Fig. 2.

Ovarian cycle regularity, defined by the frequency distribution of cycle lengths in mice and women. Redrawn from Nelson et al., 1982 [51], from longitudinal data of cycle lengths of C57BL/6J mice [51] and Treloar’s landmark study of self-reported menstrual cycles (alumna, University of Minnesota) [43]. Both show U-shaped curves of the variance in cycle length. During early adulthood, variance decreases, followed by maximum regularity during the most fertile phase of life, and then increased irregularity during follicular depletion. Perimenopausal stages are compared in Table 1.