I just came from doing a dating ultrasound and they told me I was 12 weeks and one day however I had my last period feb 11. Which is the 12th week.. I'm trying to justify this cause before my period I did not have sex! Proior to taking this ultrasound I thought I was 11 weeks according with my period when I guess I should have been ovulating and when I had sex with my husband. Please tell me why I am off a week?

Dates are all best guess based on available evidence so not hard and fast, certainly not worth undue worry.