Several patterns in menarche are evident over time and across populations which vary by ethnicity, socioeconomic status and altitude of residence: peak occurrence of menarche in January and August with a possible minor peak in April; troughs in the occurrence of menarche to either side of the peak months; and secular trends in season and age patterns of menarche. Contradictory patterns in season of menarche with respect to age at menarche and season of menarche with respect to season of birth are still unresolved. Further research is yet required to evaluate the role of the variety of endogenous and exogenous variables and their interactions on menarche in order to more fully develop an appropriate understanding of the chronobiology of developmental and reproductive phenomena in women and their dependence upon environmental quality and temporal cues.