New position

I was just blessed with an amazing job offer at St Marys children hospital. However I am 13 weeks pregnant. I did not announce my pregnancy to them when I accepted the position I do have a 90 day probation period but I will be 7 months pregnant then and I'm sure I can't put off my pregnancy that long...as of right now I do not show so I can still get away with it but how will I know when to tell my boss? And how do I tell them? Do I tell them I didn't know or do I tell them I knew but didn't disclose it since It's not mandatory too?