In part 2 of Chapter Two, the two main diagnostic tests for Down syndrome are described. The rest of the installments published so far are available here. If you had diagnostic testing, feel free to share your experience in the comments.
A. Diagnostic Testing
The two most-often used diagnostic tests are chorionic villus sampling (“CVS”) and amniocentesis (“amnio”). Both technologies were developed in the 1960’s, with amnios becoming regularly done in the 1970’s and 1980’s and CVS’s increasingly so in the 1990’s and 2000’s, though amnio remains far and away the more commonly performed procedure. Both tests involve a needle being inserted into the womb and products of the conception (the precision of that phrase will be explained shortly) being tested. Or, at least that it is the design and purpose of each test. Though called “diagnostic,” these tests are still conducted by human doctors and then processed at labs with human technicians, consequently, the human factor can result in errors. Both tests have a margin of false positives and false negatives due to errors in sampling collection and errors in laboratory processing. These margins, however, are statistically so small that practitioners and patients alike rely on CVS and amnio results as being truly diagnostic—essentially as close to definitive as any human activity can achieve.
CVS is performed in the first trimester, with the American College of Obstetricians & Gynecologists (ACOG)[1] practice bulletin on diagnostic testing recommending it be performed no sooner than 10 weeks and no later than 12 weeks gestation. With a CVS, the needle is inserted either trans-abdominally or trans-vaginally to take a sample of what is termed the “chorionic villi” from the placenta. Due to the fetus being so small at this stage of the pregnancy, samples from the fetus itself are not taken. While there have been instances of disfigurement as a result of a CVS, to include facial malformation and lack of formation of limbs, these are so rare that ACOG does not recognize it as a risk that women should be counseled about. Pregnancies become more stable as they continue and the converse is true as well, pregnancies are more delicate the newer they are. It is for this reason that many expectant couples wait until after the first trimester to reveal they are pregnant, since the risk of miscarriage is highest in the first trimester. The insertion of a needle into the sterile environment of the womb can increase the risk of miscarriage, and with that risk being highest in the first trimester, CVS has a higher risk of miscarriage associated with it than amnio.
ACOG recommends that amnios not be performed earlier than 15 weeks gestation, leaving a three week gap in pregnancy between the latest a CVS may be performed and the earliest when an amnio will be performed. The significance of these timelines will become apparent when we reach the next chapter. Now into the second trimester, the fetus and womb have grown larger. The needle is inserted through the abdomen and into the womb where it collects actual cells from the fetus, usually skin cells sloughed off just as we lose skin cells daily or cells expressed through the fetus’s urethra. Because amnio tests cells actually from the fetus and because it is performed at a safer time in the pregnancy when there is a lower risk of miscarriage, it is regularly referred to as the “gold standard” of prenatal diagnostic testing. Indeed, the National Society of Genetic Counselors (“NSGC”), in its guidelines concerning cell free DNA screening (addressed at the end of this chapter), recommends amnio as more accurate than CVS, since CVS is sampling cells from the placenta and not necessarily cells with the same DNA as the fetus.
Though both procedures can risk miscarriage of the pregnancy, the rate of that risk has been a moving target. Historically the rule of thumb quoted was that amnio has a 1% risk of miscarriage and CVS’s rate of risk was double that at 2%. These rates mean that out of every 100 procedures performed, if they were amnios, 1 would cause a miscarriage, and if they were CVS’s, then 2 would end in miscarriage. In 2007, ACOG revised its practice recommendations based on studies that reported much lower rates of miscarriage with the procedures. These studies found that when these procedures were performed by experienced practitioners at facilities with experience performing amnios and CVS’s, the rate of miscarriages for either could be below 0.5%, cutting the historically-cited rate by more than half. With the risk of miscarriage being much lower than thought, ACOG revised its recommendations such that now all women were to be offered prenatal diagnostic testing. The American College of Medical Genetics (“ACMG”)[2] issued a position statement that made it even clearer: because the risk was relatively so low, all women were first to be offered prenatal diagnostic testing and then be offered screening testing (the topic for the second half of this chapter).
[1] For the longest time, the “C” in ACOG stood for “College.” Soon after attending my first ACOG national conference, the organization announced it would be known as the American Congress of Obstetricians & Gynecologists. Then, after making that change, another press release was issued stating that depending on what issue was being addressed or act was being done, it could be done by the American Congress of Ob/Gyns or the American College of Ob/Gyns. Because I am not an Ob/Gyn and therefore perceive the organization as most outsiders do, I will continue to refer to it simply as “ACOG” since the distinction between College and Congress can confuse even some practicing Ob/Gyns.
[2] There has been no similar College/Congress confusion with ACMG.
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