When to tell employer

I am starting a new job on Monday. I am 15 weeks with baby number three and already kind of showing. I’m a bigger girl so I can pass it off for a little bit longer. I am so scared about being let go for whatever reason after I tell them. I work in an at will state so they can fire you anytime for any reason and there doesn’t even have to be a reason during the probationary period which I cannot wait until I’m out of to tell unfortunately.. am I over thinking this??