You need to walk around the cabin frequently as pregnant women are in a higher risk category for Deep Vein Thromboses. (DVT). I think the pressure socks sound like a good idea also. If I were you I would book an appointment with your doctor - either GP or OB - an ask for their advise and suggestions for how often to walk around. (I think it is at least once every hour but don't quote me!) I think you may need to keep up more fluids then normal too - which is great for a pregnant women's bladder! When you check in, or even before, contact the airline and tell them you are pregnant as they may be able to give you a bulkhead seat which has more leg room.
Best wishes for a safe journey.