Congratulations.
I went to WDW when I was 7 months pregnant. I was pretty big at the time too! Granted it was late Sept. & it wasn't very crowded--- but it was still hot & humid some of those days. Ofcourse, if you go in the summer, it will undoubtedly be hotter.
It is *doable* if you want to do it. You and your party can have fun. You just have to be prepared & flexible. It turned out to be one of our best vacations.
Things that helped me:
1.) maternity belt
2.) powder
3.) comfy sneaks & nice flat sandals for evening
4.) light weight cotton rompers --- all in one short outfits
(wore them all week)
5.) water bottle & mister fan
We stayed at the GF. It helped to be on the monorail. But that was just our preference. We planned a light morning, came back for a swim & nap and headed back out for a light afternoon. Evenings we generally did something around the resort. Listened to the piano playing in the GF lobby, strolled the shops, walked the grounds, swam.
Towards the end of the week, I used a wheelchair. I was reluctant to do it, but afterwards I was glad I did. It helped out so much. I didn't slow down everyone with my sit down breaks. And I felt better at the end of the day. I didn't feel as tired. I borrowed it from the front desk at the resort (no fee) and returned it upon check out. (FYI, If you get one in the parks, there is a fee.)
If you go, just take it easy. I enjoyed the parks so much. I didn't miss not riding most all of the rides. It was great just to be there. Especially great knowing we were having our "last hurrah", since it would be several years before we come back. We want the little one out of diapers before we return.
Good luck with your decision,
Annemarie