How to tell my mum im pregnant?

I just recently found out I’m pregnant i’m only 18 my boyfriend is 20...we told his mum, she was in shock first thinking we were joking but now shes happy as ever? But my mum is completely different and very strict at the age you have a baby etc...i’m going to the doctors first to 100% find out how far and everything, however afterwards i don’t know how to break the news without her ripping my head off ???? Help 😥😥