A paper in The New England Journal of Medicine, Cell-free DNA Analysis for Noninvasive Examination of Trisomy, reports on the effectiveness of a new proprietary method to screen for Trisomy 21, which is the cause of 90% of cases where individuals exhibit Down Syndrome. This issue is well known at this point. There are many methods to screen and diagnose Down Syndrome prenatally, but they all suffer from drawbacks, from invasiveness to false positives. Currently the vogue is for methods which analyze maternal plasma. That is, all that’s needed is a blood draw from the mother. This is one such method.

Their data set was large, N > 15,000, and not particularly old, with a mean maternal age of 31. Here’s a key point: “Among the 11,994 women with low-risk pregnancies on the basis of a maternal age under 35 years, cfDNA testing identified 19 of 19 women with trisomy 21, with 6 false positive results.” This is actually a good result, as older methods have a much higher false positive rate. But many people will not be reassured when they see that the true positive and false positive ratio is so high. Base rate neglect is always going to crop up. For many couples who get positive results confirmatory evidence is essential. That is why it is crucial that the mean gestational period of detection has to be pushed back beyond the beginning of the second trimester if these tests are to cause the minimum amount of discomfort for the couples. The later the abortion, the more psychologically and physically stressful the process.

But, I put a question mark in the title of this post because there is often an assumption that widespread screening will result in a massive decline in the number of individuals with Down Syndrome, to the point of extinction. Actually, I’m not sure about that. It seems that widespread adoption increases the pool of people who are getting tested, and fewer of these in the United States terminate pregnancies through abortion that the early adopters. As mean maternal age creeps up the number of people with Down Syndrome in the United States may not decrease nearly as much as we expect.