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.
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.