Wednesday, November 25, 2009

Non-invasive in utero tests for Down syndrome

The risk from amniocentesis and from chorionic villus sampling to test for Down syndrome is small not not zero, so non-invasive tests for it are welcome. Each of the candidate tests has a high rate of false negatives, but used in sequence, they seem to be quite successful. In the first trimester, we can measure one protein circulating in the maternal bloodstream, and in the second trimester we can measure another. These are usually used to determine whether to do further, this time invasive, testing.

One relatively new way to avoid amnio and CVS is called a genetic sonogram, which is "a sophisticated ultrasound that details the fetal anatomy ... looking for the presence of major fetal anomalies or specific anatomic features (so-called 'soft markers') that might be found in a child with Down syndrome."

In a recent study  of 8000 pregnant women screened  for fetal chromosomal abnormalities,
The detection rate of Down syndrome babies varied from 69 percent for the genetic sonogram alone to as high as 98 percent with certain combinations of the biochemical markers. More importantly, the improved detection rate was accompanied by a decrease in the screening tests false positive rates.

So using the current method of choosing whom to give amniocentesis to, and then giving them a sonogram instead of amnio, gives 2% false negatives and a smaller number of false positives than from just the blood tests. Since amnio gives no false positives or false negatives, a woman will have to weigh the current risk of amnio or CVS against the new error rates.

  • If you would want to abort a fetus if it turns out to have Down syndrome, how much risk is a 2% chance of having such a kid anyway?
  • How bad would you feel about  the chance of aborting a kid that in fact did not have Down syndrome? 
There is no way in the world I would ever make a generic recommendation on either part of that issue. It is as personal as any decision you will ever make. I wish you well but have no advice whatsoever.


  1. I recently came accross your blog and have been reading along. I thought I would leave my first comment. I dont know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often.


  2. Thank you. It's nice to know someone is listening.