Maybe she just left out from the story, but why didn't she say "But I didn't take any meth! I didn't even drink any alcohol! After a miscarriage I'd never do anything like that! Is it possible that the test is wrong?"
Doctors are like cops in that they're used to being lied to, particularly by drug users. Telling a doctor "I didn't take meth" when he has a positive test result is likely to mean exactly nothing unless you have a long term existing doctor-patient relationship.
Had they been certain the test was right, CPS would have been hovering over the birthing bed and the parents would have risked carting to a cell. That's not what happened.