Scan - Thought to be 6 weeks.. worried?

I am 7 weeks pregnant and went for a scan at 5 weeks and the tech saw a sac with a yolk sac. I went on to have a scan at 6 week and they dectected a heart beat. needless to say at about 6 weeks pregnant something should be visible I would just say your dates are off and to just wait for the following scans. Good luck.