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 :-)