Pregnancy and lactation in relation to breast cancer risk

Abstract

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.

PIP: After reviewing recent studies concerning the relationship between pregnancy and lactation and the risk of breast cancer, the conclusion was reached that the nature and the direction of the relationship remains undetermined. During pregnancy and lactation, the levels of estrogen and prolactin are increased, and some studies demonstrate that high levels of estrogen and prolactin are associated with the development of breast cancer. Other investigations support the contention that pregnancy and lactation provide protection against breast cancer. Some experts believe that decreased mitotic activity in breast tissue during pregnancy and lactation may afford protection against breast malignancy. Other experts suggest that the high levels of estriol and progesterone, which occur during pregnancy, may protect women from the carcinogenic influence of estradiol and estrone. A number of studies support the contention that the risk of developing breast cancer is reduced for women who experience their 1st pregnancy at a young age and increased for women who experience their 1st pregnancy after the age of 30; however, these findings are not accepted by all investigators.