I've been to Disneyland twice while pregnant. Once right after I found out I was pregnant with DD and then once when I was about 20w pregnant with DS.
I did a lot of Google searching to see what rides I couldn't go on, but really it all comes down to common sense. Don't go anything fast or bumpy.
Maybe make an excuse that you want to look at gift shops or eat a snack while they ride the bigger rides?