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 are also some newer tests that are being used in pregnancy to screen for Down syndrome. They have a higher detection rate of Down syndrome than the above tests and involve testing the mother’s blood between the 10th - 22nd week of pregnancy to look 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.
Ultrasound can also be used in the first trimester to screen for Down syndrome. That ultrasound can be coupled with blood testing on the mother as well to assess the risk for the fetus having Down syndrome .
The 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 eight and 12 weeks of pregnancy; amniocentesis is performed between 12 and 20 weeks; and PUBS can be done after 20 weeks.