"Women who have a decreased risk of breast cancer have things that would be associated with lower total lifetime estrogen exposure. Women - young girls who start menstruating at age 17 or older have a decreased risk of breast cancer than those who start menstruating at age 13. Early menopause - women who go through menopause in their 40's have a decreased risk of breast cancer compared to those who go through menopause in their 50's. And early age of first birth - so a woman who has a child at age 20 or younger is relatively protected. And we think that all these different things interrupt the high levels of estrogen that would be surrounding the breast tissue in life."