据统计，乳腺癌（Breast cancer）在美国是女性仅次于肺癌的肿瘤，其主要危险因素（risk factors）如下：

1、年龄增大，女性患乳腺癌之风险随着年龄增大而增加（Age - the chance of getting breast cancer rises as a woman gets older）；

2、基因异常，已知拥有BRCA1和 BRCA2两种基因之女性易患乳腺癌，建议有乳腺癌或卵巢癌之家族史的女性检测以上两种基因（Genes - there are two genes, BRCA1 and BRCA2, that greatly increase the risk. Women who have family members with breast or ovarian cancer may wish to be tested.）;

3、月经周期，如月经初潮较早（<12岁）和/或绝经较晚（>55岁）的女性易患乳腺癌（Personal factors - beginning periods before age 12 or going through menopause after age 55）;

4、超重（being overweight）;

5、接受激素替代治疗（using hormone replacement therapy  or also called menopausal hormone therapy);

6、服用避孕药（taking birth control pills）；

7、经常饮酒（drinking alcohol）；

8、生育较晚,35岁前未生育或未曾生育（not having children or having your first child after age 35 ）；

9、乳房组织较为致密（having dense breasts）。

更多资讯：http://www.nlm.nih.gov/medlineplus/breastcancer.html