I had my fourth in Sept and worked almost all through the pregnancy. We told everyone at about 8 weeks, since I was already beginning to show (booo! LoL)
I put out a slightly early edition of our monthly newsletter. Before we said anything, though; we made sure that we knew exactly what the plan was. I was due Sept 9th, so my last day was to be the end of August. I took the month of Sept off, and reopened at the beginning of Oct. My husband has very flexible hours, and he was able to be around a lot when I needed him. This was good, because she didn't arrive until the 15th, and therefore I only had a 2 week old, instead of an almost 4 week old (and we all know how much a difference a week or so can make!!).
I was fully prepared to lose some, if not all. A month is a long time to find alternate childcare. I ended up with one family saying yes, then no, then yes, then eventually they bailed on me right before I opened up again.
I warned everyone that things would be different, and on occasion I might answer the door in my pyjamas, haha. That maybe we wouldn't do as many "structured" activities, but that the kids would be getting as much care and attention as they always did.
My husband was here for prenatals, which I always scheduled during naptime. The parents all knew what was happening, and were okay with it. I even put my appts into the monthly newsletter, in case they wanted to do early pick up that day or something.