Breast cancer: potentially predisposing and protecting factors. Role of pregnancy, lactation, and endocrine status.

Risk factors of breast cancer have been correlated with the availability of estradiol and estrone. Mechanisms protecting from breast cancer are thought to be due to the mitotic rest of mammary epithelium (low deoxyribonucleic acid synthesis), as encountered during pregnancy and lactation, and to the actions of estriol and progesterone in opposing the "carcinogenic" estradiol and estrone.