I flew from the USA to Australia at 33 weeks pregnant. I moved around a lot, I had my legs up on DH's lap, I took my 3 yr old to the toilet lots and for walks around the plane, I did stretching exercises, kept my legs and feet moving, drank LOTS and LOTS of water. So, more toilet trips! I was fine. The only time I stayed in one place for an hour and half was when I was sleeping. It was such a long trip and with a large belly and a 3 year old too but it wasn't that bad considering! The staff were very understanding of my need to move around!