So, my last period was February 11 or 12, I have roughly a 24-26 day cycle. Estimating at the 28 day…

So, my last period was February 11 or 12, I have roughly a 24-26 day cycle. Estimating at the 28 day cycle from then I would be due November 19th. My dating scan happened at whay I thought was 12wks but according to that was 13+1 so the DD got changed to 11/11. So does anyone know which is more accurate for 38-40 wks.