As a general rule, during the 30 or 40 years that a woman has periods, only one ovum matures each month. However, there are exceptions to this rule. Some months, more than one matures, and some months no ovum matures.Ovulation occurs 14 days before the next menses begins. In a 28-day cycle, this means that ovulation occurs around day 14 of the cycle, as shown in Figure 23-14. (Recall that the first day of the menses is considered the first day of the cycle.) In a 30-day cycle, however, ovulation would not occur on the 14th cycle day, but instead on the 16th. And in a 25-day cycle, ovulation would occur on the 11th cycle day.