Dramatic increase in cumulative hormonal cycling of undifferentiated breast epithelium as a major driver of rising breast Cancer incidence: A hypothesis and prevention strategy

We propose that rising breast cancer incidence in modern societies is substantially driven by prolonged exposure of the breast epithelium to hormonal cycling in the undifferentiated state. Cumulative oestrogen exposure alone does not fully explain the protection conferred by early parity and prolonged breastfeeding. Lifetime menstrual cycle number, the epidemiological marker of this exposure, has increased from approximately 100–160 in ancestral populations to nearly 450 in contemporary women, owing to lower birth rates, less breastfeeding, earlier menarche, and later menopause. Cycles of proliferation and involution that fail to reach terminal differentiation leave a persistently undifferentiated, replication-exposed progenitor pool; full-term pregnancy and lactation protect principally because they drive the epithelium to terminal differentiation, and secondarily by suspending ovulation. Cycle number is thus a proxy for time spent in the vulnerable state, not the mechanism itself. The hypothesis is consistent with cohort data linking cycle number and risk, parity-dependent mutation accumulation, protection from early first birth, and animal models of cycle suppression; these findings are supportive of, rather than proof of, it, and the underlying evolutionary-mismatch pattern is particularly relevant to oestrogen-receptor-positive disease. Early first pregnancy and longer breastfeeding remain the only established reproductive exposures conferring this protection. For women who do not have children early or cannot have them, hormonal or pharmacological agents that induce the differentiation-related effects of pregnancy could be explored as intermittent chemoprevention, starting early and administered a few times across the reproductive lifespan, although current evidence is limited and the regimen remains an open question. • Rising lifetime menstrual cycles may be the main cause of rising breast cancer incidence. • Repeated epithelial proliferation and involution during cycles can lead to mutation accumulation. • Earlier menarche, later menopause and fewer pregnancies raise cumulative cycle count. • Hormonal mimicry of pregnancy/lactation is proposed as a prevention strategy. • Hypothesis is testable via epidemiological and animal model predictions.