If you follow the doctor's advice, you are not supposed to fly before the first 3 months or after the last 3 months.
According to the previous airline restriction, you could not fly after 7th month. You could, however, still be able to fly even after 7th month as long as you get a letter from your doctor confirming you are in good condition and will not be delivering your baby too soon.
From my personal experience, it should be safe to fly. I'd made six flights during my 4th month to 8th month pregnancy (that is why I called myself busy mummy!) and delivered the baby girl in October, and the baby is in absolutely perfect condition.
I got a friend who is now at about her 3 month's pregnancy, and she has been flying around many times even during the first 3 month and she is in very good condition!