If you see a pregnant woman and don't know what to say, stick to something along these lines:
1. You look so cute/pretty/radiant etc
2. I am so happy/excited/giddy etc for you
3. You're going to be a wonderful mom/parents etc
4. Your baby is just going to be cute/pretty etc
Yep, I'd say that pretty much covers the safety zone.
side note: I have (very fortunately) been feeling well and/or great, so I've been able to answer the question "how are you feeling?" with "good" or "great" ...so I can't really speak for those preggies who feel otherwise. Treading lightly is probably always the best tactic.