Carnival Cruise Western Caribbean Excursions?

Question

My husband and I are going on a cruise this summer and we are also trying to get pregnant. Do we have to sign up for our excursions now or can we wait until we get there? The Dolphin Swim & Ride sounds really neat but it says that pregnant women are not allowed to participate in that excursion. I would hate to sign up for something like that and get there and not be able to do it. Any advise would be great! Thanks