I am currently 22 weeks pregnant and we are pretty sure we are going to cancel our upcoming September trip. I was talking to my dad last night (he lives just south of Disney) about this same subject because I was thinking we could maybe reschedule for one of the cooler weather months. He says the mosquitos never really go away because it doesn't get cold and stay cold long enough. He said he actually notices them less during the hottest months, and more so from the November-February timeframe, probably because it's cooler and they are out more often. Sorry, probably not what you wanted to hear lol. If it were me I'd either delay trying to conceive until after your trip or just continue to monitor what the CDC says about the situation and discuss it with your doctor.
Sorry about your trip but you are making the unselfish and wise choice. Why risk the health of your unborn baby to go to Disney? Disney will be there for many years and you can come back when you are not risking your innocent unborn baby. It will give you more time to plan for a great vacation. Good luck with your new baby when he/she arrives.