It's happened for years one way or another. Before methadone it would be brandy or whisky.
Drac is spot on, that current processes include assessement pre birth and post delivery to assess parenting competence.
A few years ago I worked in an area with quite a high number of methadone users, and in 14 months saw at least 4 methadone overdoses. All described as "accidental" by the parents, but life threatening in two cases, history of all kids were that they'd had to undergo a withdrawl process at birth. Got a mate currently working on a special care baby unit and things have not changed, still got a lot of babies being born to drug dependent mothers needing withdrawl management.