I think I might stick to the "if you can't say anything nice..." philosophy, but I do agree with this:

I think you also have to concerns for when the baby is born. If Mum is unable or unwilling to stop drinking and smoking during pregnancy, she most likely will continue after the baby is born. Babies near a carer who is unaffected by drugs or alcohol and sounds like Mum might not be willing to do that. Is there another carer who will be able to step in when Mum is taking her time?