Depends on the airline. Most airlines wouldn't allow a 30-week+ pregnant woman (on international flights) for fear of the cabin crew becoming instant midwives.
Please find below four links that should be helpful. But, the bottomline is to contact the airline you wish to use!!
Can anyone help please? My Gambian Wife is 2 months Pregnant, and i wish to bring her over to England. My question is.. at what stage of her pregnancy, would she not be aloud to travel on a Flight??