That just goes to show that you can't go by those beta numbers. Your numbers being so high, however, mean that your chance of m/c is very low. Check out this link, at the bottom of the page where it says "Hcg levels and Outcome."
I am excited about twins, but very scared at the possibility of genetic and preemie problems. I guess your risks due to advanced maternal age are increased when you have more than one fetus in there (even though chronologically I am only 31). My DD was a month early so they said that is one strike against me as I will most likely go very early with these babies.
Best of luck to you and that healthy little one you have onboard!