How is Down Syndrome Diagnosed?
There are two categories of tests for Down syndrome available prior to birth: screening tests and diagnostic tests. Prenatal screens estimate the chance of the fetus having Down syndrome. These tests do not tell you for sure whether your fetus has Down syndrome; they only provide a probability. Diagnostic tests, on the other hand, can provide a definitive diagnosis with almost 100% accuracy.
Prenatal Screening Tests
There is an extensive menu of prenatal screening tests now available for pregnant women. 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. These blood tests, together with a woman’s age, can help estimate her chances of having a child with Down syndrome. We often perform these blood tests 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.
Prenatal Diagnostic Tests
Diagnostic tests can provide a definitive diagnosis with almost 100% accuracy.
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 nearly 100% accurate in diagnosing Down syndrome. Pregnant women usually receive an amniocentesis in the second trimester between 15 and 20 weeks of gestation, CVS in the first trimester between 9 and 14 weeks.
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 called FISH can apply similar principles and confirm a diagnosis in a shorter amount of time.