(Vanishing) Twins, slow rising hCG levels, and a lot of question marks

I have a confusing situation. Going into this I want to say I'm relatively comfortable with the reality that I will likely miscarry. I'm just tired I'm the unknowing! I had a positive pregnancy test on 12/5, went into my first appointment on 12/26 thinking I was anywhere from 7-11 weeks pregnant. Ultrasound shows gestational sacs but no yolk/fetal pole (typical at 5 weeks). I get the classic: well monitor your hCG levels maybe your dates are wrong and you're not as far along as you thought, but more than likely this is a miscarriage.

But here's the thing! There were 2 gestational sacs. As in twin pregnancy. One is smaller than the other, so the doctor comfortably feels that if this is a viable pregnancy, it wont be a twin pregnancy for long. This means vanishing twin syndrome. So my hCG levels were 10,500 the same day as no yolks. A little less than 4 days later, hCG is only 16,200. A 53% increase in 4 days normally would be enough to write off the pregnancy, but with the added layer of the vanishing twin I dont know if it means I lost one but wont lose both. 

Does anyone have similar stories or any other thoughts? The next ultrasound is a week away and I'm spending too much time on Dr. Google.