Ok so this is just a result giving an explanation how they come up with the result PPV.

That doesn’t really matter and it’s too confusing but what it essentially means is the “normal” risk at a certain age.

Except for they say “a priory” 1:20 this is age correlated to 45 years old so that PPV would be 98% by their statistic

It’s the same if you go to https://www.perinatalquality.org/Vendors/NSGC/NIPT/ And plug in ages and PPV risk for t 21

The estimated prevalence of Trisomy 21 at 16 weeks gestation for women who are 45 at EDD is 1 in 22.

Anyway this is just a chart summary of known risk at certain age in general population and it’s called A priori risk

And then the PPV associated with that statistic based on their internal studies.

So if you got a positive for trisomy 21 nIPT and they are giving you a PPV of 66% you are probably in your early 20s based by this chart and prevalence.