It will be very difficult, but the sooner you tell your mother the better. If your boyfriend is being supportive of this pregnancy have him there with you when you tell her. You may also want to have your father there with you as well.
I got pregnant when I was 15 by my boyfriend of 3 years. I was about 5 months along when I realized I was pregnant. My mom had suspected before then, but never really came right out and asked, or demanded I be tested.
When I found out I first told my boyfriend. His first reaction was to question if it was really his. Duh! I hadn't been with anyone else. Then we told my parents first. I made him be there with me for this.
They were very angry at first. For one, they didn't ever really approve of my boyfriend. Secondly, my mom had me when she was 18, and wanted a better life than what she had for all her "babies". They asked me what I wanted to do, and I wasn't for sure. My father even went as far as taking me to an abortion clinic, but I was too far along to get one.
After returning from the clinic it soon sunk in for everyone that this baby was here to stay. After adjusting to that fact my parents became very supportive. Since my mother was a stay at home mom anyway, she watched my baby so I could graduate with my high school class, and go on to finish college. It was years later that my mother told me she was secretly glad that I was too far along for an abortion, and that she herself had an abortion when she was 15.
The baby's father and I got married, but did not stay together but for another year. He became an alcoholic, physically/emotionally abused me. I moved back home with my parents. Hi parents and my parents helped to raise the baby, and helped to provide for her, along with goverment assistance so I could finish college.
However, even through all of that, it all worked out for the best in the end. I have a beautiful 14 year old daughter now, and wouldn't change anything. There were some very rough times. My parents never would babysit for me other than to attend school. Gradually I lost contact with many of my good friend because they were able to lead normal teenage lives. Occasionally I feel as if I lost out on my teen years, but I still wouldn't change a thing.