Ultrasound measurements?


I thought I was 8 weeks and a couple of days pregnant, but when we went to get our ultrasound, I was told I was seven weeks exactly. The fetus measured 9.4 mm with a strong heartbeat. Is this correct? It seems so odd to me because we had a positive ovulation test on the 26th of October, but the new due date would put my conception a week later than that?