ultrasound measuring 3 days behind

Melissa
Has one one had an ultrasound that measured a few days behind it was ok? I am supposed to be 6 weeks but I measured 5 weeks 4 days. I go back next week for follow up ultrasound to check for heartbeat. The dr said not to worry. 
I had a miscarriage in September 2016 so it is hard to hear anything is off.