Pregnancy and lactation in relation to breast cancer risk.

In the past, numerous efforts have been made to define risk and protective factors of breast cancer. Among these, pregnancy and lactation have been extensively discussed in connection with breast cancer. Unfortunately, many of the reports on the protective effects of pregnancy and lactation are equivocal; caution needs to be exercised when interpreting the results of a single publication. Development of breast cancer is often preceded by the occurrence of preneoplastic mammary lesions, which may be the result of long-term exposure to estrogens and prolactin. Since endogenous estrogen levels regulate pituitary prolactin secretion to some extent, it has been postulated that a hormonal imbalance exists in early mammary carcinogenesis. Exogenous estrogens directly increase pituitary prolactin secretion. During gestation, greatly increased levels of endogenous sex steroids efficiently stimulate pituitary prolactin secretion; during lactation, the stimulus of suckling is responsible for hyperprolactinemia. However, most studies did not reveal a cause-effect relationship between prolactin levels and enhanced risk of breast cancer. At present, the role of pregnancy and lactation in the development and prognosis of breast cancer is not determined.