle enters your skin, feel cramping when the needle enters the uterus or feel pressure when the fluid is removed. After the test, your provider may tell you to take it easy for the rest of the day and not to exercise or have sex for a day or two.

Some women may have other complications from amnio, including:

• Miscarriage. Less than 1 in 200 women (less than 1 percent) have a miscarriage after an amnio.
• Infection in the uterus
• Cramping, spotting or leaking amniotic fluid. About 1 to 2 in 100 women (1 to 2 percent) have these problems.
• Passing infection to your baby. If you have an infection, like HIV or toxomplasmosis, you may pass it to your baby during amnio. HIV is the virus that causes AIDS. Toxoplasmosis is an infection you can get from eating undercooked meat or touching cat poop.
• Rh problems. Amnio may cause a small amount of your baby’s blood to mix with your blood. If you’re Rh-negative and your baby is Rh-positive, you may get a shot called Rh immune globulin after amnio to help protect your baby.

If you have any of these signs or symptoms after an amnio, call your provider:

• Feeling a change in your baby’s movement
• Bleeding or leaking fluid from your vagina
• Fever
• Redness and swelling where your provider inserted the needle
• Strong belly cramps that last more than a few hours

Last reviewed: May, 2017