Star,
I know you weren't asking me, but I though I'd tell you something to reassure you. With my first DD, my scans right from the word go said that I was a week behind my dates, which I know were correct, due to tracking my ovulation prior to conception. Well anyway, she was born a week before my dates (39 weeks) which would have been 2 weeks early going by scan dates. She was born healthy, and thrived from the word go. So I would try not to worry about dates, just say your due around that time, that's what I'm saying this time around.
Hope this helps to settle your concerns.
Have a good Christmas