I never flew when actually pregnant, but I'm pretty sure that by 6 months, I would have found it physically very uncomfortable. I couldn't bear to have anything binding me around my middle. So anything with a waistband was bad news and seat belts were no fun at all. Obviously, I required both, but I wished I didn't! Plus, I found sitting upright awkward.
If I'd had to fly transatlantic, I'm sure I would have had to get up and walk the aisles a lot and pray for no turbulence as I'm a skittish flier at best anyway!
Just my 2p, though!