Comparison of the percent of menstrual cycles that are 21 to 45 days’ duration (red) and percent of menstrual cycles that are ovulatory (blue) by postmenarcheal age through young adulthood. Ovulation was determined by normalcy of urinary pregnanediol glucuronide in weekly samples collected during last 12 days of each menstrual cycle; cycles with clearly detectable but subnormal pregnanediol are designated here as having luteal insufficiency (green). It can be seen that most of the cycles that are not ovulatory had sufficient cyclic follicular activity to generate an immature corpus luteum, which indicates antecedent ovulation, rather than being truly anovulatory as the investigators had labeled them. Data from Metcalf et al.24