Screening for Down Syndrome and Other Chromosomal Abnormalities

“Do you want Down syndrome screening?”

This question, which I ask my patients every day, is straightforward; the answer can be anything but.

Testing options are better than ever, and are available very early in the pregnancy. But the choice women make about whether or not to test is extremely personal, and depends on her values (and her partner’s values) about whether they would continue a pregnancy affected by a chromosomal abnormality, or how they would deal with the uncertainty surrounding a test result that might (or might not) indicate an abnormality. To be sure, testing can be anxiety provoking when couples are just starting the pregnancy. What to do?

It is important to recognize that every woman is at risk for having a child with Down syndrome and other chromosomal abnormalities. There are some conditions that are related to a mother’s age with the risk rising in a steady fashion over a woman’s reproductive years. By virtue of knowing your age, you’ve already had some Down syndrome screening. However, this is not a particularly good risk-assessment tool. Most of the screening tests out there modify that age-related risk to determine an individual’s specific risk in the pregnancy.

Most tests don’t provide a definitive result, but estimate the chance that the baby may have a chromosomal abnormality. If a definite answer is desired, an amniocentesis or chorionic villus sampling (CVS) is needed to get actual genetic information about the fetus. While other types of risk assessment are entirely safe, amniocentesis and CVS occasionally cause miscarriage to the pregnancy. Most couples use risk assessment to decide whether the risk is high enough to get the amniocentesis or CVS or whether the risk is sufficiently low that they are comfortable skipping any further testing.

There are many testing options. And to confuse matters further, different labs will have different trademarked names for their version of testing.  It is important that you speak with your doctor or midwife about the tests they are offering in their office and what they see as the benefits and limitations of each. Some of the more common tests that are available:

• First trimester risk assessment: This takes your age-related risk and modifies it by a measurement of the back of the baby’s neck (nuchal translucency) and results from a blood test.
• Sequential screening: This takes the First trimester risk assessment and adds a second blood draw about a month later to refine the first trimester result.
• Integrated screening: This takes First trimester and Sequential data and reports only the results after the second blood draw. It is slightly more accurate although no results are available until after the second test.
• Quad screen: This is a blood test-only and is generally drawn between 16 and 20 weeks. It is less accurate than the other options but is available for women who miss the opportunity for first trimester testing.
• Non-invasive prenatal testing (NIPT): This is the newest technology and involves measuring and analyzing cell-free DNA from the fetus that circulates in the maternal blood stream.

The nuchal translucency ultrasound measurement is a part of many of the screening tests. I believe that the first trimester ultrasound and nuchal translucency measurement is a very important component of an early “check up” of your baby. Increases in the nuchal translucency can point to a variety of genetic or birth defects, so I find it very reassuring when I see a normal nuchal translucency.

With any screening test, there are “false positives” – when the test reports ‘increased risk’ but your baby does not have the abnormality in question. Screening tests are supposed to identify a small group at higher risk for a condition who can be offered a diagnostic test that provides a definitive answer (in this case, an amniocentesis or a CVS). Interpretation of the test results is something that your doctor or a Maternal-Fetal Medicine specialist can assist you with.

Here’s what I recommend:

• Talk with your doctor or midwife about the screening options that they offer and the benefits and limitations of each testing option.
• Have a first trimester ultrasound with a nuchal translucency measurement regardless of what you choose for screening. This is a good first look your fetus’ health and well-being.
• Strongly consider one of the newer risk assessment tests – you’ve already had risk assessment as a function of maternal age, why not have testing that is much more accurate?