Everything You Need To Know About Amniocentesis

You may have heard of someone getting an amniocentesis or “amnio” during pregnancy to test for problems with the baby. An amniocentesis procedure removes a small amount of fluid from the amniotic sac (your “bag of water”) to test for problems in your baby. But when is this necessary? And how do you know if you need to do this test?

Here we’ll discuss what an amniocentesis is and why some people may opt for this test during pregnancy.

What are the most common reasons for having an amniocentesis?

The most common reason for having an amniocentesis is to check for a chromosome problem in your baby. Chromosomes carry the genes that a baby gets from its parents. Sometimes the baby gets a flawed chromosome or the wrong number of chromosomes, which can lead to a genetic problem (like Down syndrome). An amniocentesis that looks for genetic problems is a “genetic amniocentesis.” Anyone can opt to have a genetic amniocentesis, but healthcare providers usually offer it to those with a high risk of having a genetic problem. 

Some reasons for getting an amniocentesis include:

• Your first or second trimester screening tests are abnormal. An amniocentesis will confirm if there’s really a problem.

• You’ve had a baby in the past with a chromosome problem.

• You or your partner carry the gene for genetic diseases like cystic fibrosis or sickle cell disease.

• You’re 35 years old or older. Babies born to women over age 35 have a higher risk of having chromosome problems.

• You have an abnormal finding on your ultrasound. 

Other, less common reasons to have an amniocentesis are for paternity testing and when you have too much amniotic fluid and it’s causing a problem with your pregnancy. Other reasons include checking to see if:

• Your baby’s lungs are mature (in preparation for delivering before your due date)

• There’s an infection in your uterus

• Your baby is at risk for anemia due to Rh disease 

How does amniocentesis work?

During an amniocentesis, your provider removes a small amount of fluid from the amniotic sac. This fluid contains cells from the baby’s skin. It’s an ultrasound-guided procedure, which means your doctor looks with an ultrasound during the entire procedure.  

First, your provider finds a good pocket of fluid to sample with the ultrasound. Then they’ll clean the skin on your belly to help prevent infection. Your provider then passes a long, thin needle through your skin and uterus into the pocket of fluid. They’ll remove a small amount of fluid through the needle. Usually testing needs about 3 or 4 teaspoons of fluid. Your provider uses the ultrasound to keep the needle within the pocket of fluid and away from the baby. After removing the needle, your provider uses the ultrasound to check the baby's heart rate. The actual procedure takes only a couple minutes. 

After the procedure, they’ll send the fluid sample to the lab for testing, where they use the skin cells to check the baby's chromosomes. If the amniocentesis is for a different reason than genetic testing, the lab will check for those indications.

When is an amniocentesis usually performed?

The timing of an amniocentesis depends on why it’s being performed. In general, providers can do the procedure anytime after about 15 weeks of pregnancy. Before 15 weeks, your pregnancy may not be far enough along yet to perform the procedure safely.

A genetic amniocentesis to check a baby's chromosomes occurs between 15 and 20 weeks of pregnancy.

Timing of amniocentesis for other reasons depends on the specific need. For example, to test for lung maturity, providers usually perform the amniocentesis after 32 weeks. Testing for infection or anemia occurs when your provider begins to suspect a problem.

How long does it take to get results back from an amniocentesis?

At the time of the procedure, your provider can tell you when to expect results. In general, full results of a genetic amniocentesis can take up to 7 to 10 days. It takes this long because they have to wait for the cells to grow before they can look at the baby’s chromosomes.  

Fetal lung maturity results are usually available within a few hours. Similarly, when testing for infection, partial results are usually available within a few hours. Complete results take about 1 to 2 days. 

How painful is an amniocentesis?

Pain is different for everyone. Amniocentesis may cause temporary discomfort — but not lasting pain. You will feel a pin prick as the needle passes through your skin. The needle may cause some cramping as it goes through the uterus. This usually fades pretty quickly after the provider removes the needle. 

Some providers numb the outer skin before the procedure. If so, you’ll still feel the pin prick of the needle and some burning or tingling as the medication goes in. Unfortunately, numbing medication only numbs the outer skin and not the deeper layers. It doesn’t have any effect on the feeling of the needle passing through the uterus. Because it’s such a quick procedure, many providers skip the extra step (and needle prick) of using numbing medication. 

It’s completely normal to worry about your own comfort level during and after the procedure. If you have any concerns, it’s a good idea to discuss them with your provider. 

How risky is an amniocentesis?

In general, an amniocentesis is a very safe procedure. The most common effect is mild cramping afterward. You may also have some vaginal spotting. These effects usually go away on their own within a short time after the procedure. 

During an amniocentesis the needle punctures the amniotic sac so it creates a small hole. You may experience some leaking of amniotic fluid after the procedure. This is a rare complication because the sac typically reseals itself within hours of the procedure. But if it doesn’t, or if bacteria from the skin gets into the amniotic sac and causes an infection, there’s a small chance that it can lead to a miscarriage. About 1 in 900 pregnant people who get a genetic amniocentesis has a miscarriage. 

If you get an amniocentesis later in pregnancy, it can cause you to start labor early or break your amniotic sac. These complications are also rare.

What are the best testing alternatives to a genetic amniocentesis?

Chorionic villus sampling (CVS) is the only other test that can confirm a genetic problem in the baby during pregnancy. Providers only do CVS procedures between 10 and 13 weeks of pregnancy. It uses cells from the placenta (the afterbirth) to look at the baby’s chromosomes. Though it can provide information earlier in a pregnancy, CVS has a higher miscarriage risk.  About 1 in 455 has a miscarriage after a CVS procedure.

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Written by:

Andrea Kemp, MD, MPH

Dr. Kemp is an accomplished board-certified physician with over 25 years of clinical experience. She completed medical school and residency training in obstetrics and gynecology in Cincinnati, Ohio.

Edited by:

Mandy Armitage, MD

Mandy Armitage, MD, has combined her interests in clinical medicine with her passion for education and content development for many years. She served as medical director for the health technology companies HealthLoop (now Get Well) and Doximity.

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