How do we tell her I'm pregnant?

This Saturday, my boyfriend is taking me to meet his mom, I've talked to her on the phone many times but I've never actually had the desire to meet her because she hates me. She always tells me boyfriend how I'm to young for him and immature and he can find so much better, she even called me a "tramp" one time because he went home with a hickey on his neck. The only reason she hates me is because I'm fifteen and he is eighteen, but age does not matter in our relationship. But anyways the reason I'm going to meet her is because me and him have a baby due in March! (: She already hates me so how am i going to tell her that I'm pregnant with her grandchild? And how can i prove to her just because I'm fifteen, I'm not that immature and i am in love with her son.