Amniocentesis (amniotic fluid test)

What is amniocentesis?

Amniocentesis is a test done during pregnancy. A sample of amniotic fluid is taken from your abdomen (belly). It is used to diagnose certain genetic disorders, birth defects, and other conditions in your unborn baby. The test is usually done between weeks 15 and 20 of pregnancy.

Your baby grows in an amniotic sac inside your uterus. The sac is filled with amniotic fluid, a clear or pale yellow liquid. The fluid that surrounds and protects your baby throughout pregnancy. It also helps with your baby’s development and helps keep your baby at the right temperature. The fluid contains cells that can provide important information about your baby’s health.

Amniocentesis is a diagnostic test. That means it will tell you whether your baby has a specific health problem, and the results are almost always accurate. Diagnostic tests are different from prenatal screening tests for genetic disorders. Prenatal screening tests have no risks for you or your baby, but they don’t diagnose problems. They can only show if your baby might have a health problem. If a screening test shows your baby could have a problem, your health care provider may recommend amniocentesis or another diagnostic test to find out for sure.

Other names: amniotic fluid analysis

What is it used for?

Amniocentesis is used to diagnose certain health problems in an unborn baby. It is commonly used to find:

• Genetic and chromosomal disorders, including:
  • Down syndrome, a disorder that causes delays in physical and mental development and other health problems.
  • Cystic fibrosis, a disease of the mucus and sweat glands that causes thick sticky mucus, which can lead to problems with breathing and digestion.
  • Sickle cell disease, a group of red blood cell disorders that can cause anemia and other health problems.
  • Tay-Sachs disease, a disease that destroys nerve cells, causes mental and physical problems, and often death in early childhood (uncommon).
• Neural tube defects, severe birth defects of the baby’s brain and/or spine, such as spina bifida and anencephaly.

The test may also be used to:

• Check your baby’s lung development if you have a risk of giving birth too soon (premature birth). In this case, amniocentesis is done later in your pregnancy.
• Diagnose an infection or certain other illnesses in the baby.

Why do I need amniocentesis?

Having amniocentesis is your choice. You may want this test if you have a higher risk of having a baby with a health problem. You may have a higher risk if:

• You are age 35 or older; the risk of having a baby with a genetic disorder increases with age
• You had a prenatal screening test that showed your baby might have a problem
• You or your partner have a family history of a genetic disorder or neural tube defect
• You or your partner had genetic testing that showed you carry a genetic disorder
• You or your partner have a child with a genetic disorder or birth defect

Amniocentesis isn’t right for everyone. Before you decide to get tested, think about how you might feel and what you might do after learning the results.

What happens during amniocentesis?

The test is usually done between 15 and 20 weeks of pregnancy. It is sometimes done later in pregnancy to check the baby’s lung development or diagnose certain infections or illnesses, such as anemia in the unborn baby caused by Rh incompatibility.

During the procedure:

• You’ll lie on your back on an exam table.
• Your provider will apply a gel to your belly.
• Your provider will move an ultrasound wand-like device, called a transducer, on your belly. Ultrasound uses sound waves to show the position of your baby and placenta so your provider can see where to take a sample of amniotic fluid.
• Your provider will clean your belly.
• Your provider will insert a thin needle into your belly, withdraw a small amount of amniotic fluid, and then remove the needle.
• When the sample is removed, your provider will check your baby’s heartbeat with the ultrasound.
• The sample is sent to a lab for testing. Results may take from a few days to a few weeks.

The procedure usually takes about 15 minutes. Afterward, you may be told not to exercise or have sex for a day or two.

Will I need to do anything to prepare for the test?

If you are early in your pregnancy, you may be asked not to urinate (pee) before the test to make sure that you have a full bladder. This helps move the uterus into a better position for the test. In later pregnancy, you need to have an empty bladder for the test.

Are there any risks to the test?

You may feel mild stinging, pressure, and/or cramping during the test. Afterward, you may have mild discomfort or vaginal bleeding, or a small amount of amniotic fluid may leak from your vagina.

The risk of serious complications is extremely low. Your provider will let you know if there are any symptoms that you should tell them about. These may include ongoing vaginal bleeding, amniotic fluid leakage, severe cramping, or a fever.

Amniocentesis does have a very slight risk (less than 1 percent) of causing a miscarriage.

What do the results mean?

Amniotic fluid may be tested for many different disorders. Your test results will depend on which tests your provider ordered.

• Normal results are reported as “normal” or “negative.” This means that it’s very unlikely that your baby has the disorder that was tested, but it does not guarantee your baby will not have any health problems.
• Results that are not normal are reported as “abnormal” or “positive.” This means that your baby very likely has the disorder that was tested.

Your provider will explain your test results. Amniocentesis is very accurate, but in certain cases, your provider may order more tests to learn about your baby’s health.

It may help to speak to a genetic counselor before testing and/or after you get your results. A genetic counselor is a specially trained professional in genetics and genetic testing who can help you understand what your results mean.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about amniocentesis?

In some cases, you may want information about your unborn baby sooner than 15 to 20 weeks, when amniocentesis is usually done. In that case, you might consider another diagnostic test called a chorionic villus sampling (CVS). This test takes a small sample of tissue from the placenta.

You can have a CVS test between 10 and 13 weeks of pregnancy, which gives you more time to decide what to do if the results are abnormal. Like amniocentesis, CVS is an accurate diagnostic test that can find certain genetic disorders. But it has a slightly higher risk of miscarriage.

As you make decisions about testing, remember that prenatal genetic diagnostic tests are different from screening tests. Screening tests estimate the chances that your unborn baby may have certain problems. It’s possible to have an abnormal screening test when your baby is actually healthy. But if a screening test shows that your baby has a risk for certain problems, you may have amniocentesis or CVS to find out for sure.