Epidemiological risk factors for breast cancer--a review.

The rising global incidence, morbidity and mortality from breast cancer has led to intensified efforts in the search for etiological factors of the disease. While international variations in the incidence of the disease may implicate a role for environmental factors, available evidence indicates that lifetime estrogen exposure may be a critical factor in breast carcinogenesis. While increasing age and the female sex are well-recognized risk factors, reproductive characteristics such as age at menarche and menopause, menstrual irregularity, age at first and last childbirth, parity and breastfeeding have also been linked to breast carcinogenesis. Early menarche and late menopause are associated with increased lifetime exposure to estrogens. In addition, a long period from Tanner stage breast-2 to onset of ovulatory cycles and a long period of luteal inadequacy and anovulatory cycles characteristic of the perimenopausal years creates long estrogen windows favorable for tumor induction. The intense differentiation of the terminal duct lobular unit associated with each full term pregnancy and release of various hormones, autocrine and paracrine growth factors during lactation may explain the protective effects of early age at first full term pregnancy, parity and lactation of breast cancer risk. A protective role for xenoestrogens has been postulated and evidence is emerging in support of an increased breast cancer risk with abortion and prolonged use of postmenopausal hormone replacement therapy. Appreciating relevant risk factors for breast cancer in the population is central to any preventive and control program aimed at reducing the burden of the disease through the design and implementation of culturally sensitive interventions.