I went to WDW in May 2009 @ 5 months pregnant with my DH and oldest DD (who was a little over 3 y.o.) and here's what worked for me:
-I second "figment:"
chug o' lug the bottles of water! As a consequence of this, know where the bathrooms are located! I really liked the bathrooms at the Child Care Centers (except at DHS)...don't be afraid to go in there and just "cool off!"
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Take your time: by mid-afternoon I had to take "bench breaks," preferably in the shade because the heat and sun could be quite intense. At EP, I sat in Innoventions West for almost an hour, while my DH and DD did activities. Have "the talk" now with your DH and child about the pace you'll most likely be taking.
-I second "Mrs. Jobson's" comment:
heed the warning signs at the rides...if it says, "Expectant Mothers" or "people with back problems, heart conditions," it's best to avoid that ride. For example, at AK, Kilamanjaro Safari: I skipped it while my DH and DD rode it because I felt that the bounciness of the vehicle would be too much for me. As I was sitting near the exit of the ride waiting for them, I see another pregnant woman (probably around the same months as me), being escorted by a CM under her right arm and her DH on the left...she had just rode the Safari and felt very faint and uncomfortable...First Aid came and gave her an ice pack for the back of her neck and a bottle of water to drink ...she sat there for quite some time and I overheard the CM say to her, "This is why we post warnings at the beginning of every attraction."
-Soak your tootsies: by the end of the day (I would wear sneakers for the majority of the day, and then switch to flip-flops by the evening), my feet and ankles were quite swollen from all the walking and water retention (dear lord, the retention did me in), so I would soak my feet in a cold bath back at the resort. After a good soaking (10-20 min.), I would prop my feet up in bed and relax for an additional 20 min., as though I was nursing an injury
Congrats and enjoy your vacation!!!