This is common sense, if you are with child you have a duty to keep your body clean of drugs if possible. Anything short of this is neglect and is a crime. If you are going to have a child, seriously understand, you have an option before you. Give this child the best chance at survival by keeping yourself clean or don't care and just do whatever.
I have heard first hand a mother trying to argue she needed the drug because without it she threw up much more and the baby wasn't getting the right nutrients. I am sure there are many more explanations like this for smoking mothers.
To them I would say, "Please, it is nine months of your life that will effect the entire life of the child."