Hiding my pregnancy?

I am a nurse and am in the process of getting my first nursing job. I'm 13 weeks pregnant and haven't started the job yet. I don't want to tell the nursing director because I feel like if they know they will retract their offer. At the same time, I'm already starting to really show so I'm worried that if I don't say something they will feel like I deceived them.

Should I tell them I'm pregnant before I start working or wait until after I'm working a few weeks before telling them?