I had my first u/s at 5 1/2 weeks (they thought I was 6 1/2 weeks, but turns out I ovulated late and I'm measuring at 5 1/2 weeks). They found a healthy looking gestational sac with a yolk sac inside, but couldn't see the baby, as it's too early. Then she found a second gestational sac that appears to be empty and smaller in size. Is there any chance that this really could be twins? Or because of the size difference, is it already a vanishing twin?