I would create a huge ethical problem. The mother is harming the unborn baby, no doubt. But because she’s addicted to nicotine she has lost her free will (at least to a high degree). So allow me to rephrase this to a new (hypothetical) questions:
Would you prosecute people who start smoking?
They still have the free will not to do it.
Seriously, I don’t think we need tougher laws. We need better parenting, better education, more intelligent prevention programs. We need to show how people can experience joy without wreaking total havoc with their neurotransmitter systems.