31 percent of the women in the study believed their cycle was 28 days, but only 12 percent of them had an actual 28-day cycle. 87 percent of the women had menstrual cycle lengths between 23 and 35 days. Over half the women (52 percent) had cycles that varied by 5 days or more. For those women who did have a 28-day cycle, there was a 10-day spread of ovulation, and the same was true for all the different cycle lengths they studied.
