Scan measuring behind

Went for a scan today. Measured 6 weeks 1 day. I was <a href="https://play.google.com/store/apps/details?id=com.glow.android">tracking ovulation</a> using OPKs and was pretty sure I was around 7 weeks - so this puts me 6 days behind.

Should I be worried? Has anyone experienced this?

TIA x