Breast Cancer: ... What are the Chances? In the 1940’s the chances of developing breast cancer were 1 in 40. Times have changed. Currently the American Cancer Society estimates that about 1 in 8 US women will develop invasive breast cancer over the course of her lifetime. 75% of women who get breast cancer have no family history of the disease. 15 - 20% of all breast cancers occur in women under the age of 45. And in these younger women the cancer is almost always more aggres