How to tell my parents my girlfriend is pregnant?

pretty complicated, my girlfriend is pregnant and i'm scared shitless telling people, she's dues late december and we will both be 18 when he/she is born...we didnt plan this but we both loathe the idea of abortion/adoption and want to keep it....i live with my mom(39) and sister(12),(father ped away) and well my mom will have a total freakout when/if she finds out....her parents will only be mad if i leave, which im not doing i love her too much.....i dont need any other advice other than HOW and WHEN would be a good way/time to tell both of our folks...please help me people