When do I tell nanny fam im pregnant?

Yesterday I found out I am four weeks pregnant. I nanny full time with my 12mo old, and we had planned for me to continue to nanny until the kids are three and ready for daycare. They just turned two, so that would have been another year.

I do not want to tell them yet since I did just find out, but I definitely want to be transparent with them before I am too far along. When would be the best time to tell them? I would like to wait as long as possible incase they decide to find a new nanny since I would only be available for another 8-9 months. Thanks!