When to tell work I'm pregnant? And thoughts on returning after ML?

Natalie
I know this has been asked many times before but in my situation I'm on a 3 month probation period and technically not protected by any labor laws in my region during this time. I literally found out I was pregnant just hours after I accepted the offer for this job. It's a job I had been searching a long time for and would like to continue working at after my maternity leave.
I'm 15 weeks now and not yet showing. By the end of my probation I'll be 22weeks pregnant.. If I tell my boss now, it would be the nice and courteous thing to do. But that puts me at risk for being let go for 'other reasons'. If I wait until my probation is over, I'm basically unfireable but it may annoy him because I was not open and honest about my situation. I've only been there for 5 weeks so it's difficult to tell how he'll react.
Some of my friends and family say to wait, while others say to do the honest thing. Not sure what to do.
Also, there are a few working moms out there who keep telling me that I won't want to go back after my leave because the baby will 'change everything'. How have your experiences been with returning to work? My leave will be 10 weeks. Office job so no heavy lifting or hazards.