I flew from Japan to Spain when I was 5 months pregnant and had no problems. The airline did want to be notified that I was pregnant, but they had no problems with me flying at 5 months. My doctor didn't blink an eye at it either (and the flight was 20+ hours.) You'll have to check the airline on their restrictions for pregnant women (most are for after 32 or 36 weeks), but you'd probably only need a note from your doctor OKing your travel plans (good idea to carry even if you aren't at the airline's cutoff if you are really showing).
I'd consult your doctor about flying in your first trimester (especially if you are having problems or have miscarried, etc). But I don't think it'll be a problem otherwise.
Do consult your doctor about any flying concerns. If he approves the travel, ask for recommendations on how to keep in good health during the flight. Read up on flying while pregnant. Make sure you are well hydrated during the flight and move (walk the isles, roll your legs, etc) to make sure you are having good circulation during the flight.