I don't think it's the most wise thing to do.... My mom quit cold turkey from cigarettes and ganja when she was pregnant with me because the doctor told her it could have ill-effects on developing cells, so she took the advice...
But think about it, this is a new creature being made by EVERYTHING the woman that's pregnant puts into herself... So if she doesn't smoke and keeps up a healthy habit then the baby should be just fine, but if you ate at mcdonalds everyday and a bunch of GMO's along with a toke, then yeah, your gonna have some health problems with your baby...
but then again i don't know for sure, this is just from my knowledge