I flew from Europe to OZ when I was 22weeks and didnt find it as uncomfortable as I had thought (granted I didnt have much of a tummy yet). I also managed to get an exit row seat which was great, as I could stretch out my legs. See if you can either prebook one of these seats or ask when you check in. On my return trip I was 32 weeks pregnant and was in business class which was heaven. Your going for work right? Any chance they would pay for a business ticket? Like the others have said, walk around a lot and drink heaps of water.