Down syndrome blood test: How does it work?

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Down syndrome blood test: How does it work?

Question

How does the Down syndrome blood test work?

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Answer

The Down syndrome blood test is a new prenatal screening test for women at high risk of having a baby who has Down syndrome. The test is used to evaluate your risk of carrying a baby who has Down syndrome. In some cases, the Down syndrome blood test can help you avoid testing that's more invasive.

During the Down syndrome blood test — which can be done as early as week 10 of pregnancy — a maternal blood sample is taken and sent to a lab. At the lab, fetal DNA circulating within the blood is analyzed for extra chromosome 21 sequences. A higher than expected number of chromosome 21 sequences indicates an increased risk of trisomy 21, the most common cause of Down syndrome. Trisomy 21 occurs when a person has three copies of chromosome 21, instead of the usual two copies, in all of his or her cells. The Down syndrome blood test doesn't specifically address other genetic or chromosomal conditions or other rare causes of Down syndrome. Your health care provider will be notified, however, if extra sequences of chromosome 13 (trisomy 13) or chromosome 18 (trisomy 18, also called Edwards syndrome) are found. Typically, test results are available within two weeks.

If a traditional prenatal screening test — such as first trimester screening or the quad screen — indicates that you're at high risk of carrying a baby who has Down syndrome, a more invasive test is typically offered to confirm the diagnosis. For example:

  • Amniocentesis. During this procedure, a sample of the fluid that surrounds and protects the baby during pregnancy (amniotic fluid) is removed from the uterus for testing.
  • Chorionic villus sampling (CVS). During CVS, a sample of the wispy projections that make up most of the placenta (chorionic villi) are removed from the placenta for testing.

Amniocentesis and CVS both carry a slight risk of miscarriage. You might be able to avoid amniocentesis or CVS, however, by taking the Down syndrome blood test. If the Down syndrome blood test results are normal, you might not need further testing. If the Down syndrome blood test detects a high risk of trisomy 21, amniocentesis or CVS will be offered to confirm the results.

If you have risk factors for having a baby who has Down syndrome — you're an older mother or you have a family history of Down syndrome — ask your health care provider about the Down syndrome blood test. Also ask about the blood test if you've gotten positive or worrisome results from another prenatal screening test. Your health care provider or a genetic counselor can help answer any questions you might have and discuss what the results might mean for you and your family.

Last Updated: 2012-02-22
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