I flew at about 27 weeks and it was fine, I was advised to wear support stockings too. I was also told to get a letter from my doc to say when i was due - just in case they decided to refuse me. Apparently they CAN refuse you if they really want, so it's good to have a letter proving that you're only 20-something weeks and not further along. And btw, I found it was a great time for a trip and I'll be doing it again - wasn't too uncomfy at all.

Have a great time!