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Prenatal Testing for Down Syndrome

Diagnostic Testing Options

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Updated May 09, 2014

What is amniocentesis?

An amniocentesis is a diagnostic test usually done between 15 to 20 weeks of pregnancy. This test involves removing small amount of amniotic fluid (the fluid surrounding a developing fetus) from the uterus. The amniotic fluid can be tested to determine if the fetus has certain kinds of birth defects.

An ultrasound is used to visualize the baby and a very thin needle is inserted into the lower abdomen (not through the belly button) and small amount (about one ounce) of amniotic fluid is removed. The amniotic fluid contains fetal skin cells that have sloughed off and are floating in the amniotic fluid. These skin cells are taken to a laboratory and a karyotype test is performed to give a picture of the baby’s chromosomes. FISH testing may also be performed.

What does an amniocentesis test for?

In general, an amniocentesis tests for two things -- chromosome abnormalities such as Down syndrome, and neural tube defects such as spina bifida and anencephaly.

A karyotype or FISH test is done on the fetal skin cells to look at the fetal chromosomes. The most common chromosome abnormality is Down syndrome which is cause by having an extra #21 chromosome.

An amniocentesis also tests for other chromosome abnormalities including trisomy 18 and trisomy 13. Testing detects over 99% of all chromosome abnormalities. An amniocentesis tests for open neural tube defects, such as spina bifida and anencephaly, by measuring a protein called alpha-fetoprotein (AFP) which detects 96% of all open neural tube defects.

What Doesn't an Amniocentesis Test For?

Neither amniocentesis or chorionic villi sampling can test for all birth defects. Every pregnancy has a risk of about 3% to 5% for different types of birth defects including things such as autism, cleft lip, forms of mental retardation other than Down syndrome and heart defects. No prenatal test is available to test for all birth defects. So while normal results from an amniocentesis are reassuring, they are not a 100% guarantee that your baby will have no health problems.

If you are worried about a specific genetic disorder that you are worried about, you may want to ask your doctor or genetic counselor if testing is prenatal testing is available for that disorder.

What does an amniocentesis really feel like?

Most women are surprised at how painless an amniocentesis is. Some women have an uncomfortable pressure or cramping feeling during the procedure but most feel no pain at all. The procedure is generally quick -- about 30 seconds to one minute. An ultrasound is used throughout the procedure to monitor the position of the baby and to make sure that the needle doesn't touch the baby.

How does an amniocentesis diagnose Down syndrome?

Amniotic fluid contains fetal skin cells that have sloughed off. These skin cells are taken to a laboratory and tested to determine if the fetus has Down syndrome. Two different tests -- FISH testing and karyotyping -- can be done to determine if a fetus has Down syndrome. Both FISH testing and karyotyping directly analyze the fetus’s chromosomes and determine whether or not the fetus has Down syndrome or another trisomy. FISH testing does not give you a complete picture of all the baby’s chromosomes, but it can give you a quick answer about trisomies. Results of a FISH test are generally available in 3 to 4 days. A karyotype takes longer (up to two weeks), but gives you more complete information about the baby’s chromosomes.

Results

If your results are abnormal, you should talk with your doctor or with a genetics specialist about what your results mean. It is important to get accurate, up-to-date information about any diagnosis that you receive from an amniocentesis.

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