When we went in 2005, I was just shy of 6 months pregnant. I searched every website & the boards for all the pregnancy advice I could get--and talked to my OB!! Here is what my advice is:
Always good, but especially important when pregnant--walk, walk, walk before you get there (I did 3 1/2 miles a day, pushing my older two in the jogger). It gets you more prepared for all the walking when you are there. Also, walk in the shoes you will be wearing at Disney!! I wore crosstrainers, the whole trip. Fashion be d@m*#d. It gave me room for any swelling, and my whole body was used to the shoes. Best thing I did.
At night, I soaked my feet in bath salts for tired feet. Another great bet; it made a HUGE difference. I was always ready to go the next day.
Avoid all the pregnancy restricted rides. There are a whole host of reasons on why you are supposed to avoid flume rides and rollercoasters beyond pre-term labor & miscarriage. I always say on that type of stuff, better safe than sorry. Who wants to question themselves later if their child's learning disabilities are from their brain being shaken around in utero later even if it is probable that it wasn't anything you did? But, of course, talk to your OB on it. ONly the two of you can make those decisions. This also gives you a great time to get in rest while the rest of your party rides. Besides, its very little you miss.
You might want to try a belly belt thing. A lot of people have sworn by them, even those in the "just starting to show" stage.
Stay hydrated.
Only plan lightly, that way you make adjustments for what your body tells you. Listen to that body! Don't go commando at the parks. Feel free to sleep in late and head back to the hotel early.
For any food cravings & aversions, know your restraurants and plan those carefully.
GEt familiar with the location of bathrooms.
Don't listen to other people tell you that you are miserable. I never felt any worse off than I did at home, but complete strangers felt the need to tell me I was crazy and felt horrible.
If you can, stay on site. Repeat-- STAY ON SITE. My picks would be Poly, Grand Floridian, Contemporary or the Wilderness Lodge for convience sake. For that matter, it might pay off to have a car (or shelling out $ for cabs), especially if staying at a different hotel. Did I mention, stay on site?
OK, that's all I can think of. Those were the very important things when I was pregnant.