I had the same thing on Friday - thought I was 6 weeks 6 days, measured 5w 5d, radiographer said it was a couple of days too early to see a heartbeat. I was going to let it go and wait til the NT scan, but now I'm slightly worried and think I might see if I can have another scan next week. My blood test came back as 4 weeks, when I thought I was 5 weeks too.