Menstrual cycle patterns and breast cancer risk factors.

Using a data set of women who longitudinally recorded menstrual and reproductive events, we examined menstrual cycle characteristics in relationship to early and late menarche, early and late menopause, and deferred parity, three variables epidemiologically related to breast cancer incidence. Women with late onset of menarche had longer and more variable cycles in the 10 years after menarche than did those with early onset. Women with late onset of menopause had longer and more variable cycles in the premenopausal interval than did those with early onset. Cumulative fertility in women after marriage did not differ according to cycle length and variance. Late menopause may be a breast cancer risk factor due to relative estrogen excess and progesterone lack as reflected in longer, more varied cycle patterns. Observed cycle differences between women with early and late menarche await further study of the endocrine physiology of the menstrual cycle in those groups.