I think it can also depend on how.much fluid you are carrying. :)but doc knows best usually. :)
I wouldn't worry to much long as baby has good strong heartbeat, mines measuring bigger than he should, they can measure 2 weeks either way the 40 week mark, why its 38-42 week. Baby's grow at different rates, some bigger some smaller. I'm measuring 1 week and 3 days ahead now and they just say he's a big baby. Go by your first u/s for your due date, it's more accurate
I know I was told about 15 weeks that the measurements they took confirms original due date so that I wouldn't have to worry about it again.