Perhhaps you could ask your GP to contact your mam on your behalf if your worried or even a teacher that you trust.
I know its scary,but you need to tell her asap because you are going to need alot of support and im sure that your mam would be more upset over you not telling her than you being pregnant. I fell pregnant with my 1st at 18 and like you i felt worried about telling my mam, but i knew i had to. I simply asked my mam if we could have a chat, just the 2 of us, i got really emotional and burst into tears and with that she knew what was wrong. I thought she would go mad but if anything she showed me the most affection that night than she ever has my whole life. good luck.