When we were looking at the CA prescreening data for our fetus, it came to my attention that a "positive" test for one of these blood levels was, nevertheless, still an indication that IN ALL LIKELIHOOD our fetus was normal. That is, the vast majority of people who got a positive test were false alarms, and only a small minority of those positive tests actually indicated a problem. (This wasn't a problem with the criteria of the test itself, there was just no good place to draw the line to get ONLY correct rejections and "true" positives). However, it's incredibly easy to think that your fetus is "sick" if you get a positive test. Intuitively, it's very compelling. I don't know what the hit rates and false alarm rates are for the tests Saletan is talking about, but if they are anything like the blood screening or nuchal fold tests then people need to realize that if they get a positive then it's more LIKELY that their fetus has a problem than if they had a negative test, but it's more likely STILL that the fetus is normal.