Go with your doctors. Most due dates change between 1-2 weeks anyway. Very few women have their baby on the original due date.
Yep, I agree with Pam. Even though they date your pregnancy as 40 weeks from first day of LMP, we all have slightly different cycles, and ovulation times. If you have a 26 day cycle, that can change things a bit. The ultrasound is more accurate. Your Dr. would have let you know if he was worrying about measuring behind, and it sounds like all is just fine!!