I had a great time in WDW when I was a little over 20 weeks pregnant. First, if your doctor allows and you are beyond your first trimester, I would recommend a pre-natal message over at the Grand Floridian. It was absolutely wonderful. After the message, I sat by the Grand Floridian pool, which you are allowed to use if you are getting a treatment the same day.
The nice thing about the WDW parks is that there are plenty of rides that are low impact and can be enjoyed while pregnant. Some of my favorites are the Haunted Mansion and It's a Small World. I also went on
Soarin' while I was about 8 weeks pregnant over in Disneyland and enjoyed it. If you are not far along and in good health, you may want to check it out.
Be sure to keep yourself hydrated and take a lot of breaks. Most of the shows are inside and air-conditioned. Also be sure to see the 4-D shows like Mickey's PhilHarMagic or an interactive show like Monsters Inc, Laugh Floor.
Congratulations and I hope you enjoy your trip.