Well if you got a pos test in november and youre anything like me, you're probably already getting thick and full in "suspicious places". And I think you're right that the longer you wait to tell your mom the harder its gonna be. I was just the other way around from you. I was more scared about telling my dad than my mom, so I told my mom before dad got home and she told him.
When I told my mom her first reaction was "if this is a joke i'm not laughing". I told her it wasn't a joke then went to my room and started to cry. Mom came in and told me she loved me and it'd be ok, and she'd be there for me. I told her I was really scared how dad would take it and asked mom if she'd tell him. I was upstairs in my room scared to death when she told him. I could hear they were both really upset, but then after a little while they both came in and said they were disappointed but they still loved me and would stick by me no matter what.
Going by what i've heard from other girls I know i'm lucky. I hope you'll be as lucky too. Maybe you can tell your dad and let him tell your mom.
Oh yeah, i'm 17 but i'll be 18 on the 27th. At least you're old enough you can make your own decisions and no one can say much about it.
Good luck!!