How does a baby test + 4 drugs, Mom hair test is -?

If a pregnant mother ingests a drug during the first trimester of pregancy, at birth of the baby can it still be in the baby's system? I know a new mom who tested negative for all drugs, yet her baby tested positive for drugs she has not touched since 3 months pregnant. In fact a drug test by hair, which goes back 90 days on the new mom came back negative, why is baby positive? Someone please find the answer.