An amniocentesis — also known as an amnio — is a type of prenatal test that can detect and diagnose chromosomal conditions. The results of the test can provide you with valuable information about your baby and help you make decisions about her health.

Not every expecting mother-to-be will need this test, however. Here's what you need to know about amniocentesis and how to determine whether it's right for you.

What is an amniocentesis?

An amniocentesis is a procedure that extracts amniotic fluid (i.e., the fluid that surrounds your baby) from the uterus. This fluid, which contains your baby's cells and other chemicals, can provide a wide range of information about her genetics, health and level of maturity. 

amnio test

Amniocentesis can detect several hundred genetic conditions, including Down syndrome, Tay-Sachs disease and sickle cell anemia. Unlike chorionic villus sampling (CVS), in which a sample of tissue is taken from the placenta, an amnio can also rule out neural tube defects such as spina bifida.

An amnio does not, however, detect every kind of abnormality, including cleft lip or palate. And it can't determine the severity of the problem.

Why is an amniocentesis performed?

Like all genetic testing, amniocentesis is optional. It is generally offered when:

  • Results of a screening test (the combined first trimester blood work, the quad screen or noninvasive prenatal testing, for example, or an abnormal ultrasound during an anatomic screen) turn out abnormal and you've missed the window for CVS (or opted not to get CVS because that test can't detect neural tube defects). Because amnio is a diagnostic test, it can give an accurate answer as to whether or not there actually is an abnormality. 
  • You already have a child with a condition that can be diagnosed with amnio, like Down syndrome or cystic fibrosis (CF).
  • You or your partner has a family history of a genetic condition like Tay-Sachs disease or sickle cell anemia (unless you've been screened and found out you're not carriers).
  • Toxoplasmosis, fifth disease, cytomegalovirus or another fetal infection is suspected.
  • You're 35 or older and want to rule out that your baby has Down syndrome, since there's a slightly higher risk of it in older moms (though if you've gotten reassuring screening results, you may opt to skip the amnio after you've consulted with your practitioner).

When is an amniocentesis done during pregnancy?

Amniocentesis is usually performed between week 15 or 16 and week 20, though it can be performed up until you give birth.[1] (It's not recommended before week 15 or 16 because there are risks involved, including miscarriage.)

Do amniocentesis tests carry risks?

The risks from amniocentesis — to both you and your baby — are rare. However, some complications that can occur include:

  • Miscarriage: With an experienced care provider, the risk that a second trimester amnio will result in a miscarriage is estimated at about 0.1 percent to 0.3 percent.[2]
  • Uterine infection: This is exceptionally rare.
  • Passing an infection to your baby: If you have an infection such as HIV, hepatitis C or toxoplasmosis, it might be transferred to your baby during the procedure.[3]
  • Rh problems: During the amnio, some of your blood may mix with your baby's (though this is rare). If you're Rh-negative and your baby is Rh-positive, you'll receive an injection of Rh immune globulin to prevent your body from producing antibodies that can harm your baby.
  • A leakage of amniotic fluid through the vagina
  • Cramps or spotting

What happens during an amniocentesis procedure?

You'll lie flat on your back for this procedure, with a drape placed over you, leaving just your belly exposed. Your abdomen will be cleaned with an antibacterial solution to prevent infection. An ultrasound will be performed to locate the position of the amniotic sac along with the fetus and placenta so your doctor can steer clear of them during the procedure.

A long, very thin, hollow needle is inserted into your abdomen and uterus, into the fluid-filled amniotic sac. Your practitioner will look for a spot away from the baby, where there is a good pocket of fluid to withdraw — about two tablespoons are all that's needed. (Don't worry, your body will quickly produce more amniotic fluid to replace what's withdrawn.) You may feel a sting from the needle, and some pain and cramping, but this is normal and tends to be mild.

If you're carrying multiples, your practitioner may need to perform the procedure more than once to take a separate sample from each baby. If you're Rh negative (which is determined through Rh factor testing), you'll receive an injection of Rh-immune globulin (RhoGAM) after amniocentesis to be sure the procedure does not result in Rh problems.

From start to finish, an amnio takes about 30 minutes (though most of the time even less), but most of that is the setup and ultrasound — the needle part only takes a couple of minutes. After the procedure, you'll be able to drive yourself home, though some doctors recommend that you have someone else there to drive you just to be on the safe side.

You'll probably be told to take it easy for a few hours to a full day. You may wish to avoid sex, heavy lifting, strenuous exercise and flying for the next one to three days.

How accurate are amniocentesis tests?

The results from an amniocentesis tend to be accurate. For example, the test is more than 99 percent accurate in diagnosing Down syndrome.

Interpreting your amniocentesis test results

Amnio results are available within one day to several weeks. Labs that offer the FISH — fluorescent in situ hybridization — method, which quickly counts the number of certain chromosomes within cells to get a faster result, can come back within a day or two. It's always followed by the usual chromosome analysis in the lab. Results of amnio that's performed any time during the last trimester to assess the maturity of the fetal lungs are available within a day.

More than 95 percent of the time, amnio will find nothing but a healthy baby. (And if you'd like to know the sex of your baby, amnio will give you that news, too.)

Although most of the conditions that are detected with amniocentesis can't be cured, the test can let parents know about them in advance. This gives parents the time to become informed about a condition, as well as make decisions about their baby's future health care or make the difficult decision not to continue the pregnancy.

When to call your doctor after an amniocentesis

Complications from an amnio are rare, but you should discuss them with your practitioner when considering whether or not to get the test. 

You might experience minor cramping, but if cramps become severe or persistent, call your practitioner. Also call if you notice that you're leaking amniotic fluid, bleeding or spotting or have developed a fever over 100.4 degrees Fahrenheit.

Chances are good, though, that you won't have to worry about any of those symptoms, and you'll feel like yourself again in no time.