How is Down syndrome diagnosed?

Prenatally

There are two categories of tests for Down syndrome that can be performed before a baby is born: screening tests and diagnostic tests. Prenatal screens estimate the chance of the fetus having Down syndrome. Most of these tests only provide a probability. Diagnostic tests can provide a definitive diagnosis with almost 100% accuracy.
   
Most screening tests involve a blood test and an ultrasound (sonogram). The blood tests (or serum screening tests) measure quantities of various substances in the blood of the mother. Together with a woman's age, these are used to estimate her chance of having a child with Down syndrome. These blood tests are often performed in conjunction with a detailed sonogram to check for "markers" (characteristics that some researchers feel may have a significant association with Down syndrome). New advanced prenatal screens are now able to detect chromosomal material from the fetus that is circulating in the maternal blood.  These tests are not invasive (like the diagnostic tests below), but they provide a high accuracy rate. Still, all of these screens will not definitively diagnose Down syndrome. Prenatal screening and diagnostic tests are now routinely offered to women of all ages.
  
The diagnostic procedures available for prenatal diagnosis of Down syndrome are chorionic villus sampling (CVS) and amniocentesis. These procedures, which carry up to a 1% risk of causing a spontaneous termination (miscarriage), are practically 100% accurate in diagnosing Down syndrome. Amniocentesis is usually performed in the second trimester after 15 weeks of gestation, CVS in the first trimester between 9 and 11 weeks.

At Birth

Down syndrome is usually identified at birth by the presence of certain physical traits: low muscle tone, a single deep crease across the palm of the hand, a slightly flattened facial profile and an upward slant to the eyes.  Because these features may be present in babies without Down syndrome, a chromosomal analysis called a karyotype is done to confirm the diagnosis. To obtain a karyotype, doctors draw a blood sample to examine the baby's cells. They use special tools to photograph the chromosomes and then group them by size, number, and shape. By examining the karyotype, doctors can diagnose Down syndrome.  Another genetic test called FISH can apply similar principles and confirm a diagnosis in a shorter amount of time.

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