How is Down syndrome diagnosed?

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 photograph the chromosomes and then group them by size, number and shape. By examining the karyotype, doctors can diagnose Down syndrome. Another genetic test,  fluorescence in situ hybridization (FISH), can apply similar principles and confirm a diagnosis in a shorter amount of time.

This content originally appeared on the National Down Syndrome Society website.
Down syndrome is usually suspected based on distinctive findings noted on the physical examination and then confirmed by performing a blood test called chromosomal analysis or karyotyping. This test determines the number and types of chromosomes in the cells and confirms the diagnosis of Down Syndrome which is a genetic disorder. Prenatal tests can be done during pregnancy to detect Down Syndrome in the fetus as well.

If a newborn baby is showing characteristic physical signs of Down syndrome (slanted eyes, low muscle tone, a flatter than typical face, abnormality in the ears, among others), a doctor will recommend that a blood test be taken to verify the diagnosis. This test is called a chromosomal karyotype. It takes a few weeks because new cells need to be formed, or grown, from the blood. Then geneticists can study the cells to look for the extra copy or portion of chromosome 21 (the genetic cause of Down syndrome).

Expectant mothers may also get a diagnosis of Down syndrome through three different prenatal tests (amniocentesis, chronic villus sampling, and percutaneous umbilical blood sampling). In each test, cells are captured and scrutinized for the extra genetic material.


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