Maternity leave

So what's the best way to inform your job that you are pregnant. I work with 1 year olds at a day care and will be 16 weeks next week. I'm just not sure how to go about informing my director. What do I say? What do I ask? Are there any specifics I should mention or ask ? I'm just not sure. This is my first child and I've been told you should let them know before you start showing a lot.