When should I tell?

I’ll be 24 weeks pregnant tomorrow. I applied for a probation officer position Feb 13, and after three interviews, was offered a position last Friday. She told me she’d call back in a few weeks with my start date. Since I am just transferring departments, I don’t have to fill out any paperwork. I haven’t told them I’m pregnant, and wasn’t showing during my interviews. Should I tell them after they give me a start date or after I’ve already started the job? I know legally they can’t rescend an offer because of pregnancy, but it makes me nervous to tell them ahead of time.