New Job - Should I Disclose?

I'm a teacher out for the summer. I just got hired part time for a summer job at a retail store. I'm only 5 weeks pregnant. Should I disclose to my manager that I'm pregnant? We haven't even told family yet. I have no symptoms that would give me away, so I think I could hide it for quite a while until I start showing. At what point do you normally disclose to employers? I'm planning on telling my principal when we go back in August so we can plan maternity leave.