One thing I did both times I flew in my 7th months was request a copy of my ob file to bring with me. That way in case the airline questioned how far along I was I had documentation. Also, if something had happened I would have had my information with me. My doctors in Austin and Phoenix were glad to do it, your ob chart is usually 4 pages or so with all your progress notes on it.
Adding: I made sure I flew in my most comfortable, loosest outfit, used a bag to raise my feet so my feet didn't swell and drank twice my normal water - I had to force it down but I was glad I wasn't dehydrated the next morning!