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Amniocentesis (AM-nee-oh-sen-TEE-sis) — or amnio for short — is a medical test for pregnant women. It is usually performed between the 16th and 22th weeks of pregnancy to see if the baby has certain conditions, such as Down syndrome.
Amniocentesis is a genetic test. Genetic tests are used to look for certain genetic diseases, chromosome problems, and birth defects. Your doctor and/or a genetic counselor can help to determine which test or tests are right for you. This depends on factors such as your age, your family and medical history, and environmental exposures. Genetic testing cannot answer all questions about the health of your developing baby, but it can provide information to help you and your partner make informed decisions.
Each cell of your body has chromosomes that carry the blueprint for your whole body. The chromosomes carry the traits (such as eye or hair color) that you inherited from your mother and father. Normally, each cell contains 46 chromosomes. Having too much or too little chromosome material often results in health problems.
To find out if a baby has a chromosome problem before birth, we test the fluid around the baby. This fluid is called amniotic (am-nee-OT-ik) fluid.
The baby sheds some cells, with the baby’s chromosomes in them, into this fluid. Careful testing of the fluid finds the cells and looks for problems in them.
Brain and spine problems, such as open skull (anencephaly) and open spine (spina bifida), also can be found by measuring the level of AFP (alpha-fetoprotein) in the fluid.
A sample of the fluid is taken with a thin needle. The needle is guided by ultrasound (see the UPMC patient education page Ultrasound: Pelvic, Pregnancy, and Abdominal). Ultrasound uses sound waves to produce a “picture” of the baby. It appears as an image on a small TV screen (monitor). Ultrasound helps the doctor determine how many weeks the pregnancy is along. It shows if there is more than one baby. Also, it shows where the placenta is located in the uterus.
For the ultrasound, you will lie on your back on a table in an exam room. A gel will be spread on your belly. The gel helps the sound waves pass through your belly. You will probably be able to see your baby’s image on the screen.
A thin needle is placed through your belly and through the wall of your uterus. The needle takes a small amount of amniotic fluid from the sac surrounding the developing baby. The ultrasound is used to show where the baby and placenta are so the needle can be placed where it won’t touch the baby or placenta.
Amniocentesis, including ultrasound, takes less than 1 hour. You will not have to stay in the hospital overnight.
You should take it easy the rest of the day but may go back to your usual activity the next day. You will be told to watch for any unusual occurrence. If you have bleeding or loss of fluid through the vagina, severe cramping, or a fever, call your doctor.
The fluid will be tested in a lab. Results from the test are usually available in 10 to 14 days. Ask your doctor when you should call his or her office for your results.
As with many medical procedures, there are risks with amniocentesis. Recent studies suggest that the risk of miscarriage isapproximately 1 pregnancy loss in every 1,000 procedures performed. Before agreeing to have amniocentesis and genetic testing, you will be asked to sign a consent form that explains the risks involved.
No. It is important to remember that all couples have a 2 to 3 percent risk of having a child with a birth defect. This means that out of every 100 pregnancies, on average, 97 or 98 babies will have no birth defects, and 2 or 3 babies will have a birth defect. Many of these birth defects are not due to a chromosome disorder. So amniocentesis would not find them.
If a disorder is found, your doctor will explain your options. At UPMC, counseling and support are available to help the couple make an informed decision. Several medical options are available, including referral to experts who are familiar with the treatment and management of specific conditions.
If you have additional questions or think you may want amniocentesis, please call the UPMC Center for Medical Genetics at 412-641-4168 or 1-800-454-8155.
Revised September 2011