Gosh, I know you are scared and worried especially with the other miscarriages. Please don't give up hope yet. Many women have what appears to be an empty sac at this stage and sure enough, they go on to have a full pregnancy. I would not yet believe that this is a miscarriage in the works. So often, at the stage you are, women will go back in two weeks and there the baby is. I'd hold off from any things that assume you are or will miscarry to make it happen for another couple of weeks. That's just me. It IS really hard to say for certainty if this will turn out okay but there is still a chance it will and I wouldn't give up that chance. Talk to your doctor but see if you can wait another couple weeks before making any determination. How far along were you with your two miscarriages? (very sorry for your losses, by the way).
So sorry you are going through all this and for your previous losses! I'm with specialmom, and I wouldn't jump to the conclusion of miscarriage just yet. I'm guessing that the doctor is assuming you are further along due to the high HCG levels in the bloodwork- however, that's not an assumption that is fair to make. There is no solid HCG number that you will have each week of pregnancy- there is an estimate of it should be between X amount and Y amount around whatever week you are, but it really varies for each woman.
Is the doctor going to take more blood to check your HCG levels again soon? They should within a few days- if they are going up, that will indicate a healthy pregnancy is more likely then a miscarriage. The ultrasound results really don't show anything conclusive if you are only 5 weeks- a follow up of one of those can be done in a few weeks in order to show a bit more, but even then I would lean more on the results of bloodwork and HCG levels to indicate what's going on.
If your doctor didn't order another blood test, request one. Let us know how you are doing- wish all the best for you in this pregnancy!