No embryo, just yolk sac?

Is that normal at 6 weeks? Did it happen to anyone else and have a viable pregnancy? This was 10 days ago in the ER and it is still bugging me, follow up ultrasound is Monday at 8 weeks. There was a yolk sac and the sac measured perfectly, just not embryo. Er dr said it wasn’t normal, but that my levels were still raising fine. I saw an OB (not my OB) yesterday for iv fluids who also said it was a bit strange, but because I’m still suffering HG and me levels rose fine, that he still thinks it may be viable. This is my hopeful rainbow after a 17 weeks loss, so I’m paranoid as ever.