Measuring a week behind on my first U.S?

i was woundering if anyone else was having this problem and what due date there doc used...I had my last period on march 12 2011 and i no for a fact i ovulated and concived march 25th which would make me due dec 17..i went for my first us at 7 weeks and 4 days and they told me the baby measured 6w4d so excalty a week diff if they go based on my us im due xmas eve :) i no for a fact i concived on the 25th though bc i can tell when i ovulated and thats the only night me and my boyfriend had sex...can anyone help me when do u think im due???