Girls, whose ultrasound showed different dates of pregnancy/gestation?…

Girls, whose ultrasound showed different dates of pregnancy/gestation? I had an embryo on my last week's ultrasound at 6-7 weeks. Today exactly one week later, I did an ultrasound, saw the embryo but didn't hear the heartbeat. According to my cycle, it should be 8 weeks, but according to the ultrasound, they put 5-6 weeks.