It's pretty common to either be a couple of weeks behind or ahead. I always measure ahead. With my son I measured 3-4 weeks ahead at one point. At my 37 week check up with him I measured 40-41 weeks. Had him a week later and he was 8lbs 4oz. Couldn't imagine going full term!! You could just have a bigger baby...time will tell! =)
when i was pregnant with my son i always measured larger. at a scan they gave me at 34.5 weeks my little man was already weighing in at 7.5lbs. on the day of delivery, which was right on his due date, my uterus measured 49cm. he was born weighing in at 8lbs 14oz and 20 inches long.
best of luck! :D