When I had my mc it was confirmed at 6/7 weeks that the baby had stopped developing and my HCG levels were not doubling. They were continuing to rise but very slowly and not in the way they should be. I continued to have blood tests for the next 3 weeks as nothing was happening and they still continued to rise but only by a very small amount. My doc offered me a Dand C but suggested that it was better for recovery to let things happen naturally - so I did and at 9 weeks I finally started to cramp and bleed and then it all happened in 2 days and I lost the baby. So Somtimes like mentioned above it can take awhile for you body to begin to mc as the sac can continue to grow while the baby does not. This can trick your body into thinking you are still pregnant and not let you mc for a while. I am very sorry you are having a hard time and hope all works out for you.
I would get another hCG test done. If it has risen since the last one, you could assume you had a vanished twin. But if it is continuing to go down, then your doctor is probably right. You don't necessarily experience cramping and bleeding right away.
Yeah, I'm sure they could send you for a scan seeing as you would be 6 weeks at this point, you should be able to see if there is still a baby inside you or not
I guess may that could be possible. BUT I'd say a miscarriage is more likely.