Screening versus Diagnostic Testing
There are two kinds of testing that can be done during a pregnancy - diagnostic testing such as amniocentesis and chorionic villi sampling (CVS), and screening testing such as ultrasound (sonogram) and maternal serum screening tests. A screening test can never give you a firm diagnosis - a screening tests simply tells you if you have a slightly higher risk to have a baby with Down syndrome. A diagnostic test on other hand, gives you a firm diagnosis.
Down syndrome can be diagnosed in pregnancy by diagnostic tests such as amniocentesis or chorionic villi sampling (CVS). These are considered optional tests during pregnancy which means that it is your choice whether or not to have this testing. It is recommended that women considering testing speak with a genetic counselor to review their risks and to discuss risks and benefits of these procedures before making their decision about testing
The results of amniocentesis and CVS will give you a picture of the fetus’s chromosomes and the diagnosis of Down syndrome is very straightforward. Infants with Down syndrome have an extra number 21 chromosome for a total of 47 chromosomes rather than 46.
An ultrasound, also know as a sonogram, is a test done during pregnancy which uses sound waves to generate a picture or image of the fetus. Occasionally, but not always, infants with Down syndrome show signs on an ultrasound that can make your doctor suspect that the fetus may have Down syndrome.
Some of these signs seen on an ultrasound include:
- a decrease in femur length (a bone in the leg)
- an increase in the skin behind the neck (called nuchal translucency)
- choroid plexus cysts (cysts in a section of the brain that produces spinal fluid
- heart defects
- gastrointestinal defects
While any of these signs may make your doctor suspicious, Down syndrome cannot be diagnosed based on ultrasound findings alone. These findings simply suggest that there is a slightly higher risk that the fetus may have Down syndrome and your doctor will recommend that your consider having an amniocentesis or CVS to confirm the diagnosis.
Maternal serum screening tests
Because a mother and her fetus share a circulation system, it is possible to look for certain substances made by the fetus in the mom’s bloodstream. By measuring the amount of these substances at a specific point in pregnancy, it is possible to estimate the risk for the baby to have Down syndrome. There are now numerous different maternal serum screening tests some of which are done in the first trimester and some of which are done in the second trimester. A relatively new form of screening is called integrated screening. This screening test uses a combination of maternal serum screening and ultrasound to generate a estimate of the chance for the baby to have Down syndrome.
It is important to remember that screening tests cannot tell for sure if your baby has Down syndrome. The test results simply indicate if your risk is higher than someone else your age. If your screening test is positive, it simply means your risk of having a baby with Down syndrome is higher than other women your age. But even with a positive screening test, most women will have babies without Down syndrome. Conversely, a negative screening means that the chance of Down syndrome is low but not zero. It doesn't guarantee a baby without Down syndrome. It is up to each woman to decide which approach - screening, diagnostic testing or no testing at all - is right for her.
National Institute of Child Health and Human Development (NICHD), Facts about Down Syndrome. 2008.
Newberger, D., Down Syndrome: Prenatal Risk Assessment and Diagnosis. American Family Physician. 2001.