Yep - this is really, really normal. I asked the sonographer with the last baby why my dates vs scan dates were off and he said not to worry, that the earlier scans are more accurate and the dates from the later scans don't mean much in the long run. The measurements on the scan are against average with a deviation on either side. For example, I was 20 weeks pregnant according to dates and early scan yesterday, but the baby is measuring between 21 and 22 weeks in size. Unfortunately it doesn't mean that he will come earlier and I'll get a shorter pregnancy, just that he is sitting on the upper quarter for baby sizes so far (95th percentile *sob*). So, my scan means that I am probably heading to have another 9lb plusser and you're heading for a much more sociable 7lb-er. That being said, the doctor also said that there is room for error and that the baby has growth spurts - so don't worry at all - you won't get an extra week in there!