Sex Positions and the Uterus

Ok, this may be a stupid question but does the penis go into the utehrus or is only reach the opening. I not huge but I am 5 11 and my girlfriend is 5 1. When ever we do it doggy style or her on top she complains about it hurting. I wanted to know if the head of my penis is going into the Uterus. I would not say she has an unusual swivel to her hips

Question answered here.