How can a baby be born positive for meth and going through "withdrawl", yet the mother be negative?

Someone I know had a baby and it was having seizures and screaming fits, so they tested the baby and he was positive for meth, yet the mother was negative. The mother was given Tylenol 3 and fed, but that's the extent of any drug relations. The baby also has an infection of some sort. They scanned his brain with an MRI and found a white spot in the left hemisphere, the specialists have not told my friend anything yet so I suppose they have no clue. Any ideas? The only thing that I could come up with is that maybe is could be an infection that was ped to the baby in utero and that it somehow created an interference with the meth testing by possible kidney damage or infection... ?? Sound plausible?