Measuring small

So I had an ultrasound and the baby was measuring at 5 weeks 1 day they wanted me to go 2 days after that and the baby was measuring one day ahead at 5 weeks 4 days. I went in for another ultrasound to see if we could hear a heartbeat and we could! It was at 124. My question is I was supposed to be 7 weeks but the baby was measuring 6 weeks 4 days. Is this a cause for concern? If not can someone explain why the baby is measuring 3 days smaller.