10 or 11 weeks


So I went to my apt today got a “ confirmed due date “ but I’m sorta iffy because I put in the calculator like the first day of my last period which is dec 23 and they had said first time due date was sept 29th now their saying it’s oct 7 ?? And that I’m 10 weeks but ultrasound clearly says 11 weeks ?? Should I just go based off the first due date cause I read pregnancy can last up to 42 weeks which oct 7 puts me at 42 so idk how they are calculating??