Help please! Possible MC

My last period was 7/7/14. I know conception date was around 7/23. Had 1st MD appt today and they said based in LMP I should be 8 weeks 4 days but baby only measuring 6W3D. They are thinking I will have a MC. But if I know 100% date my husband and I conceived would baby really only be about 6W? I go back next week to see if baby has grown but I'm so nervous! Anyone else had similar experience and positive results!! I'm praying for a miracle!!!