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;
assess immediate medical needs such as heart and gastrointestinal conditions.
- 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
- Shortened humerus (arm bone) or femur (leg bone) length measured via
ultrasound, detects about 31% of Down syndrome cases.
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
- 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
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