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Down Syndrome: Tests and Diagnosis

What tests are used to diagnose Down syndrome?

Prenatally – At this time, 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 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 diagnostic procedures available for prenatal diagnosis of Down syndrome are chorionic villus sampling (CVS) and amniocentesis.  

After Birth – Down syndrome is usually identified at birth by the presence of certain physical traits: low muscle tone, a single transverse 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 with 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 (Fluorescent In Situ Hybridization) can apply similar principles to provide an initial diagnosis of Down syndrome in a shorter amount of time.  It provides limited information about other structural chromosome abnormalities such as translocations and deletions.

How do providers at Children’s Hospital Colorado make a diagnosis?

Tests to confirm Down syndrome are often done before a baby is born through amniocentesis or chorionic villus sampling (CVS).  There are now non-invasive prenatal tests that evaluate the DNA of the fetus in the mother’s blood and it provides a high accuracy rate of a fetus having Down syndrome. In considering these prenatal tests, pretest counseling regarding these limitations is recommended.

Referral for genetic counseling is suggested for pregnant women with positive test results. Because false-positive test results can occur, confirmation with amniocentesis or CVS is recommended by the American College of Obstetricians and Gynecologists.  After birth, diagnostic evaluation begins with a thorough medical history and physical examination of your child.