What causes HCG to rise?

Being pregnant causes HGC to be in your body. If you want more detail than that; it's a hormone that your body produces when you are pregnant. That is what home pregnancy tests detect. If you have HGC in your urine, you are pregnant.

P.S. its actually HCG... if you'd like info on HGC, please go to this wikipedia link:

http://en.wikipedia.org/wiki/HGC