It was explained to me that the early scans are most accurate for dating, by the 18wk scan, baby has taken on its personal characteristics like long legs, short legs etc. Measuring a week behind now is ok. You might have also caught baby right before a growth spurt which if you had the scan a few days later might see baby measuring a few more days ahead. As long as baby follows its curve, its ok. If they are concerned they will order more scans.