I flew long haul this summer was about 16 weeks on the way out and 20 weeks on the way back, 10 hours each way, just me and my 2 year old! It actually wasn't that bad at all I just had to plan and prepare more.
I did have a seat for DS which made life easier but then he had to have one as he was 2+. Could you get the bulkhead seat? I don't know about other airlines but BA does a bassinet to strap onto the fold down shelf of babies 0-6 months and then they have car seat-type thing for babies 6-24 months. This means that you get space for your DC to sleep without paying for an extra seat. DS last used one when he was 13 or 14 months and it worked well. Even if you can't get that car seat-thingy, a bulkhead seat would be a good idea as you'd have a bit more space and your DC could get down onto the floor to play.
When I fly long haul with DS I make sure that I have a ton of things to entertain him, I load the Ipad with shows and apps, take new little toy, playdough, snacks (eating's an activity!) all kinds of stuff. He eventually gets tired and sleeps for most of the journey.
As for the whole pregnancy thing, I did find it a bit tricky to managed all the bags and walking but, through the kindness of strangers mostly, I managed. I also found that my lower legs and feet really did swell, I'd probably take compression socks next time.
It might be madness and the journey might not be wonderful but it's a fixed amount of time and hopefully your destination will be worth it. For mean spending 4 weeks with the in-laws was well worth it. They lavished attention on DS, I lay on the sofa and ate 