Motherless child

I'm 16 and I've made some horrible mistakes in my life so far and I believe I really did fuck it up! My mom is in prison, n well my dad walked out on me when I was 3 n never came back. I want to tell my mom in a letter that I'm pregnant but I don't know how. She knows she fucked up my life with the choice she made, and idk how to tell her this, or if she asks questions I can't answer them. I've hadn't live with my since I was 9 but I know this will break her heart. I'm so alone right. My story is really long and I know you guys don't like reading well that's why people do polls more then topic. I need help with this. What do you think I should do?