I have had a couple of friends who have had fraternal (non-identical) twins in the last year and they were both told that they could have conceived their twins up to a week and a half apart. Because you have to release 2 eggs in the case of fraternal twins, from what I gathered, it would be possible to ovulate twice a few days apart. That would mean that if what the sonographer saw was a baby, it might have just been because you ovulated a second time and/or it implanted later. I would think that not all embryos implant at the exact same time either, even if your body did make 2 eggs. I think it is still possible that you have a viable (such an awful word isn't it) second baby in there but you'll have to wait it out to see.

I really hope that it is any of the other alternatives to an empty sac. You so deserve this pregnancy and I don't want anything to rain on your parade . I hope the next week or two pass quickly so you can get some answers.