Unless you know when you ovulated, which it sounds like you don't, then I wouldn't worry too much about it this early.
With DS, I knew EXACTLY when I ovulated, almost down to the hour thanks to fertility treatments, and he measured about 5 days behind for our first ultrasound at 6 weeks, 4 days behind a week later, and by 9 weeks he was exactly on track, by 13 weeks he was measuring several days ahead and continued to get more and more ahead as my pregnancy went on. (He was born at 36 weeks weighing 7.4 lbs.) Our daughter had severe IUGR and so I was really really worried in the early days with DS when he measured behind, and I spent a lot of time worrying about nothing. (I did not get pre-e at all in that pregnancy.)
The baby/embryo is so so little at 6-7 weeks, they are just a few millimeters big on the screen, so even the barest fraction of a millimeter off of the cursor can be the difference of "days" of measurement, that early. More importantly this early, is that you saw a nice strong heartbeat. I would try not to stress too much about measuring behind/ahead this early though! (Easier said than done, I know!)
Me (29) DH (30)
#1-Olivia Caetlyn-9-28-09-9-28-09, 23+2 wks, emergency classic c-section, class I HELLP, IUGR
#2- Lucas Oliver (rainbow baby)- April 2011, 36+2 wks, HELLP and pre-e free! (lovenox and LDA pregnancy)
#3-Matthew, late October 2012...mostly normal, 37 wks, (lovenox and LDA again)
My blog: http://www.butterflies-and-rainbows.blogspot.com/