What Is Amniocentesis?

An amniocentesis—sometimes called amnio—is a type of prenatal test used primarily to diagnose genetic abnormalities and birth defects, as well as neural tube defects. Armed with the results, you’ll be able to make important decisions regarding your baby’s health and your pregnancy.

“Practically-speaking, [an amniocentesis] involves placing a thin needle in the sac around [your baby], usually while watching on ultrasound for precision and safety, and then removing a small amount of amniotic fluid,” explains Amber Samuel, M.D., M.B.A., F.A.C.O.G., a board-certified maternal fetal specialist and medical director for Obstetrix-Maternal-Fetal Medicine Specialists of Houston.

Amniotic fluid is the liquid that surrounds your baby during pregnancy, and it contains some fetal cells with genetic information.

If your health care provider has suggested an amniocentesis, you may be wondering how safe this test is, why it’s used, and if it’s right for you. Below you will find the answers to these questions and more as we explore the details of amniocentesis, its accuracy, and its safety.

What Is Amniocentesis?

Amniocentesis is a prenatal procedure that’s usually performed between 15 and 20 weeks of pregnancy. It involves testing amniotic fluid to diagnose genetic conditions or birth defects.

Reasons for Performing Amniocentesis

Most commonly, amniocentesis is performed because a genetic or fetal abnormality is suspected, says Dr. Samuel. “This is most likely due to abnormal non-invasive blood test results in a patient, or an abnormality seen on prenatal ultrasound.”

Amniocentesis may be used to diagnose the following:

  • Chromosomal disorders caused by missing, damaged, or extra chromosomes, including trisomy 13 (Patau syndrome), trisomy 18 (Edwards syndrome), trisomy 21 (Down syndrome), and Turner syndrome
  • Genetic disorders resulting from gene mutations, such as cystic fibrosis, sickle cell disease, and Tay-Sachs disease
  • Neural tube defects, which include spina bifida and anencephaly

An amnio may also check for certain prenatal infections and Rh disease. Finally, your doctor may order an amnio to treat polyhydramnios (a build up of amniotic fluid).

Amniocentesis vs. Prenatal Screening Tests

Amniocentesis is called a prenatal diagnostic test because it determines whether your baby has a genetic disorder. On the other hand, prenatal screening tests, including blood tests and ultrasounds, can only tell you if your baby has a higher risk of a disorder.

In the past, amniocentesis was used to examine your baby’s lung development, but it’s rarely done for that purpose any longer, says Sammy Tabbah, M.D., a maternal fetal medicine physician with OhioHealth Maternal Fetal Medicine Group. “Part of the reasoning is that delivery indications have become better standardized over time and we would make that decision irrespective of what the test would show.”

Who Gets Amniocentesis?

The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant people be given the option of having prenatal tests, like an amniocentesis. That said, people who are recommended to get amniocentesis usually have one of the following conditions:

  • Advanced maternal age, because chromosomal condition risk increases after age 35
  • A family history of genetic conditions
  • A previous child or pregnancy with a birth defect
  • Abnormal prenatal screening test results
  • Unusual ultrasound findings

Sometimes the decision to get an amniocentesis is more about patient choice; parents want to determine if their baby’s chromosomes are normal, says Michael Cackovic, M.D., a maternal fetal medicine physician and clinical associate professor of Obstetrics and Gynecology at The Ohio State University Wexner Medical Center.

“But, sometimes parents wait until a baby is at risk and decide about the test based on that,” says Dr. Cackovic. “It’s a very personal choice that is about gathering important and useful information about the pregnancy.”

When Is Amniocentesis Performed?

Amniocentesis is the most commonly performed prenatal diagnostic test offered in the second trimester to pregnant people.1 Typically, it’s performed between 15 and 20 weeks of pregnancy. Having the test before 15 weeks could be harmful to a developing baby.2

That said, amniocentesis can also be done up until you give birth. “There is not a hard and fast upper limit when performing amniocentesis,” says Dr. Tabbah. “You just need enough time for the information [or test results] to come back, which is most often around one to two weeks ahead of delivery. If an infection is suspected amniocentesis can be done at any time, though.”

What Happens During an Amniocentesis Procedure?

Once you have consented to an amniocentesis and you understand the risks, you will likely be scheduled for the procedure. Amniocentesis can be performed by a general OB-GYN or a perinatologist, who specializes in maternal-fetal medicine. While every practitioner is different, you can usually expect the following things to occur during amniocentesis:

  1. You will be asked to lie on an exam table.
  2. Some providers apply a numbing medicine to your abdomen, while most do not. The numbing medication may cause more discomfort than the procedure itself. If this is something you want, you can talk to your provider about it.
  3. An ultrasound transducer shows the position of your baby and the placenta, so the provider can find a pocket of fluid to draw from where neither are located.
  4. The provider will clean your abdomen with an antiseptic.
  5. A thin needle will be inserted into your abdomen, and then through the uterus, to withdraw a small amount of amniotic fluid (about one ounce).
  6. Once the sample is removed, your baby’s heartbeat will be checked with the ultrasound.

The entire procedure will take about 30 minutes, according to the Cleveland Clinic, but the needle is only inside for about one or two minutes.

What to Expect After Amniocentesis

After amniocentesis, some people may experience cramping. There also is a chance of spotting or increased discharge, says Michael Platt-Faulkner, D.O., an OB-GYN at St. Elizabeth Healthcare. Most people do well with over-the-counter painkillers and rest following the procedure, though.

You’ll likely be told to avoid any hard exercise or physical activity for the next 24 hours, says Dr. Tabbah. “Typically, I tell families to take the day off work and rest the day of the procedure. They can go back to their normal activities the day after.”

You also should watch for any abnormal side effects, though these are rare. For instance, if you experience any contractions, you will want to let your provider know, says Dr. Tabbah. You also should contact them for the following symptoms after amniocentesis:

  • Bleeding or spotting
  • Leakage or gush of fluids
  • Extreme abdominal pain or cramping
  • Fever
  • Changes in discharge
  • Differences in your baby’s movement

Risks of Amniocentesis

While amniocentesis is a safe and highly accurate procedure, it does come with some risks, including the following:

  • Miscarriage. There is a very low chance risk of miscarriage (less than 1%) after amnio. But, when compared to losses in pregnancies without amniocentesis, the risk is about the same.
  • Uterine infection
  • Leaking amniotic fluid, which happens in about 2% of cases3
  • Vaginal bleeding, which occurs in about 2% to 3% of cases3
  • Transmission of a blood-borne infections, such as hepatitis or HIV, to your baby
  • Preterm labor. “As you get closer to when the baby can survive outside of the uterus, the risk becomes preterm labor,” says Dr. Cackovic.
  • Rh sensitization. If the baby’s blood gets in the pregnant person’s bloodstream, and their blood types are different, complications can occur.
  • Needle injury to the baby, which is very rare

“Risks are a difficult thing to study well because people undergoing this procedure are more likely to have a pregnancy loss due to the reason that is prompting the procedure in the first place,” says Dr. Samuel. “There also are cases when the water is broken (also rare) but because the entry is small, that opening may seal in the future so the pregnancy may still continue normally.”

How to Prepare for Amniocentesis

Usually, you do not need to do anything special to prepare for amniocentesis. You can eat and drink as normal beforehand, but if you are taking any blood thinning medications, you may be asked not to take those the day of the procedure, says Dr. Samuel.

The biggest challenge some people face is coping with their anxiety surrounding the procedure and the possible results. But Dr. Cackovic reminds patients that amniocentesis is a safe procedure. “It will help you by ruling out bad things and allowing you time to accept things you were not expecting. That is an important thing because it allows you to have a better second half of pregnancy.”

Interpreting the Results of Amniocentesis

After your amnio is done, the sample is sent to a lab. Fetal cells in the amniotic fluid are grown and cultured and the chromosomes are analyzed for abnormalities. Most of the time, you’ll learn about results in 10 to 14 days. “It really just depends on what is being studied,” says Dr. Samuel. “Sometimes preliminary results may be available as soon as 48 hours or may take up to a few weeks.”

How Accurate Is Amniocentesis?

Amniocentesis is about 99% accurate at diagnosing genetic abnormalities or birth defects, though it doesn’t indicate the severity of these conditions. Amniocentesis can’t diagnose all birth defects and genetic diseases because some conditions can’t yet be detected prenatally.

Here are some possible results you may get following an amniocentesis:

  • Normal: If you get a normal result, this means that the test did not find any evidence of the conditions your baby was being screened for.
  • Abnormal: If you get an abnormal result, this means that the test identified a genetic difference in your pregnancy. When this occurs, your health care provider will discuss both the results and your options.
  • Uncertain: This result indicates that a genetic difference was found, but its impact on your pregnancy is not clear. Your health care provider will discuss the next steps with you and may recommend additional testing.

While you wait on the test results, try not to dwell on the “what ifs.” After all, there is a pretty good chance your results are going to come back normal. In fact, Dr. Cackovic indicates that their group does about 50 amnios a day and only about 2% come back with abnormal results.

Amniocentesis vs. CVS: What’s the Difference?

Chorionic villus sampling (CVS) is a diagnostic test that evaluates cells from the placenta that contain the same genes as the fetus. The advantage of CVS is that it is done in the first trimester, typically between 10 and 13 weeks, which gives you more time to process the results.

Although CVS can detect genetic disorders and chromosomal abnormalities, it can’t provide information about neural tube defects. If you have CVS, you might still need a blood test at 16 to 18 weeks to screen for neural tube defects, and you may be advised to have a follow-up amniocentesis.

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