Measuring ahead?

So on Thursday we had our first scan and saw our little squirt for the first time. At the time I was 8weeks2days (according to my last period). Before we went in I prepared myself for measuring at 5 or 6 weeks gestational, leaving some room for the two or so weeks to be taken off for ovulation/period etc. Quite surprised when she told me I was actually measuring at 9 weeks! What does this mean? Does this mean my baby has grown really fast, or is this a problem? Or, does it just mean I'll have a big baby? Help!!