HCG is growing so fast! Twins???

Elena

Hello ladies, I'm 5w3d today with our rainbow baby!!! I had HCG test done last Wednesday and it was 572, today it is 15,634. By my calculations, if HCG is supposed to double every other day, it should have been around 5,000 today. Does it mean I might be pregnant with twins? How fast is your HCG growing? I'm having my first US next Friday, so I'll know for sure then!