Cycles

As explained in previous sections of this unit, once spermatogenesis begins at puberty in males it is relatively constant throughout the remainder of life. Granted there is a gradual decline after about age 50 but the process continues virtually until the death of the man. In women, on the other hand, the process is much different and beginning at puberty follows a cyclic pattern with increasing and decreasing hormone levels as well as changes in the ovaries and uterus, repeating itself about every 28 days. This series of events is referred to as the menstrual cycle. Since this cycle involves changes in both the ovaries and the uterus it can be divided into two cycles, the ovarian cycle and the uterine cycle. We will explore the cycles separately but keep in mind they happen simultaneously and are intimately connected. For both of the cycles, we start counting the days on the first day of menstrual flow or menses. The culminating event in the cycles is release of the ovum (ovulation), which typically occurs on day 14 in a typical 28-day cycle. Note that not all cycles are exactly 28 days and there can be differences between women, and even differences from one cycle to the next in the same women. Normally the most consistent portion of the cycle is the second half, the time from ovulation to the onset of the next menses. This portion is expected to last 14 days. In women whose cycles are not the typical 28 days, ovulation usually occurs 14 days prior to the onset of the next menses. Please refer to the image above as we discuss the Ovarian and Uterine cycles below.