Here in my hometown, it’s Derby week. With the greatest two minutes in sports upon us, it naturally has me thinking about odds. Indeed, prenatal testing for Down syndrome has that same focus. So, what are the odds that you are having a child with Down syndrome? Not only is this answer important, but it will surprise many.
It has long been known that the chance for having a child with Down syndrome increases with the age of the mother. See the graph at this post for the chance based on maternal age. While this relationship has been known, the reason for it remains unknown. Moreover, the most recent guidelines from the International Society for Prenatal Diagnosis (ISPD) state that maternal age, alone, is not enough to rely upon for the chance of having a child with Down syndrome.
This is where prenatal screening comes in. Screening tests provide a reevaluation of the odds that a pregnancy is positive for Down syndrome. Initially these tests could only be provided in the second-trimester. Then, in 2007, professional guidelines recognized the nuchal translucency-combined test, which can be offered in the first trimester.
As an example of these type of screening tests, a mother who is 25-years old has a baseline chance of having a child with Down syndrome of about 1-in-1,250; if she were to receive a screen result for Down syndrome, then her odds may be readjusted to 1-in-250, or some other revision of her baseline chance for having a child with Down syndrome. But, these tests have been likened to “tarot cards” because they are imprecise, with false positives outweighing the number of pregnancies with Down syndrome that the screening tests actually detect.
As has been covered at length on this blog, now we have a new era of prenatal testing known as Non-Invasive Prenatal Screening (NIPS). These tests essentially turn the odds of the previous screening tests on their head. Whereas under the “tarot cards” false positives outnumber true positives, that is not the case for NIPS; instead, a NIPS result for Down syndrome can mean the odds are that the pregnancy actually is positive for Down syndrome (when the mother is 30 years or older–see the graph at this post).
But, as every professional statement and even the NIPS testing companies themselves have emphasized, a NIPS result is not enough to rely upon because there remain false positives and false negatives. Therefore, diagnostic testing is required to truly know whether you are having a child with Down syndrome and only an amniocentesis provides the most accurate results, since CVS testing can return the same false positive as NIPS.
But (and here’s the kicker), even a diagnostic test will not tell you your odds for having a child with Down syndrome.
Odds are, you are not going to have a child with Down syndrome. There is no maternal age where the odds of having a child with Down syndrome exceed the chance of not having a child with Down syndrome. In fact, almost every pregnant woman in the world is not going to have a child with Down syndrome.
This is because the incidence of actually having a child with Down syndrome is very low. Of the 6 million pregnancies each year in the United States, there are at most 20,000 pregnancies that are actually carrying a child with Down syndrome. But, even for those pregnancies the odds are they are not going to have a child with Down syndrome.
How can this be?
Because most pregnancies positive for Down syndrome naturally miscarry. Reports vary on the percentages, but the ones that I’m familiar with state that 50% of pregnancies carrying a child with Down syndrome miscarry in the first trimester and 40% of those pregnancies that make it to the second trimester miscarry. This means, receiving a CVS result in the first trimester means the pregnancy has as much of a chance of miscarrying as continuing into the second trimester. Once the pregnancy is into the second trimester, when an amniocentesis can be performed, the odds of giving birth to a child with Down syndrome are still 6 out of 10.
So, the odds are that even with a diagnosis for Down syndrome, it does not mean that a child will be born with Down syndrome.
I wonder how often this actually is shared with expectant mothers considering prenatal testing. While it is discussed in recognized resources like “Diagnosis to Delivery,” for those mothers who choose to continue their pregnancy after a prenatal diagnosis, I wonder if it is shared with mothers considering termination?
It certainly would seem relevant that a woman considering an invasive medical procedure would be advised of the possible outcomes if the procedure were not performed. Considering that the whole exercise of prenatal testing is to determine the chance of having a child with Down syndrome, shouldn’t these odds also be shared?