I think the main problem you might face is the long flights - you will need to get up and walk around the plane a lot to keep your circulation moving. Can you break up the long flights with some overnight stops just so you're not stuck on the plane so long? And drink LOTS of water on the flights as planes dehydrate you even more than pregnancy does...

Once you're there you'll be able to pace yourself - spend one day resting somewhere pretty for every couple of days of tearing around sightseeing.

Have fun - it sounds like a great trip!