> Right, but a positive screen is usually followed up with a more specific test (karyotype, cvs, amniocentesis) which can give a definitive diagnosis.

I was thinking more of false negatives than positives, but reading it back, there was no way for you to know that from what I wrote!

The test for our daughter came back negative (well, something like 3,000,000:1 chance of Down's), but she did actually have Down's. Later, talking with other parents of kids with Down's, multiple had had similar odds, some even crazier (and in a small country with <50k births each year).