Jinx246,
i too have been for a few scans that have been out, almost a week, now just remeber its a guide and if you know your due date from your last mensural cycle then i wouldnt worry.

Here is what they told me& my husband while doing the scan-
in the 1st trimester they give you a couple of days either side of the timing, in the second trimester its a week either side and in the third three weeks so if you are like me im in the 2nd trimester and i was 4 days behind but all bubs are going to grow at differnt rates, the main thing is they are checking for devleopment of the heart, brain & organs normally done around 18-19 weeks.

so Just relax and let the little miricle grow.
Hope this helps
colleeg O