Cogratlations, kassiemom! I'm in a very similar situation - 13 weeks pregnant due October 13. I know for sure that I am closing my daycare and taking at least a year off with my new baby. I agree that you need to have a solid plan in place, decide first what you want to do and to wait as long as possible before you tell your clients.
Also I would prepare yourself for the possibility of a family or two leaving as soon as they find alternate care (all the more reason to wait as long as possible to tell your clients).
I am telling my children this weekend and going to tell my clients after Easter. My contract states that I require four weeks notice if they decide to fine alternate care and I will remind them at that time that that agreement is still in effect.
Good luck and let us know how it goes.