Has anyone have wrong dates?

Can you please tell me if you miss diagnosed your pregnancy days. I dont get how can it be like a week off. I though i was 7 weeks and 2 days but dr did an us and said the sac measured 6 weeks. He couldnt see the baby so im really sad cus i dont want this to be a blightem ovum again. I just hope god gives me the opportunity to have my baby