In 1940, the lifetime risk of a woman developing breast cancer was 5%, or one in 20. 1940年，终生风险的妇女患乳腺癌的5 ％ ，或20 。 The American Cancer Society estimates that risk to be 13% in 2005, or almost one in eight.据美国癌症协会的估计，可能是13 ％ ，在2005年，几乎八分之一。 In many cases, it's not known why a woman gets breast cancer.在许多情况下，它不知道为什么一个女人得到乳癌。 In fact, 70% of all women with breast cancer have no known risk factors.事实上， 70 ％的女性乳腺癌没有任何已知的危险因素。