According to my last period, I'm am suppose to be 7 weeks and 5 days, giving me the due date of December 15th. When I got my ultrasound at the ER, they measured me a whole week earlier, making me 6 weeks and 5 days, then the tech told me that my due date would be December 23rd but she also said not to be surprised if my ob/gyn keeps my due date at December 15th. Why is this?