Due date

Molly
I had my 20 week scan last week when I was 20weeks 3 days... After my ultrasound the chart of his measurements said he measured 21weeks 3 days! Also the Estimated due date printed on my ultrasound says 11-6-13 but my doctor said she was sticking to my original due date of 11-13-16 ... Shouldn't my due date follow his measurements?