You can never go wrong with "Congratulations!" and "How are you feeling?" (although the latter does get old and hard to answer as it gets closer to the big day).
Here is a helpful list of things that pregnant women you work with wouldn't mind hearing from you:
1. You look great.
2. Can I help you carry that?
3. Here's a chair for you.
4. Would you like me to bring you back something for lunch?
5. No really, you look great.
6. Whatever you decide will be the right decision because you made it, and it's what's right for you.
7. Why don't you go take a break, I'll finish up here.
8. I have a ton of cute maternity cothes and baby stuff I won't be using, you're welcome to it.
9. Where are you registered?
10. You're going to be a great mom....