And say how many false positives there are.

If someone gets a 1:300 chance and then it turns out that they don't have Down's that doesn't mean it was a false positive.

A 1:300 chance is not a positive at all. It is just the odds.

A false positive would be a 1:1 chance and the baby comes out fine.

Correct me if I am wrong please. Maybe I am confusing this test or something.