Years ago I dated a NICU (Neonatal) nurse and I spent a fair amount of time at the hospital watching what goes on. Unfortunatley, mother who smoked, drank, or were on drugs had sick, low birth rate babies. Esp. the drinkers.....and the drug users babies could be born with their addictions. One thing to consider is these smokers don't stop when they have the child. So you are also talking about second hand smoke in the home.
I dated a woman that had two children, 10 & 5. She smoked while she was pregnant with her first son. He was born with severe asthma and was in and out of the hospital all his life. She took him to the bar one dart night (where everyone smoked) because she could not get a sitter. He was in the bar area on his nebulizer and I went insane (I came home early from a business trip and stopped by the bar). That lead to our break-up and she moved out two weeks later. A month later, she found him dead in his bed one morning. She was a selfish POS that I hold a great deal of pitty for.