I'm going to tell you a secret right now.
Here it is: Pregnant women don't want to hear your comments about how huge they are and how you can't believe that they still have x amount of days or weeks before they are due. I don't care if the woman has gained 70 lbs and has to be rolled around on the floor because she is so huge. I don't care if it looks like she has a fully inflated beach ball down the front of her shirt. I don't care if she has walk around with both hands under her stomach to support the weight of the baby. No pregnant woman wants to hear "Oh, you look like you are ready to pop at any moment!" Particularly if said pregnant woman has just disclosed that she has more than a month (or two or three) to go before her projected delivery date. If you are shocked when she says she has 8 week, SHUT UP. Keep your face neutral and stick to these stock responses:
You look beautiful
Variations on these responses will work, but don't get too crazy. You can also combine any of these options if you feel the need to say more. For example, "Congratulations, you look beautiful" or "That is very exciting, congratulations. And, by the way, you look beautiful." Under NO circumstances should you say "Good luck with that" or "I'll be praying for you." As for the last comment, I'm certain many pregnant women appreciate the sentiment that you will be praying for her and her baby, but, put into this context, the comment is too easy to interpret as "I'll be praying for you, you are so huge with your giant mutant child that you are going to need it."