The date as determined by the scan is normally the most accurate. Especially if it was done between week 6 and 20. The date as determined by your LMP can not take into consideration the variations you might have had in your cycle.

If it is any consolation, even the most accurate estimation can be off by as much as a week (or maybe even more). Statistics have shown that various things like your health, weight, age, race, nutrition and other factors will influence the true due date. You will know it is time when the baby shows its face.

Best of luck!!