Mayo Clinic states that the menstrual cycle is the process a woman's body goes through over the course of about a month to prepare for a possible pregnancy. The menstrual cycle includes the release of an egg cell, the thickening of the uterine lining in preparation for a possible pregnancy, and if no pregnancy occurs, the shedding of that lining during the menstrual period.
In the absence of pregnancy, the normal menstrual cycle repeats every 21 to 35 days, according to the Mayo Clinic. The length of each cycle is counted from the day the woman's menstrual flow begins to the day her flow begins the following month. Menstrual cycles are more irregular in teens and women approaching menopause.
WebMd states that hormones, especially progesterone and estrogen, control the menstrual cycle. At the end of a woman's menstrual period, an increase in estrogen causes the thickening of the uterine lining. After one of the ovaries releases an egg, progesterone production increases. The progesterone works with the estrogen to keep the uterine lining thick. If a fertilized egg does not implant in the lining, progesterone levels drop abruptly, and the lining sheds.
WebMd notes that many factors influence the menstrual cycle. These include hormonal birth control methods, low body weight, rapid body weight changes, obesity, stress and regular strenuous exercise. Some women experience pain and discomfort during the menstrual period portion of the cycle.