I was just over 10 weeks pregnant when we went to WDW and for the most part I was fine. However I found I had an aversion to chicken (which I normally love) and it made me ill anytime I ate it, I didn't figure this out until I was several days in. The cravings also started and I spent 90 minutes searching DTD for a ham sandwich! I also was pretty tired and we were on the Deluxe Dining Plan but we ended up missing a lot of our reservations as I didn't feel like eating that much. I found there was an awful lot of rides I couldn't go on, some were quite surprising as I really thought they were quite mild. Morning sickness started the day before we returned home and it came with a vengeance!
To be honest, if I had a do over I wouldn't have gone. While I will always enjoy Disney, it was an awful lot of money to spend when I felt so tired and couldn't enjoy most of the rides or food that I had spent so long planning for. It really does depend on the person but unfortunately for me tiredness hit me hard during the first trimester and although flying wasn't a problem for me, I found I was worried an awful lot and it was the only time I was glad to be coming home. I found out I was pregnant when we only had 3 weeks to go, so we would have lost a lot of our money. However I wouldn't advise it if you had the choice. But that is just my opinion and I am sure others loved going to WDW when pregnant.