2nd trimester: I took a flight of 15 hours followed with 4 hours of local bus transport on my 16th week. Then do the same on my 18th week to return back. Everything was fine; but I must admit that I kept looking for toilet on the route. I did get a medical letter from my ganae that mine was of low risk pregnancy; also I spent more money to book a more comfortable seat on the plane.
3rd trimester: I took a flight of 5 hours followed with 3 hours bus transport alone on my 27th week. I book business class to make myself more comfortable.
Advice: get medical confirmation from you gynae. Research local medical resources in case of emergency. Spend money to get yourself a seat nearer to toilet.