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CVS testing - Prenatal Testing for Down Syndrome

Diagnostic Testing

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Updated May 28, 2009

What is chorionic villi sampling or CVS test?

A chorionic villi sampling (CVS test) is a diagnostic test usually done between 10 and 12 weeks of pregnancy. This test involves removing a small piece of the chorionic villi (outer layer) from the placenta. This piece of the placenta can be tested to determine if the fetus has certain kinds of birth defects.

A CVS test can be done either through the cervix rather like a a pap smear or through the abdomen like an amniocentesis. You don’t get to choose which way you want the test performed. An ultrasound is used to determine where the placenta is within the uterus. The doctor then decides which method -- transcervical or transabdomenal -- would be easiest to reach the placenta.

If you have a transcervical CVS (through the cervix), a small catheter is inserted into the cervix and a small piece of the placenta is removed and sent to the lab for analysis. If you have a transabdominal CVS (through the abdomen), a small needle is inserted through your abdomen (not through your belly button) and a small piece of the placenta is removed. The placenta and the fetus have developed from the same fertilized egg, so the chromosomes in the placenta are the same in the chromosomes of the fetus. These placenta 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 a CVS test for?

A CVS for chromosome abnormalities such as Down syndrome.

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

A CVS also tests for other chromosome abnormalities including trisomy 18 and trisomy 13. Testing detects over 99% of all chromosome abnormalities.

What doesn't a CVS test test for?

A CVS test does NOT test for open neural tube defects, such as spina bifida and anencephaly.

Neither CVS nor amniocentesis can test for all birth defects. Every pregnancy has a risk of about 3% to 5% for all 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 a CVS test 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, you may want to ask your doctor or genetic counselor if testing is prenatal testing is available for that disorder.

What does A CVS really feel like?

Most women report that a CVS test is relatively painless.

A transcervical CVS test, done through the vagina, feels much like a pap smear but last a little longer. Some women experience mild cramping and some spotting after a transcervical CVS.

A transabdominal CVS test, done through the abdomen like an amniocentesis, is also relatively painless although it does take longer, up to 1 to 2 minutes depending on the location of the placenta. An ultrasound is used throughout the procedure to help the doctor quickly locate the placenta. Some women have an uncomfortable pressure or cramping feeling after the procedure but most feel no pain at all.

How does an CVS diagnose Down syndrome?

The placenta and the fetus have developed from the same fertilized egg, so the chromosomes in the placenta are the same in the chromosomes of the fetus. These placenta cells are taken to a laboratory and tested to determine if the fetus has Down syndrome. Two 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 of the baby’s chromosomes, but it can give you a quick answer about some of the most common trisomies (13, 18, 21, X and Y). 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 a CVS test.

Related Video
How Does Chorionic Villus Sampling Work?

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