I'm of the belief that the early the scan the most accurate the dates. Having a scan at around 8 weeks I would think would be quite accurate. I'm the opposite to you in that I am measuring a week ahead for my dates, but I think that is correct as I think I ovulated earlier, otherwise it is a miracle conception as the was no DTD near to ovulation at around day 14. If there is a chance you may have ovulated later then I think that the scan maybe correct.

Will be intersting to see what your 12 week scan says.

Glad to hear you had a strong heartbeat