They used 48 samples
* 2 male -> no grow
* 6 non-pregnant females -> no grow
* 40 pregnant females -> 12 no grow | 5 poor grow | 23 grow
Mi guess is that in the press article they calculate 28/40 = 70%.
By repeating the test every few days, and looking at the patterns of grow/no-grow you would probably get a much better result.
Regarding false positives - since you’re detecting hormones that are not produced outside of pregnancy time, then the only possible reason for false positive could be some genetic condition, or maybe some serious endocrinologic disease, maybe ovarian cancer.