What Tests Determine if My Baby Has Down Syndrome?

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Q: What testing can be done to screen for down syndrome? Why is testing recommended? What are the risks?

A:  Every pregnant woman, regardless of age, should have the option to do different types of testing to determine the baby's risk of Down's syndrome and other chromosome-number related abnormalities (extra or too few copies.) Every pregnancy has an age-related chromosome abnormality risk that increases with age. But with testing, a much more accurate risk assesment (first trimester screen, second trimester triple or quad screen) can be performed that usually reveals a significantly lower risk. Likewise, diagnostic tests can be performed, which assess not the risk, but obtain the actual chromosome number.

The first opportunity of both types of tests is the same, between 11 and 13 weeks gestation. Screening tests refer to a non-invasive assessment of the risk a certain fetus has of having an abnormality. This is refered to as First Trimester Screening test, or Nuchal Fold test, or BUN test. A specialist measures the fold in the back of the fetus's neck, as well as certain proteins in the mother's blood, and then puts the results into a formula to result with a risk assesment. For example, A 35 year old woman's fetus has an age related risk of Down's syndrome of aproximately 1/450, but the first trimester screening results usually lower that to anywhere from 1/500 to up to 1/10,000 or lower. When combined with a second trimester triple or quad screen (15-20 weeks) paired with a structural anatomy ultrasound at 20 weeks, we approach a very high percentage of certainty that Down's or other chromosomal abnormalities are not present. 

If a patient desires a more certain result then a risk assesment, or if a previous risk assessment returns with an increased risk, then there are diagnostic tests available that retrieve the actual chromosome number. These tests do carry some risk as they are invasive. Again, the first opportunity for this type of test is Chorionic Villus Sampling or CVS, and is also performed between 11-13 weeks. The specialist retrieves cells from the placenta, usually through the cervix, under ultrasound guidance, to obtain the exact chromosome number. Because this is an invasive test, it carries a miscarriage risk of about 1/200, though the risk is usually much lower in the hands of an experienced expert. But the advantage of this testing is this leaves virtually no doubt (over 99% accuracy) as opposed to a risk assement.  The second opportunity for this type of diagnostic testing is after 15 weeks and is by amniocentesis, which obtains the chromosomes through amniotic fluid retrieved through the abdomen. This carries a risk of miscarriage as well, though lower than with CVS (approximately 1/500.)

Lastly, there are certain soft markers for Down's syndrome that can be seen on a detailed anatomic survey ultrasound around 20 weeks that may prompt diagnostic testing. These can be certain characteristics seen in the heart or bowel or bones, but are soft markers only and increase the risk of Down's usually only when more than one marker is present at the same time. 

It is very important, usually at your first meeting with your healthcare provider after confirming intrauterine pregnancy, to fully discuss these options and feel comfortable that you are being informed and making the testing decisions that are right for you.