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Down Syndrome Genetic Testing

The genetic code of an organism determines how the organism will be put together and will function in the world. Sometimes mistakes occur in the human genetic code. This can result in physical and mental deficiencies that are hardly noticeable, or it can result in conditions with specific symptoms and characteristics that are readily apparent and problematic. Down syndrome falls into the latter category, and this disorder can be confirmed through genetic testing.

  1. Definition

    • Down syndrome, or Trisomy 21, is a genetic condition in which an individual has an extra 21st chromosome. The extra chromosome means that the affected individual has 47 chromosomes instead of the normal 46. The extra chromosome results in a flat face, mild to severe mental retardation and stunted growth. The condition occurs in approximately 1 out of 800 births.

    Screening

    • Genetic testing is the first step in screening for Down syndrome. The first genetic test is a blood test known as expanded AFP. It is done at 15 to 20 weeks gestation and combined with the age of the mother to give an approximation of risk for delivering a child with Down syndrome. The second method of screening is called nuchal translucency (NT). This is done at 11 to 14 weeks gestation, and it involves a simple ultrasound of the fold of skin on the back of the fetus' neck (the nuchal region). Neither of these tests can determine for sure that a fetus has Down syndrome. They are used as a means to assess the risk of the fetus developing the condition.

    Amniocentesis

    • Amniocentesis is a primary means of genetic testing for Down syndrome. This test is done at 15 to 20 weeks gestation. It involves puncturing the abdomen with a long needle and drawing amniotic fluid from the amniotic sac. The fluid from the test is sent to a lab and a chromosomal analysis is done. The test is highly accurate, but it does carry a risk of miscarriage, which results in about 1 out of every 100 tests (this rate varies by facility). Results are available in 2 weeks.

    CVS

    • Chorionic villus sampling, or CVS, is another method of genetic testing for Down syndrome. In this test, a piece of tissue is taken from the placenta for chromosomal analysis. The tissue is obtained either through puncturing the abdomen (similar to amniocentesis) or through using a catheter through the cervix. The risk of miscarriage for this procedure is the same for amniocentis, and the results take the same amount of time. The advantage of this test is that it can be done earlier in the pregnancy (10 to 12 weeks).

    New Methods

    • A new means of early screening for Down syndrome involves only a simple blood test. The test is less risky than other tests and works on the principle that cells from the placenta release fetal DNA as they enter the mother's bloodstream and are broken down. The test can be done as early as 5 weeks gestation.

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