Maybe when you next see her smoking ask her if she can feel her baby coughing.
That is awful - I know it is sooo hard but you have to ignore the bitch and think that you are doing much better by your baby - dieting, smoking & drinking and then being proud of your under-sized bump is truely sickening. Ok your diet may not be text book perfect (whose is!) but your weight gain is NORMAL and your baby will be much healthier because of it. If I were you I would be proud to show off my healthy bump at the party and rise above it.
Good luck
Kx
Thanks everyone I've decided to go to the party on Saturday and just pretend I haven't seen her comment, but when she starts smoking my OH has already said he's going to ask her to do it away from me because he doesn't want his baby harmed by the smoke (he was as annoyed as me by the comments!)