Telling ur work?

So when did everyone tell there work? I am starting a new job August 15th as a school nurse...however I wasn't done with my 1st trimester when I interviewed so I didn't mention it. I got the job and sign paperwork with hr today. When should I tell them I'm pregnant? I am currently 15 weeks and due in mid January. I really wanted this job and it was a very hard job to get into so that's why I didn't offer up the info in my interview. I don't start till August 15th so when is the right time to tell them? 😁 thanks!