When to tell your boss of your new job you're pregnant, and how?

Shai-Anne

I just got a different, better paying job. I was advised not to mention being pregnant in my interview, as it's not incredibly noticeable yet.

Well I got the job, and it so happens I have my 13 week check up the same day as my first day of work..

How long should I wait to mention it to my boss? How should I say it?