What prenatal tests for Down syndrome are available?

A pregnant woman has a few options when it comes to testing for Down syndrome prenatally. She can opt for a blood test that looks for certain substances that have been soft markers for the abnormality and an ultrasound that looks for certain physical characteristics of Down syndrome. These "screens" are not meant to be diagnostic in themselves; they simply give a pregnant woman a probability of having a baby with Down syndrome. Based on the results, a woman can undergo a number of diagnostic tests that carry a chance of miscarriage. In each one, cells are extracted and studied for signs of the extra chromosomal material that indicates Down syndrome. The three tests are:

Chronic villus sampling: Cells are taken from tissue that has yet to form into the placenta during this first trimester test.Amniocentesis: Cells are taken from the amniotic fluid during this second-trimester test.Percutaneous umbilical blood sampling: Cells are taken from the umbilical cord during this second trimester test.
There are several tests available in early pregnancy to help determine if a baby might have downs syndrome.  The easiest test is a blood test called a quad screen which is done around the 18th week. However this test only gives a percent risk that the infant could have down syndrome. If you wanted to know for sure if your infant has down syndrome or not then a more invasive test is needed. You can either do chorionic villas sample which requires a needle to be put into a part of the placenta ad is completed at around 10 weeks of gestation or an amniocentesis in which a needle is placed into the fluid around the baby and a small amount removed which is completed between 15 and 17 weeks of gestation. Both of these methods will look directly at the babies DNA and can diagnose downs syndrome, however, there is a small risk with these proceedures of miscarriage and so any proceedure should be discussed with your obstetritian before any decision is made.
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. 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 percnet accuracy.

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. 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). Prenatal screens are able to detect chromosomal material from the fetus that is circulating in the maternal blood. These tests are not invasive (like the diagnostic tests are) 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 one percent risk of causing a spontaneous termination (miscarriage), are nearly 100 percent accurate in diagnosing Down syndrome. Amniocentesis is usually performed in the second trimester between 15 and 20 weeks of gestation; CVS is usually performed in the first trimester between nine and 14 weeks.

This content originally appeared on the National Down Syndrome Society website.

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