8 weeks measuring 6??

This will be my 4th baby. Based off my last period I should be 8 weeks I have regular cycles always have but my first ultrasound showed I measured 6 weeks 2 days. We could see a flicker like a heart beat but the dr did not check it he seemed a little surprised by the dates being off too. I was rescheduled to go back in 4 weeks. Does this seem to be a viable pregnancy? With my other children I had early ultrasounds at 6 weeks we had a fetal pole and the picture was much better it was just Soo hard to see in there. Any thoughts?