Prenatal Screening & Gene Tests

In the news: Fact or Fiction?

Prenatal screening, which is used to identify fetal problems, or the chance that a baby has a chromosomal abnormality like Down Syndrome, has been continuously improving. Years ago we screened based on a mother’s age; the problem was, 2/3 of the babies with Down Syndrome were born to younger women. Even though they had a lower chance of having a baby with Down Syndrome, they were having most of the babies and as a result they were having most of the babies with Down syndrome. This led to better testing which initially involved a blood test after 15 weeks. Now it involves an ultrasound around 12 weeks to measure the thickness at the back of a baby’s neck (called nuchal translucency) plus one or more blood tests. These screening tests have been getting more and more accurate with fewer and fewer false positive results. A positive result could lead to a diagnostic test…essentially a test that tells us definitively if the baby has the condition we are looking for. Unfortunately, the diagnostic tests (amniocentesis or (CVS) chorionic villus sampling) are associated with a small risk of pregnancy loss, but we are now on the verge of being able to offer a maternal blood test that specifically looks at a baby’s DNA, further improving the accuracy of the screening test. It’s still considered a screening test and may require further follow up with an amnio or CVS.

Up until now, most of the prenatal screening has been directed at problems with Chromosome numbers (like an extra chromosome #21 which is Down Syndrome, but also an extra chromosome #13 or #18 or the X or Y chromosome) or for spina bifida, which is where the spine has not formed properly. However, there are many more potential genetic problems that can occur and are often not picked up until after delivery - and some are never picked up because they have no significant implication on the baby’s health. As we learn more about the human genome and the technology improves, we are better able to screen the entire genome for problems.

A new study just published suggests that techniques to look for single gene defects can identify babies previously thought to be normal based on the usual chromosomal studies. This advance in gene testing is especially useful in situations like stillbirth where we’re looking for information on the cause of death to provide parents with some degree of closure and help determine any future risk. I think these new tests are exciting, but I also think we need to be cautious. Some of the gene defects that can be found through this new testing are of unknown significance and the results of the testing can cause more questions than it can provide answers.

There is no doubt that in the future, prenatal screening will be heavily influenced by the advancing technology of gene testing. But is this necessarily a good thing? Potentially, yes. On the other hand, there is the real potential gene testing will only cause further anxiety for parents. As an obstetrician, I firmly believe that if we are going to adopt these techniques, we should be using them not as a screening test for all pregnancies, but only in follow up to an ultrasound identified fetal abnormality, or where there is a previous adverse obstetrical history like stillbirth. Providing test results without being able to provide information on what those results might or might not mean is of questionable benefit.