Amniocentesis Prenatal Test

If you’re wondering what amniocentesis is and why it may be recommended, you’re not alone. Many expectant parents are curious about this optional procedure and want to know whether they need it, what it tests for, and what the risks are. Take a look at some of the most frequently asked questions about amniocentesis, but keep in mind that ultimately your healthcare provider will be able to help you weigh your options based on your situation. And, for more helpful information about what to know and prepare for before your little one arrives, download our Pregnancy Guide.

What Is Amniocentesis?

Amniocentesis, also known as amnio, is a common prenatal test that involves removing a little amniotic fluid from the uterus for use in either testing or treatment. Amniotic fluid surrounds your baby in the amniotic sac. Analyzing this fluid, which contains fetal cells, can provide crucial information about your baby’s health, and can help your doctor diagnose or — more often — rule out certain birth defects and genetic or chromosomal disorders. Amnio is also used to assess lung maturity. In the lab, different tests are performed on the fluid based on what the underlying concern is.

Prenatal screening and diagnostic tests are offered to all pregnant women, but because amniocentesis carries small risks, some doctors may prefer to offer it to parents if there is an increased risk of chromosomal or genetic conditions. In general, screening tests (like ultrasound exams and carrier tests for parents) give your doctor information about the chance your baby has a condition. Diagnostic tests, like amniocentesis, give doctors more definite information about whether your baby actually has certain conditions.

Amniocentesis is optional, and there is no right or wrong choice about having the test. Some parents-to-be would like to have as much additional information as possible, while others would prefer not to know. In some situations, having more information at an earlier stage may help you plan your child’s healthcare in advance, and in a small number of cases, doctors may be able to treat certain disorders before birth. Your doctor will review the risks and benefits of the procedure, based on your situation, to help you make a decision you’re comfortable with.

Amniocentesis is sometimes used as a treatment. If you have too much amniotic fluid (a condition known as polyhydramnios), for example, your doctor may recommend amnio as a way to drain the excess fluid.

What Does Amniocentesis Test For?

Amniocentesis can be used to assess many aspects of your baby's health, including:

  • Some genetic and chromosomal disorders and diseases: Amniocentesis cannot detect all potential conditions, but it can test for things like

    • Down syndrome

    • cystic fibrosis

    • sickle cell disease

    • Tay-Sachs disease

    • muscular dystrophy

  • Fetal lung maturity. Amniocentesis can help determine whether your baby's lungs are mature enough for birth.

  • Fetal anemia: If your baby has Rh sensitization, amniocentesis may be used to determine the severity of anemia.

  • Fetal infection: The test may be recommended if your baby is considered at a higher risk of infection.

  • Paternity testing: Amniocentesis may be used to obtain the baby's DNA for prenatal paternity testing.

The fetal lung maturity test may be recommended if an early induction or cesarean section is being considered in a non-emergency situation. In this case, amniocentesis is typically recommended between 32and 39 weeks of pregnancy to see if the baby's lungs are ready for birth.

Amniocentesis can be recommended for other reasons, too. For example, it may be suggested to some moms-to-be in the third trimester to diagnose uterine infections. It can also be recommended to check on the severity of anemia in a baby who has Rh disease.

Why Might My Doctor Recommend This Test?

There are many reasons your doctor may recommend amniocentesis. The genetic amniocentesis test may be recommended for the following reasons:

  • Abnormal results from a prenatal screening test or ultrasound. If your doctor is concerned about the result of a prenatal screening test, amniocentesis can confirm or rule out some conditions.

  • You or your partner have had a child or pregnancy with a chromosomal condition or a neural tube defect. Amniocentesis can confirm or rule out potential conditions in your current pregnancy.

  • You or your partner’s family history. If you or your partner have a family history of birth defects or genetic disorders, or if either of you are carriers of a genetic condition, the test may be recommended.

  • You’re older than 35. Maternal age can increase the risk of chromosomal conditions for the baby.

The fetal lung maturity test may be recommended if an early induction or cesarean section is being considered in a non-emergency situation. In this case, amniocentesis is typically recommended between 32and 39 weeks of pregnancy to see if the baby’s lungs are ready for birth.

Amniocentesis can be recommended for other reasons, too. For example, it may be suggested to some moms-to-be in the third trimester to diagnose uterine infections. It can also be recommended to check on the severity of anemia in a baby who has Rh disease.

What Do the Results Mean?

Most of the time, the results will be normal (negative). Keep in mind, the genetic test can reveal some genetic or chromosomal conditions, but not all. The results of amniocentesis are nearly 100 percent accurate, and your healthcare provider will help you understand the outcome. If the results are positive, your doctor can help you understand your options and may also refer you to a specialist. There may also be support groups available, so ask your provider to put you in touch if this is something you might like. In some cases, you may be referred to a prenatal genetic counselor who can help you understand what to expect, how to prepare, and what your options are. The fetal lung maturity amniocentesis may help reassure you and your medical team that your baby’s lungs are mature enough for an early delivery.

What Are the Risks of Amniocentesis?

Serious complications are rare, but there are risks associated with amniocentesis, which your doctor can help you evaluate. Risks include:

  • Leaking amniotic fluid. In rare cases, a small amount of amniotic fluid leaks out through the vagina. Most often, the leak stops within a week, and the pregnancy continues normally.

  • Needle injury to baby. Sometimes the baby’s arm or leg might move into the path of the needle, but serious injuries are uncommon.

  • Rh sensitization. Occasionally, if the baby's blood cells enter the mother's bloodstream during the procedure, the mother’s body may start producing antibodies that can pass through the placenta and damage the baby’s red blood cells. The doctor can test to check whether the mom-to-be’s body is developing antibodies and can administer Rh immune globulin to prevent the mother’s body from making Rh antibodies.

  • Triggering a uterine infection. This is rare, and occurs in fewer than 1 in 1,000 cases.

  • Transmitting an infection to the baby. Amniocentesis may not be recommended if the mother has an infection such as hepatitis B or C, toxoplasmosis, or HIV/AIDS.

  • Miscarriage. There is only a slight risk of miscarriage — about 0.6 percent — if amniocentesis is carried out in the second trimester. The risk is higher if the procedure is done before 15 weeks of pregnancy.

What Happens During the Procedure?

Genetic amniocentesis is typically performed between 15 and 20 weeks of pregnancy. If performed any earlier, there is a higher risk of complications. It is sometimes also performed during the third trimester. Ask your doctor to confirm how to prepare. Typically, if you’re having the test before 20 weeks of pregnancy, experts recommend having a full bladder to help support the uterus, so you'll need to drink lots of water beforehand. If you’re having the test after 20 weeks of pregnancy, an empty bladder may be recommended to help avoid a puncture.

During the test, your provider will ask you to lie on your back on a table. An ultrasound will help your doctor locate your baby within the uterus, and your abdomen will be swabbed with antiseptic. Generally, an anaesthetic isn’t used as most women find it’s only mildly uncomfortable or not painful at all. Using the ultrasound as a guide, the doctor inserts a thin needle through your abdominal wall and into the uterus. A small amount of amniotic fluid (about 1 ounce or 30 milliliters) is removed. You’ll need to lie nice and still, and you might feel some mild cramping during and shortly after the procedure. The doctor will monitor your baby’s heartrate via the ultrasound for a little while after the test. The whole procedure takes only a few minutes, and results are usually available within days or weeks — depending on the exact conditions you’re testing for.

Are There Any Side Effects?

It’s generally best to go home and relax after amniocentesis and to avoid strenuous activities for a day or so. You might also experience mild cramping and pelvic discomfort. Leakage of amniotic fluid and slight bleeding can both occur, but usually stop on their own. Contact your healthcare provider if you

  • have unusual vaginal discharge or bleeding

  • feel severe cramping that goes on for more than a few hours

  • have a fever

  • see redness and inflammation at the spot where the needle went in

  • notice your baby is moving unusually or that there is a lack of movement (keep in mind that, depending on how far along you are, it may be too early to feel your baby move).

FAQS at a Glance

How do I know if amniocentesis is right for me? After consulting with your healthcare provider and a genetic counselor, who will explain the procedure, its risks, and its benefits to you, the choice to pursue genetic amniocentesis is ultimately up to you. The test may be suggested later in pregnancy to asses fetal lung maturity if an early delivery is planned.

What birth defects can amniocentesis NOT check for? Amniocentesis can only check for genetic and chromosomal abnormalities, so heart conditions and physical deformities like cleft palates will not be detected.

How do I know if I need to have amniotic fluid drained (polyhydramnios)? Your healthcare provider can measure the amount of fluid in your uterus during an ultrasound and will let you know if some amount needs to be drained. Polyhydramnios can come with some abdominal pain and shortness of breath, but these symptoms are common in pregnancy and not always indicative of excessive amniotic fluid.

Like all parents, you want what’s best for you and your baby, and it can be tricky to weigh your options. Your healthcare provider is the expert, and you can also discuss your situation with people you trust when deciding if amniocentesis is something you want to do. Remember, this is an optional test that can provide you with more information, but it’s important to do what feels right for you.