due date off by 10 days???

So I just had my first ultrasound and the ultrasound tech told me the baby was measuring at 5weeks 5 days. I tracked my cycle and I know I should be 7 weeks 1 day. I immediately started crying because I was fearful that the baby stopped growing at 5 weeks. My doctor has assured me that it's very common to be off on ovulation by a week or so and that it's hard to know the exact date of conception and implantation. I'm still worried and of course have to wait until Monday to check my hcg levels. Has this happened to anyone?? I can't help but worry through the weekend