Ultrasound measurements

Jessica
I just had my 20 wk ultrasound today ..I'm 21 wks+1 but the baby is measuring more around 20 at least in her body ..her head is a little smaller of course ! The tech didn't mention anything but I asked if its normal for the baby to measure smaller than the due date and she said its typical for them to measure a week or so off !! 
I'm not too worried , the tech didn't seem worried and she said the baby is super active and she was able to check all the babies organs etc !! 
Was just wondering if any of you ladies have measured a week or so behind ? And was that just normal ??