Dated Early at US and Sac Empty. Anyone else?

Talitha
I went I yesterday for an ultrasound and thought I was 6w3d. All they saw was a gestational sac that measured closer to 5w2d. I know it is possible my dates are off. I go back for another ultrasound in a week. I have had 3 miscarriages before and I am terrified that I'll lose this one too. Anyone else have this happen and all turned out ok??