Although a newborn's physical characteristics may lead doctors to suspect Down syndrome, a genetic test called a karyotype must be done to confirm the diagnosis. A karyotype involves testing a small amount of the baby's blood.
It is also possible to screen for and diagnose Down syndrome during pregnancy. A screening test estimates a woman's risk of having a child with Down syndrome. A diagnostic test determines whether the fetus has Down syndrome.
Two blood tests, the triple screen and the alpha-fetoprotein, are common prenatal screening tests and are often done in combination with a detailed sonogram (ultrasound). These tests are usually offered to a pregnant woman between week 15 and week 20 of pregnancy. However, these tests also have a high number of false positives, or results that indicate Down syndrome when the condition is not really present. Any time the test comes back positive, further testing must be done to confirm the diagnosis. Also, some women may receive false-negative results; in other words, the test may fail to detect Down syndrome when it does exist.
There is a newer test being used in pregnancy to screen for Down syndrome called the Harmony test. It has a higher detection rate of Down syndrome than the above tests do and involves testing the mother’s blood from the 10th to the 22nd week of pregnancy. The test looks at the broken down fetal cells in the mother’s circulation (also called “cell-free DNA”). This test has a 99% detection rate for Down syndrome.
Other prenatal tests used to diagnose Down syndrome are chorionic villus sampling (CVS), amniocentesis, and percutaneous umbilical blood sampling (PUBS). These tests do carry a small risk of miscarriage because they require insertion of a needle into the placenta or umbilical cord to examine the baby's chromosomes. These tests accurately diagnose Down syndrome 98% to 99% of the time. CVS is typically performed between 8 and 12 weeks of pregnancy; amniocentesis is performed between 12 and 20 weeks; and PUBS can be done after 20 weeks.