the nice thing about flying when pregnant is you get special treatment, even on those airlines that have no assigned seating. Long haul flights require a doctor's letter saying how far along you are, and they each have different cut offs, so check with your airline (then you'll feel more informed/reassured too).

Beware the danger of being a bit vague: for instance at 28wks pg about to fly jakarta-sing-melb i left in the taxi the bag containing ALL our I.D./valuables/doctors letter and EVERYTHING but our passports as we arrived at the airport. irreplaceble documents were lost forever and i was distraught, but as the passports were in my hand, we still got the flight to have the baby in australia (in a labour so rife with complications we probably wouldn't have survived it elsewhere). Then the next day i walked through customs into singapore (transit) oblivious to the signs and tv screens everywhere saying cigarettes need to be declared... I was carrying a carton for DH and was detained, interrogated and nearly arrested and fined $1000, until i could convince them through copious genuine tears that it was an accidental oversight because i was so vague! (it still cost me $300 in taxes tho.)