Advice please?

Hi, I have a regular cycle of 26 day’s, it’s varied in the past between 24-28 but never less. I reckon I concieved in December 21st , but I had a scan on the 24rth of feb and it says 10 weeks four days? I had sex on the 12th December the day after my period, but I think I concieved the 21st as it would be closer to ovulation? Any one else had a dating scan out by nine days? X