goflippr wrote:
Let me google that for you, KOTJ.

Spaying greatly reduces the chances of a female dog developing mammary cancer. In those females spayed prior to their first heat cycle, mammary cancer is very, very rare. The risk of malignant mammary tumors in dogs spayed prior to their first heat is 0.05%. It is 8% for dog spayed after one heat, and 26% in dogs spayed after their second heat. It is believed that the elimination or reduction of certain hormonal factors causes the lowering of incidence of the disease in dogs that have been spayed. These factors would probably be estrogen, progesterone, a similar hormone or possibly a combination of two or more of these.


http://www.peteducation.c...cle.cfm?c=2+1638&aid=460

Took me about 2 seconds to find it.
Thanks GoFlip!  I was remembering the 8% from my notes at school but I have those buried deeply in my move from school to home and I just totally forgot to Google!