OP, not to get way off track here, but are you taking a longer than usual maternity leave? I was counting backwards on my fingers, and if you plan to take a 12 week leave you are about 4- 6 weeks pregnant. I think it's great you are looking ahead, but 10 months out from the potential start date is a lot!
That said, if you know friends who know people looking to help their nanny find a new position, it never hurts to network through neighborhood listserves and so on. A nanny whose charges are headed to FT school who doesn't want to be the housekeeper/nanny might be interested in making a change.
As a nanny, I would be willing to meet a family up to 3 months before they would need me to start, and IF they were willing to prepare and sign a contract AND to pay a significant retainer, I might even sign on as much as 8 weeks in advance of the start date.
More than 8 weeks out though, for me, is too much time for people to change their minds on all sorts of things, including whether mom is actually going to go back to work.
Good luck with your search next summer!