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What is amniocentesis?

An amniocentesis is generally used to get a small sample of the amniotic fluid to identify chromosomal disorders and open neural tube defects (ONTDs). Sometimes, amniocentesis is performed to test the fetus for infection, fetal lung maturity or blood type. In the past, genetic testing focused on detecting chromosomal abnormalities, such as Down syndrome. Now, microarray testing can be requested on amniotic fluid to check for many other genetic defects that are too small to be detected by conventional tests.

An amniocentesis will generally be offered to you between 16 and 20 weeks of pregnancy, especially if you are at increased risk for chromosomal abnormalities. It will also be offered if you’ve had an abnormal maternal serum screening test, which indicates an increased risk for a chromosomal abnormality or neural tube defect. Sometimes amniocentesis is recommended later if there's a new problem because it can provide useful information and help you better prepare for delivery based on the risks or potential for complications.

The approach for offering an amniocentesis is no longer determined by maternal age; any pregnant woman can decide for herself that she would like to have an amniocentesis. During an amniocentesis, a long, thin needle is inserted through your abdomen into the amniotic sac to withdraw a small sample of the amniotic fluid for examination. The fluid contains cells from the fetus, which include genetic information. To perform an amniocentesis, your abdomen will first be cleaned with an antiseptic. Then, an ultrasound is used to direct the needle into the amniotic sac where a small sample of fluid is taken and sent to the lab for analysis. The amniocentesis procedure is brief and causes minimal discomfort. Local anesthetic is rarely needed.

If you are having twins or multiples, a sample is usually taken from each amniotic sac in order to get results for each baby. Depending on the position of your baby, placenta, amount of fluid, or your anatomy, sometimes the amniocentesis cannot be performed. The fluid is sent to a genetics laboratory so that the cells can grow and be analyzed. Results are usually available in about seven days to two weeks, depending on the types of tests requested.

Our approach

Determining whether or not to have genetic testing, such as amniocentesis, is a big topic of discussion for many who are expecting. If you wish to obtain more detailed information about the health and well-being of your baby and his or her genetics, our OB/GYN experts will offer an in-depth overview about your options during your first prenatal visit. While the options can be overwhelming, our expert team, which includes maternal fetal medicine specialists, genetics specialists and genetic counselors, will help guide you to make an informed decision.


Amniocentesis is a relatively safe procedure with little risk to the pregnancy. The risk of miscarriage is less than one in 500.

Also, if your blood type is Rh-negative (where there is a lack of the rhesus protein on your red blood cells), you will need to get a vaccine to prevent your immune system from making antibodies against the baby’s red blood cells. Your physician will discuss this with you in more detail should you need this vaccination.

How to prepare

You do not need to do anything special to prepare for an amniocentesis, but do plan the procedure on a day where you can take time off work to rest afterward.

What to expect after treatment

Strenuous activities and heavy lifting should be avoided for about 24 hours following an amniocentesis. You may feel some cramping during or after the procedure. If you experience any vaginal bleeding or amniotic fluid leakage, please contact your physician immediately.

After you receive the results of the test, you will be advised to return for post-test counseling if there is an abnormality. Sometimes your results will warrant additional testing. Our maternal fetal medicine and genetics experts will be there to help you interpret the results, counsel you and your family on your options and recommend next best steps for care.

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