As everyone else has suggested, they should be checked again to see for sure what they are doing. But generally, if they drop, that usually indicates a miscarriage or impending miscarriage. The other possibility is that it could be what is called "vanishing twin." This is where you were pregnant with twins, and then one "vanishes." Basically, you lose one twin. This sometimes does cause bleeding, and it does explain the drop in hcg without total miscarriage. Good luck.