Vanishing twins? Or just too small?? Ugh

Today, I went in for my first ultrasound. According to my LMP, I would be 7 weeks today.

Anyway, so during the ultrasound, there were two sacs. One had a little baby and they were able to get a heartbeat of 129, measuring at 6 weeks. The other sac (same size as the first sac) she couldn’t see anything.. no baby.

I have another ultrasound in two weeks.. but my question is— could the other baby just be measuring a little behind?? Or is it more than likely a “vanishing twin”?