The Aerospace Medicine Association (USA) indicate that in their Medical Guidelines for airline travel "Pregnant women can normally travel safely by air, however most airlines restrict travel in late pregnancy"
After 28th week, doctor's/midwife's letter confirming EDD is needed. Usually, in single pregnancies, flying is permitted to end of 36th week and in multiple pregnancies, flying is permitted to end of the 32nd week.

AsMA. Medical Guidelines for airline travel. 2nd Ed. 2003. http://www.asma.org