I did Costa Rica to London at 36 weeks (two planes, 18 hour trip total) I did manage to get the priority seats on one flight, it made a huge difference, they are the ones in front of a wall with loads of leg room (and you can rest your feet up against the wall
) They are kept "reserved" for priority passengers right up until boarding, so you have to be at the gate early and be prepared to "out-prioritise" other passengers who may be fighting over them, especially if it is a crowded flight.
I second the use of flight socks, I also took baby aspirin as the risk of thrombosis increases with pregnancy anyway, adding the flight will up it even more. It MUST be low dose (81mg) aspirin though, not full dose. Also keep an empty water bottle with you that you can fill up from fountains once past security, and ask the stewards on the plane to fill it for you as soon as you get on.
I found at the Costa Rican end there were lots of priority queues or signs saying I could queue jump which took a lot of the strain of travelling away, things not so good in USA and Europe though, ask where you can if you can queue jump as it will save you a lot of energy.
Pillow and blanket is also a great idea - I invested in a nice comfy neck pillow and was glad of it.
good luck and have a nice trip.