Probability of a December baby?

Guys FTM here so please don’t judge for my question that may or may not sound dumb 😂 seriously though, I’m a very tiny girl with a tiny frame. Literally weigh 98-100 pounds pre-pregnancy. I’ve put on about 25 pounds and now I’m currently 27w 6d. I have a feeling I’m going to have a big baby. I don’t have gestational diabetes or anything. I’m due January 17th But what are the chances I go early? Does body shape matter or do all first time moms go past their due dates? At my 19 wk private scan the tech said my baby’s head was measuring about 2 weeks ahead. I’m not sure what that means but I wanna make sure my shower is done and over with and his nursery is ready just in case. I’m known to do everything last minute but I really wanna be prepared forbaby even if he comes early.