From Encyclopedia of Sex and Sexuality
A woman’s menstrual cycles are the result of an intricately balanced process of communication between the hypothalamus (an area of the brain), the pituitary gland, the ovaries, and the uterus. The messengers for these communications are hormones, chemicals produced by the body’s glands and transported through the blood to exert their effect at a distant site. Although these processes are obviously interdependent, women’s hormone cycles are most easily considered in three sub-groupings:
- Hormonal communication between the hypothalamus, the pituitary gland, and the ovaries (hormonal cycle);
- The development of the ovarian follicle containing the egg and the corpus luteum, the shell of the follicle after an egg has been released (ovarian cycle);
- The endometrial, or uterine cycle.
Women’s cycles will be discussed from these three perspectives, but it is necessary to remember that they are integral parts of the complete female reproductive cycle.
 The Hormonal Cycle
The hypothalamus releases one hormone, gonadotropin releasing hormone (GnRH), that causes the pituitary to release the two hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH), that affect the woman’s ovaries. During (or even shortly before) a menstrual period, the levels of FSH start to rise and cause the ovaries to begin development of one or more follicles—fluid-filled cysts in which an egg is developing. The wall of the follicle is made up of cells that, in response to FSH, produce estrogen, the female hormone associated with women’s sexual excitement and secondary sex characteristics. In response to the increasing levels of FSH, one follicle becomes dominant and its growth exceeds all the others. This follicle produces increasing amounts of estrogen, and the rising estrogen levels cause the pituitary to decrease the amount of FSH produced.
When a certain level of estrogen has been produced for a critical period of time, it causes the pituitary to release a large amount of LH (the LH surge). This causes the egg to undergo its final maturation and preparation for fertilization and it initiates the chain of events that results in ovulation (rupture of the follicle and release of the egg) about thirty hours later. The LH surge also transforms what had been the follicle into what is termed the corpus luteum. While the follicle’s primary hormonal product was estrogen, the corpus luteum produces a large amount of progesterone in addition to estrogen.
The corpus luteum has a limited life span unless the woman becomes pregnant. If she does, the corpus luteum continues to produce estrogen and progesterone. Otherwise it stops, and the rapid fall in estrogen and progesterone leads to a rise in FSH, starting the cycle anew.
 The Ovarian Cycle
A female infant is born with all the eggs she will ever have. These eggs are surrounded by a layer of granulosa cells and are called follicles. The follicles remain in a resting state until they are selected to begin developing, ten, twenty, thirty, forty, or even fifty years later. It has been estimated that by the time a young girl starts to experience secondary sex development (puberty), she has approximately four hundred thousand developed follicles—each containing an “oocyte” or primitive egg—in her ovaries. Each month, approximately one thousand of these follicles begin development. This is true regardless of whether she is having menstrual periods, taking birth control pills, attempting pregnancy, pregnant, or breast-feeding. The initial stages of development of these follicles appear to occur without any external hormonal stimulation. Once the follicles reach a certain state, a fluid-filled cavity develops within the granulosa cells. It is approximately at this stage of development that the follicle requires increasing levels of FSH for continued growth and development. The one follicle that is at a perfect state of development when the FSH levels begin rising is able to achieve dominance, and its growth and development will exceed those of all the other follicles. This follicle produces increasing amounts of estrogen and the oocyte within this follicle is destined for ovulation. The other 999 follicles are destined to atrophy, or die, without their egg ever being released.
As noted, when the dominant follicle reaches a certain stage of development, it produces a certain amount of estrogen for a critical period of time, inducing the pituitary to release a surge of LH. The LH causes the final maturation of the egg so that it can be fertilized; it triggers the rupture of the follicle and release of the egg, and transforms the follicle into the corpus luteum. The corpus luteum has a limited life span of approximately twelve days, during which it produces both estrogen and progesterone. If pregnancy does not occur, the corpus luteum stops producing hormones and fades away. If a pregnancy does result, the early pregnancy produces hCG, another hormone, which stimulates the corpus luteum to continue producing the amounts of estrogen and progesterone necessary for its early maintenance. In this situation, the corpus luteum continues to be fully functional throughout the first trimester of pregnancy. The first phase of the ovarian cycle during which the follicles are developed until ovulation is termed the follicular phase (referring to what is occurring in the ovary) or the proliferative phase (referring to changes in the lining of the uterus). The second phase of the cycle, from ovulation until menstrual bleeding, is termed the luteal phase. The third phase, during the menstrual cycle, is called the “menstrual” phase.
 The Uterine Cycle
The uterine cavity is lined by a tissue called endometrium approximately one quarter of an inch thick. The endometrium exhibits a striking series of changes throughout the menstrual cycle.
During the proliferative stage of the cycle, in response to estrogen the endometrium grows and thickens. With ovulation, and more specifically, the initiation of progesterone production, the endometrium exhibits little additional gain in thickness but a dramatic amount of development in preparation for potential implantation of a fertilized egg. Among the changes is the secretion of fluid from the endometrial glands to nourish the embryo during the three days in which it is floating freely in the uterus, before it attaches to the endometrium. In the absence of successful implantation, the production of estrogen and progesterone ceases about twelve days after ovulation and, without continued hormonal support, the menstrual period begins. The menstrual period actually involves the shedding of the superficial lining of the endometrium. With the initiation of hormone production from the next “crop” of growing follicles, endometrial development begins anew for the next cycle.
In summary, the hormones produced by the hypothalamus, pituitary, and ovaries orchestrate the interrelated development of the ovarian follicles containing a maturing egg and the uterine endometrium into which the egg will implant if fertilized. If there is no pregnancy, the female cycle repeats at about twenty-eight day intervals.
 See also
- Cleanliness and Sexual Odors
- Premenstrual Syndrome (PMS)
- Tampons, Pads, and Sponges
- Toxic Shock
 Cultural and Psychological Aspects of Menstruation
During ancient times, attitudes about menstruation alternated between repulsion and celebration. Some cultures revered this monthly event as magical and intimately connected with the renewal of life. But it was more common for societies to regard menstruation with disgust. In many places menstruating women were sequestered away from men, sometimes in huts designated for this purpose. In ancient Sumeria women wore a visible towel, a “blood bandage,” for the days of their period. During this time they were forbidden to touch plants or crops and were viewed as a source of evil and disease.
Throughout history many have believed that menstrual blood was tainted, and that a man who came into contact with it, as during sex, would be poisoned by it. For that mistaken reason, some societies forbade sex with women during their monthly period. The belief that there is something unholy or unhealthy about menstruation is, of course, pure superstition. Menstruation is a perfectly natural biological process. Yet echoes of these ancient myths still appear in beliefs that forbid women to swim, climb, or follow any of their usual pursuits, including sex, during menstruation.
In fact menstruation is not physically disabling. Exercise, including swimming, is actually beneficial during one’s period; women athletes generally report fewer complaints of cramping than their sedentary counterparts. By stimulation of the circulatory system, physical activity helps ease muscle tensions and congested blood vessels. As for sex, while many women experience a loss of interest during their periods, others enjoy it very much. Orgasm may even temporarily relieve mild cramps by accelerating menstrual flow and relieving tensions.
Another false but persistent attitude about menstruation is that it is intellectually or emotionally debilitating. An old wives’ tale holds that during menstruation all of the woman’s blood rushes out of her brain. While people no longer believe this today, the belief still exists that women are less rational during their periods. Such a belief may, in fact, be a self-fulfilling prophecy. Negative mental attitudes can effect the experience of one’s period, making it both physically and psychologically more unpleasant.
Because a good, healthy mindset about one’s menstrual period is important, adolescents should be taught about the natural process they are undergoing, one which will someday allow them to choose to have children. Girls should receive specific guidance about bodily changes and hygienic practices. It is most important for parents to demonstrate their pride in their daughters’ passage from childhood to sexual maturity, perhaps even celebrating menarche (the first menstruation).