If you are seeing your Dr anyway I would get a Dr's certificate so that you are armed with it at check-in should anyone query it. I would also ring the airline once you have booked and tell them that you are pregnant and ask if you could be allocated a seat with easy access etc. If you are feeling up to it maybe be sweet to the person at checkin and ask how full the flight is and if there is any possibility of an upgrade or at least a vacant seat beside you so you can spread out (never hurts to ask - sometimes they say yes )

Hope that you can get everything sorted out.