The NF testing shows a risk factor that can determine if more invasive testing is warranted.
Sometimes a NF test can show up high risk but testing such as amnio or CVS will rule out Downs Syndrome. The problem with these tests is that as they are invasive then there is a rick of miscarraige.

Other times the NF test shows a low risk but then other markers show that the baby does have it at the 18 weeks U/S.
They still look for these markers at the U/S even if you have a low risk.

No test is fail safe though and some babies are born with DS even though all the tests have been done.