Measuring behind...

I had my first appointment at 11 weeks exactly and went in for the ultra sound where my doctor said baby was measuring at 10 weeks 5 days. He said that was great, but worries me yet. Is it ok that he or she was 2 days behind what I thought they were suppose to be? Anyone else ever experienced this? I've had two miscarriages so I'm always paranoid up until second trimester!