Two sacs, one empty...anyone else?

Allie

So according to the first day of my last period I should be 9 weeks. I went for an ultrasound at Hello Little One because my doctor wasn’t getting me in for a while and I wanted to confirm that all was well since I had also had some light bleeding. The tech and I were able to see that there were two sacs and she could barely see something in one but said I was measuring about 5 weeks so that was normal to not see very much. A week later I went to the doctor who ordered an ultrasound and they said they also saw two sacs but one was empty and I was measuring about 6 weeks. I’m just confused about the difference in how according to my period I am 9 weeks but they are saying 6. Has anyone had any experience with this? Has anyone gone back and there were babies in both sacs? Thanks! 💗