If you are flying a US airline they require pregnant stewardesses to stop flying after 28 weeks. They will at that point require a note from your Dr. clearing you to fly, so it will be up to your Dr.
It also depends on where your going, I'll be flying when I'm around 34 weeks to but I'm only going 2 provinces over so they're more relaxed.
it's best to check with individual airlines as they have different policies on how far into the pregnancy you can travel. i think most airlines have a cut off point of 36 weeks, but you will need to check with the airline you want to travel with what their policy on it is