I think it's just an issue of being up in the air, not near a hospital.
I just asked my OB last week and she said no worries up until 30 weeks. Then she would want to at least see me before the flight if it was after 30 and then said that I should try to only do "necessary" trips after that. But if something was deemed necessary by me that she was ok up to about 35-36 weeks.