measuring 9 days behind during anatomy scan

Hi ladies I just went in for my anatomy scan today, I am supposed to be 20 weeks and 2 days today. The baby was measuring 19 weeks. Is this a cause for concern? My only other ultrasound was when I had my first one at 9 weeks. Of course the ultrasound tech could not tell me if anything was wrong but she didn't sound negative at all through the appointment. I'm just curious if this is a common thing or should I worry?