Telling my boss about my pregnancy soon. HELP!

Karen
I recently received a promotion at work after constantly being in my boss' office "reminding" him that I can do more and even tried leaving once since I found a better opportunity (he raised my wage so I stayed). I was 7/8 weeks when hell broke loose at my workplace and a great position opened up and was offered to me. Of course, I took it. 
I am now 13 weeks, belly popped and ready to tell my boss about my rainbow baby. HOW DO I TELL HIM? This is my first time doing this and part of me feels intimated since I now have this administrator job and in a matter of months I'll go on maternity leave. 
PLEASE HELP! What was your story? What are some do's and don't's?