When do you tell your boss you're pregnant?

Ha

I'm a nanny. So a Mom is my boss, right now I've been dealing with nausea in the morning so the three little boys under 4 years old are a little much for me. But I love my job. How do I go about telling her? I feel like she'll react like "oh no... so you'll be gone in 8 months?" And that's definitely not the case. Maybe I should start it by reminding her I am planning on staying. Help!