19 weeks pregnant and have not told my new job

I live in California, and just accepted a part time job at a bank. I'm 19 weeks pregnant and I'm certain they can't fire me for being pregnant but it is at at will employer.

I don't know when I should tell them that I'm pregnant, it's a bank that I will be working at, and the location itself only has about 20-30 people who work there, but company wide, there's maybe 200 within a 25 mile radius.

I'm really nervous because I don't know if they will give me a maternity leave, though they do offer medical benefits as a part time employee .