If pregnant with more than 2 babies and miscarry one will hcg levels decrease from 1300 to 700?

I'm not sure, but I am meant to be 9 weeks pregnant and went for scan and they could only find gestational sack at 6 week size, no fetal heartbeat. Prepared me for miscarriage. Then had bloods done and HCG levels were 18000, then 18900. My thoughts were that my body still thinks I'm pregnant, and hence still producing HCG...but doc wants to wait and see as he said if you had a twin and it perished, this could explain why the gestational sack is still small - so it could be a viable pregnancy....

I wish he didn't give me false hope!