How do I tell my new job I'm pregnant?

So I started a new job as an assistant teacher at a day care last week. I didn't tell them at my interview that I was pregnant because I really needed a job, and I was small. However, I am now 17 weeks and I have nothing to wear that doesn't show off my stomach. Also, yesterday my boss told me to tell her asap if I ever need time off. How should I tell my boss? Can I get fired?