If you can afford it, not opening would be a benefit, but that's just me! I've read on here many who go through pregnancies without too much trouble, so I guess it all depends on you! You could also consider just taking in 1-2 wee ones, that would help you manage your time and stress a bit better...if they are good kids of course!
I'd also guess it would be harder to find families once you tell them you are preggo, and IMO I think you really should offer that up front. I could be wrong, but I'm thinking from my point of view and if I knew a potential provider was pregnant and wanting to take some time off, I'd likely look elsewhere to avoid having to search again in 9 months or less! It wouldn't be so much the appointments, it would be the time after the baby was born I'd be worried about.