I was 3-4 mos preggo when we went in Jan. My then 3 yr old son didnt go for the "big" ticket rides thankfully. I rode just about everything in Fantasyland, skipped the tea cups (spinning not good), was very concerned/cautious on the Indy Cars (bumping can be bad), etc. I avoided anything that bumped along, spun around a lot, roller coasters, and smelled funky (morning sickness, though I never got sick).
Try to avoid the sun as much as possible (can cause a funky mask on your face), avoid the heat and humidity, and drink tons of water (which means you will also need to know where the bathrooms are). I get so dried out walking around WDW, even in Feb and when I wasnt preggo, so keeping hydrated is important. Take frequent breaks and try to put your feet up to avoid swelling (especially later in pregnancy). Smells (food, perfume, etc) may or may not be a problem the first few months (morning sickness). Also eat well.
alison