Lifetime reproductive and anthropometric risk factors for breast cancer in postmenopausal women.

Hormonally-linked adult reproductive and anthropometric risk factors have been well established in the etiology of postmenopausal breast cancer, though early life exposures have been evaluated only more recently. Here, we examine the evidence for associations between lifetime reproductive and anthropometric risk factors for postmenopausal breast cancer. The review finds some evidence for the hypothesis that breast cancer risk is determined by the number of susceptible stem cells, modified by the hormonal environment. The in utero experience of an infant may be associated with postmenopausal breast cancer; preeclampsia may decrease and greater birthweight increase risk, but more evidence is needed. Earlier and more rapid childhood growth appears to increase postmenopausal breast cancer risk and childhood obesity to decrease risk, but very few studies have yet examined these associations. Increased final height and earlier age at menarche are consistently associated with increased risk for postmenopausal breast cancer. Later age at first birth, decreased parity, later menopausal age, use of hormone replacement therapy (especially progestin containing), and increased postmenopausal adiposity are well-established risk factors for postmenopausal breast cancer. The effect of a woman's own pregnancy conditions and lactation are not established. Further investigation is needed to identify whether events occurring early in life modify later events or accumulate over the life course. Many aspects of this research can be conducted by examining the influence of early life events on intermediary events without the need for longitudinal data.