How unfortunate for both of them. She's playing roulette with her child's health. Here is what some studies have found

Previous studies have found that in utero exposure leads to poor fetal growth, abnormal brain developments, birth defects, visual deficits, and physiologic stress.[1] After birth, these infants have been found to experience behavioral problems, hyperactivity, and memory and learning deficits.

A Swedish study found 

They found that 8 year olds displayed aggressive behavior and social adjustment issues, which were positively associated with the amount and duration of meth exposure in utero. By age 14, their language acquisition, athletic abilities, and mathematic skills were statistically lower than those of their classmates..

Its possible and even probable that simply living in a household with a meth addict for a mother is, by itself, contributes to many of these issues. 

She should not breast feed as the meth is passed into the baby in her milk.  This is very rare but one woman was convicted of killing her infant by passing so much meth into the child through her breast milk that it died.