Posted here are a NYT Article about genetic testing companies and the value of the results they provide along with an additional explanation of the math behind the issues.
Altogether, there are 415 positive tests: the 15 true positives, and 400 false positives. So if you get a positive test result, the chance that the fetus is actually affected is about 3.6%.
On its face, this sounds like a really good test! It detects 75% of cases, with only a 0.5% false positive rate. That seems like it should be helpful.
Further discussion from this author about Bayes’ rule