when should I stop working!

I'm a full time nanny for a 4 year old and an 18 month old. I work about 40 hours 5 days a week. And I am 21 weeks pregnant today. I'm trying to decide when I should tell my boss when I'll be ready to leave. She already knows I'm pregnant and understands I'll have to leave. I told her I'd stay until it gets too hard to chase the kids around. I just don't know how to judge that and still give her enough time to find someone else. Any thought?