Prenatal Testing & Diagnosis
There are two types of tests for Down syndrome that can be performed before your baby is born: screening tests and diagnostic tests.
Prenatal screenings estimate the chance of the fetus having Down syndrome. These tests do not tell you for certain 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.
There is an extensive menu of prenatal screening tests now available for pregnant women. Most of these 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, including alpha-fetoprotein, inhibin A, plasma protein A, estriol, and human chorionic gonadotropin. Together with a woman's age, these tests are used to estimate her chance of having a child with Down syndrome. Typically offered in the first and second trimesters, maternal serum screening tests are only able to predict about 80% of fetuses with Down syndrome. It is important to note that none of these prenatal screens will be able to definitively diagnose Down syndrome. Instead, mothers should expect results such as, "You have a 1 in 240 chance of having a child with Down syndrome" or "You have a 1 in 872 chance of having a child with Down syndrome."
These blood tests are often performed in conjunction with a detailed sonogram (ultrasound) to check for "markers" (characteristics that some researchers feel may have a significant association with Down syndrome). Recently, researchers have developed a maternal serum/ultrasound/age combination that can yield a much higher accuracy rate at an earlier stage in the pregnancy. Still even with the ultrasound, however, the tests will not definitively diagnose Down syndrome.
A prenatal screening test named "MaterniT21" has been made available by the company Sequenom. This test, which can only be ordered by a physician, involves blood being taken from the expectant mother, as early as 10 weeks of gestation, and relies on the detection of cell-free DNA that circulates between the fetus and the expectant mother. According to the latest research, this blood test can detect up to 98.6% of fetuses with trisomy 21. A "positive" result on the test means that there is a 98.6% chance that the fetus has trisomy 21; a "negative" result on the test means that there is a 99.8% chance that the fetus does not have trisomy 21. The turnaround time for the test is about 8-10 days, and approximately 0.8% of patients do not receive a result due to technical standards.
While the initial clinical trials were conducted just on cases with trisomy 21, one form of Down syndrome, a "positive" result cannot distinguish between trisomy 21, translocation Down syndrome, and high-percentage mosaic Down syndrome. Put another way, a "positive" result will pick up all forms of Down syndrome, except when mosaic Down syndrome has a chance of 33% or lower. All mothers who receive a "positive" result on this test are encouraged to confirm the diagnosis with one of the diagnostic procedures described below.
Prenatal screening tests are now routinely offered to women of all ages. If the chance of having a child with Down syndrome is high from prenatal screening, doctors will often advise a mother to undergo diagnostic testing.
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. Amniocentesis is usually performed in the second trimester between 15 and 22 weeks of gestation, and CVS in the first trimester between 9 and 14 weeks. Amniocentesis and CVS are also able to distinguish between these genetic types of Down syndrome: trisomy 21, translocation Down syndrome, and mosaic Down syndrome.
*All information above is provided on the website of the NDSS.