I'll tell you this: I found out I was pregnant on 25th August. We were flying to Hawaii (7000 miles away) on
31st August. When we flew out I was 5 weeks pregnant, and when we flew home I was 8 weeks pregnant. Each way was a 30 hour trip, and I am absolutely fine. Just make sure you don't do your seatbelt too tight, and that if you're going long haul, you remember to stretch your legs ...