Like the other girls said, definately tell her sooner rather than later. Thats what I did, when I found out I felt the same as you, terrified of telling my mum because I knew she would be saying I needed to get an abortion and how dissappointed she was, but I figured she would have to find out sooner or later, and if I did it quick then I would worry less about it and just get it over with. Plus it then gave her time to calm down and get used to the idea sooner. I ended up telling my mum the day after I found out I was pregnant, and she did get mad and said I needed an abortion. I told her that it was my decision and it was up to her whether she wanted to be involved or not. She came over later that day and she had calmed down a bit. She's used to the idea now and is excited about having a granddaughter. Good luck!!!
xoxox