I know how you feel I was pregnant with my son at 15 and I was scared as hell when I was going to tell my mom. Just come straight out and say it. Hope for a good reaction but expect a bad one.
The best way to tell her being onky five weeks along would probably be to get her a worlds greates grandma coffee mug or t shirt. I think thats an adorable idea and wish I had done it for my mom. The news was a bit harder to break to my mother though. Now she is as excited as I am :) Good luck sweetheart
Personally, I think the BEST way is to show her an ultra sound, that's how I did it and even though she was mad she got over it real quick just looking at the ultrasound, my mother is really quick but I've come to learn they will never stay mad at you moms will always accept and understand you the most