[gwern]

Presumably what he is leaving out is that the increase in white-spots led to more amniocentesis, which then confirms the Down syndrome. If you did amniocentesis on all babies, it would of course increase the diagnosis rate even more.

Whether this is a bad thing, as he claims, depends on whether you believe screening was being done optimally before, and that will depend quite a bit on things left out like the utility of not having a Down baby. (He doesn't present his working out the entire scenario, as it's just an aside, but hopefully before Jordan went around telling people how to change their prenatal screening systems, he did work it out a little bit more than back-of-the-envelope.)

[lifeisstillgood]

Actually you would not expect it to increase the Downs rate at all - the null hypothesis is that there are X% Downs babies born and Y% identified through amniocentesis (where X-Y is z% the percentage of parents choosing termination)

edit: actually there is a Zt (percent of parents choosing termination after detection) and Zu (percent age of undetected cases going to term). Zt is a social / moral thing and won't change based on better pixel resolution, but Zu should not be expected to change either - we are assuming there has been no change to the real rate of Downs (which requires something else) and no change to rate of parents choosing termination (see morals)

so ...

If Y% increases a lot (better detection of an underlying true rate) then either X% must increase or z% must. Neither of which i think we expect or know about.

So what I hope happened was dramatically better training for operators on the new ultrasound (kind of like exactly what did not happen onthe 747Max)

So either the OP was one of the first to spot this issue, and tipped off the whole medical industry, or, and this is where my money goes, he followed the reasoning of dozens of professionals who were several years ahead of him (naturally) and was reassured by someone who just saw "anxious parent" in front of him.

But that's fine too :-)

[gwern]

"Actually you would not expect it to increase the Downs rate at all - the null hypothesis is that there are X% Downs babies born and Y% identified through amniocentesis (where X-Y is z% the percentage of parents choosing termination)"

No, see, you're misunderstanding and making the exact same mistake by assuming that the nurses sat down and crunched the spreadsheets, which they obviously did not, and implicitly conditioning. Yes, if you slice the multivariable data just the right way to extract the conditional %, the increase would be the same. Screening more women won't make the conditional percentage go up.

However, if you are just a nurse, observing # of women coming in, and # of confirmed Down babies coming out... Screening more women will mechanically make the # of confirmed Down babies go up. And that's what the rate is, it's a count.