I flew from Sydney to Brisbane when I was 34 weeks and returned to Sydney at 36 weeks. It wasn't bad at all but then I was flying for a holiday and had a two week break in between flights. What I did find difficult was the higher humidity in Brisbane (we went up for Christmas 2005) and was fairly miserable much of the time we were there. I would feel completely different about flying if it were going to be a regular thing during the pregnancy.