They go by your last period unless baby is measuring at least a week off from that at your ultrasound, then they may change it. They probably will be able to see a heartbeat then but I've seen some people say they had scans at 7 weeks and didn't see anything, so I guess it depends. I had one at 6 weeks with twins and they were able to see both heartbeats. You won't be able to hear it until around 15 weeks though.
Usually you calculate it from the first day of your last period. I'm sure they'll be able to calculate it from what your baby looks like. By the way I had a scan at 6 weeks too, just to have a check on something. It's amazing what you will see. Good luck xx
From the first day of your last period