Different due date

Ashley • Her children arise and call her blessed; her husband also, and he praiseth her.
Put my info into glow for my due date, date of last cycle 12/14/14, number of days in cylce 21. So it gave me a due date of 9/15/15,(7 weeks along yay!) But went to the doctor yesterday had a sono and they said 9/20/15 only 6 weeks. I know it's only a week off but that extra week counts when you are excited.