Due date confusion

Hey , my lmp started on Feb. 28th , so that would make me 2months and five days in or close to it. I have the due date of Dec.3rd but when I went for my ultrasound four days ago, she said I was 7wks and 3d and that my due date is Dec.13th. So according to us, today I am two months exactly. I'm confused as to how the dates are different as I'm sure first day of my lmp was Feb 28th. I'd be 9wks and a couple days according to cycle. I don't know which to go with. Any help on breaking this down to me???