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Down Syndrome (cont.)

Down Syndrome Screening

Diagnosis of Down syndrome before birth can be very useful. Parents can learn about Down syndrome before the arrival of their baby and prepare accordingly; particularly to assess immediate medical needs such as heart and gastrointestinal conditions.

Prenatal Screening

  • Expanded alpha-fetoprotein screening (AFP) is the most widely used test to screen for Down syndrome. A small blood sample is taken from the mother between 15 and 20 weeks of pregnancy. The levels of AFP as well as three hormones called unconjugated estriol, human chorionic gonadotropin, and inhibin-A are measured in the blood sample. Altered levels of AFP and the three hormones can indicate Down syndrome. A normal test result does not exclude Down syndrome.

  • The nuchal translucency test measures the thickness of the neck fold via ultrasound. Combined with the mother's age, it identifies about 80% of Down syndrome fetuses.

  • Shortened humerus (arm bone) or femur (leg bone) length measured via ultrasound, detects about 31% of Down syndrome cases.

Prenatal Diagnosis

Several invasive diagnostic tests reliably detect Down syndrome. Most of these procedures carry a small risk of pregnancy loss.

  • Amniocentesis is usually performed between 16 and 20 weeks of pregnancy. A needle is inserted through the abdominal wall and a small sample of amniotic fluid is collected for analysis.

  • Chorionic villus sampling (VCS) is another reliable test to detect chromosomal abnormalities such as Down syndrome. The main advantage over amniocentesis is that it can be done earlier, usually between 11 and 12 weeks of pregnancy.

  • In percutaneous umbilical blood sampling, fetal blood is collected from the umbilical cord and examined for chromosomal abnormalities such as Down syndrome. It is performed after 17 weeks of pregnancy.

  • Fluorescent in situ hybridization (FISH analysis) can be done quickly to determine how many copies of a particular chromosome are present. This test is usually performed on the same sample taken from blood, during amniocentesis, or during CVS.
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In 1866, Down described clinical characteristics of the syndrome that now bears his name.

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