In north america when are obs doing blood vs genetic amnio tests on babies?

Depends .: Both are being performed for different risks.

Screening vs confirm: Blood screening is a relatively non invasive way to get information on significant problems but by definition it is screening. Screening is an effort to narrow down the list of people that need a specific confirmatory test.By example, for every 1000 screened, 10 may be positive, and offered the amnio to find 1 true positive. An amnio is more invasive & risky but gives more definitive results.