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Prenatal Testing for Down Syndrome: Understanding Two New Studies

Down syndrome Trisomy 21
Ten years ago, when I was 28-years-old and pregnant with my first child, I didn't spend much time thinking about prenatal testing. I was young and healthy, which meant it was unlikely for me to be carrying a baby with a trisomy (three copies of a chromosome instead of the typical pair) or any other genetic condition.

Still, when my doctor offered a simple blood test to screen for trisomy 21, also known as Down syndrome, I shrugged and stuck out my arm. It couldn't do any harm to the baby, and it would give me more information. What harm could a little more information do?

Over the course of the past decade, the prenatal testing industry has advanced and shifted. New screening tests called Non-Invasive Prenatal Screening ("NIPS") have come on the market. The companies who market these tests promise pregnant women 99% accuracy in identifying fetuses with trisomy 21 and other genetic conditions. These tests analyze placental DNA in a pregnant woman's blood in order to identify fetuses with Down syndrome.

On the one hand, these tests provide women with a more accurate, non-invasive screening test than those offered in the past. On the other hand, these tests are marketed without regulation by the FDA, and their accuracy has been questioned by many. But a new study, released this week in the New England Journal of Medicine, offers a more conclusive assessment. The study concludes that this type of testing "had higher sensitivity, a lower false positive rate, and higher positive predictive value than did standard screening."

For pregnant women who want to know if their baby has Down syndrome or another trisomy, the new blood tests are the most accurate option. Still, because these are screening tests, and not diagnostic tests, they can include false results —some babies with Down syndrome are not identified and others without Down syndrome will be misidentified. Even now, the only conclusive way to identify babies with Down syndrome is through the invasive and somewhat risky procedure called amniocentesis, which can only take place later in pregnancy.

Regardless of accuracy, the only information these tests—screening or diagnostic in nature—can offer is an indication of whether or not a baby has Down syndrome. They cannot predict with any specificity the physical or cognitive condition of the child, nor can they predict the degree of familial, medical, or social support that child might receive. In our case, we didn't learn that our daughter had Down syndrome until after she was born, and yet even that surprising but definitive diagnosis told us very little about who she would be or the context in which she would develop.

Another study came out this week in the American Journal of Medical Genetics that sheds some light on the significance of the cultural context of prenatal testing. This study analyzed decades of data about prenatal testing and the prevalence of live births of babies with Down syndrome in the United States. It concluded that the rate of live births of babies with Down syndrome is approximately 30% lower than it would be without prenatal testing, because some women choose to terminate their pregnancy after discovering their baby has Down syndrome.

But even within the United States, the termination rates vary from population to population and from region to region. Among ethnic groups, for example, non-Hispanic whites and Asians/Pacific-Islanders have the most significant rates of reduced live births of babies with Down syndrome. In addition, women within the Northeastern United States are more likely to choose termination than those in other parts of the country, and particularly when compared with the South.

This study suggests at least two conclusions. One, the United States as a whole contains many women who choose to have babies with Down syndrome, whether by forgoing prenatal screening tests, forgoing diagnostic tests, or choosing to continue a pregnancy with a prenatal diagnosis. Two, cultural factors play a significant role in determining who decides to terminate pregnancies once trisomy 21 has been identified.

Still, this study ends in 2010, the year before the newest NIPS tests came on the market. The prevalence of the new prenatal tests may change the number of babies with Down syndrome yet again. But we find ourselves in an era of medical advances that allow for high quality of life, and with social supports that allow for inclusion in classrooms and the workplace. Perhaps, even with the new prenatal screening tests, women who know their child will have Down syndrome will simply use that information as a way to prepare well for the birth of their baby.

Amy Julia is the mom of three kids who love broccoli and hot dogs, and who ask for lollipops every day! Her guilty pleasures are Chardonnay and Diet Coke. She is also the author of Small Talk: Learning from my Children about What Matters Most and A Good and Perfect Gift: Faith, Expectations, and a Little Girl Named Penny. Visit her at amyjuliabecker.com.

Should You Have an Amniocentesis?

Photo of Down syndrome/Trinomy 21 via Shutterstock