Hi,
I was told that dating scans are only accurate before week 11. I went through a similiar situation, and got very worried when I was told at 18 weeks that DS was measuring 2 weeks behind my dating scan at 7 weeks. I had to go back 2 weeks later to check measurements again, but he was back on track again, the sonographer just put it down to a growth spurt.
I would go by your first dates.