I completely understand where you are coming from. My Mother in Law and Sister in Law smoke as well as a bunch of other extended family members.
I simply sat down (easier said than done.... it took me 6 months to get the courage) with her and tell her your feelings. My Mother in Law now smokes only outside and she has to change her shirt or wear a coat when she smokes. In result she has cut down her smoking overall and especially when she comes to visit.
My doctor also wrote a "Rx" that said ... no one is to smoke around Dominick at any time. He may get asthma if not followed or something to that extent. I had that on my fridge as a gentle reminder.
This may not work for all but seemed to work overall... and no she did not take it very well at first...but, once she had a chance to think about it and the health of her grandsons (number three is on the way by her daughter) she has more than accepted it...it has become habit and automatic.
HOpe this helps a little. Good luck with your delivery... and cherish your little one!!