What is menstruation?

Process and Effects

Menstruation is the monthly discharge of bloody fluid from the uterus. It occurs in humans and in other primates (apes and monkeys), but not in all mammals; for example, horses, cats, and dogs do not menstruate. The menstrual fluid consists of blood, cells, and debris from the endometrial lining of the uterus, and mucus and other fluids. The color of the discharge varies from dark brown to bright red during the period of flow. The menstrual discharge does not normally clot after leaving the uterus, but it may contain endometrial debris that resembles blood clots. The flow lasts from four to five days in most women, with spotting (the discharge of scant fluid) possibly continuing for another day or two. The volume of fluid lost ranges from ten to eighty milliliters, with a median of about forty milliliters. The blood in the menstrual discharge amounts to only a small fraction of the body’s total blood volume of about five thousand milliliters, so normal physiological functioning is not usually impaired by the blood loss that occurs during menstruation.

The first menstruation (menarche) typically begins between the ages of eleven and fourteen, when a girl goes through puberty; the last episodes of menstruation occur some forty years later at the time of menopause. Menstruation does not occur during the months of pregnancy or for the first few months after a woman has given birth.

Menstruation is the most visible event of the woman’s monthly menstrual cycle. The average length of the menstrual cycle in the population is about 29.1 days, but it may vary from sixteen to thirty-five days, with variation occurring between different individuals and in one individual from month to month. Girls who have just gone through puberty and women who are approaching the menopause tend to have more variation in their cycles than do women in the middle of their reproductive years. There is also an age-related change in cycle length: Cycles tend to be relatively long in teenagers, then decrease in length until a woman is about forty years old, after which cycles tend to lengthen and become irregular.

Hormones cause menstruation to be coordinated with other events in the menstrual cycle. Uterine function is regulated by two hormones,
estrogen and progesterone, which are produced in the ovaries. In turn, the production of estrogen and progesterone is controlled by follicle-stimulating hormone (FSH) and luteinizing hormone (LH), both of which are produced in the pituitary gland. The hormones from the ovaries and from the pituitary have mutual control over each other: they participate in a feedback relationship. The fact that females produce ova only once a month, in a cycle rather than continuously, is the result of a change in the feedback relationships between the ovarian and pituitary hormones as the menstrual cycle proceeds.

In the first half of the cycle, the follicular phase, a predominant negative feedback effect keeps pituitary hormone levels low while allowing estrogen to increase. Day one of the menstrual cycle is defined as the day of the onset of the menstrual flow. During the days of menstrual bleeding, levels of estrogen and progesterone are low, but FSH levels are high enough to cause the growth of follicles in the ovary. As the follicles start to grow, they secrete estrogen, and increasing amounts are secreted as the follicles continue to enlarge over the next five to ten days. The estrogen exerts negative feedback control over the pituitary: FSH and LH production is inhibited by estrogen, so levels of these hormones remain low during the follicular phase. Besides producing estrogen, the growing follicles contain ova that are maturing and preparing for ovulation. Meanwhile, estrogen acts on the uterus to cause the growth of the endometrial lining. The lining becomes thicker and its blood supply increases; glands located in the lining also grow and mature. These uterine changes are known as endometrial proliferation.

As the woman nears the middle of her cycle, a dramatic change in hormonal feedback occurs. The increasing secretion of estrogen shifts the hormonal system into a positive feedback mode, whereby an increase in estrogen stimulates the release of LH and FSH from the pituitary instead of inhibiting it. Thus, at the middle of the cycle (around day fourteen), simultaneous peaks in levels of estrogen, LH, and FSH occur. The peak in LH triggers ovulation by causing changes in the wall of the follicle, allowing it to break open to release its ovum. Although a group of follicles has matured up to this point, usually only the largest one ovulates, and the remainder in the group die and cease hormone production.

Following ovulation, negative feedback is reestablished. The follicle that just ovulated remains as a functional part of the ovary; it becomes transformed into the corpus luteum, a structure that produces estrogen and progesterone throughout most of the second half of the cycle, the luteal phase. During this phase, the combined presence of estrogen and progesterone reestablishes negative feedback over the pituitary, and LH and FSH levels decline. A second ovulation is prevented because an LH peak is not possible at this time. The combined action of estrogen and progesterone causes the uterus to enter its secretory phase during the second half of the cycle: The glands in the thickened endometrium secrete nutrients that will support an embryo if the woman becomes pregnant, and the ample blood supply to the endometrium can supply the embryo with other nutrients and oxygen. If the woman does in fact become pregnant, the embryo will secrete a hormone that will ensure the continued production of estrogen and progesterone, and because of these hormones, the uterus will remain in the secretory condition throughout pregnancy. Menstruation does not occur during pregnancy because of the high levels of estrogen and progesterone, which continually support the uterus.

If the woman does not become pregnant, the corpus luteum automatically degenerates, starting at about the twenty-fourth day of the menstrual cycle. As the corpus luteum dies, it fails to produce estrogen and progesterone, so levels of these hormones decrease. As the amounts of estrogen and progesterone drop, the uterus begins to produce prostaglandins, chemicals that act as local signals within the uterus. The prostaglandins cause a number of changes in uterine function: blood flow to the endometrium is temporarily cut off, causing the endometrial tissue to die, and the uterine muscle begins to contract, causing further changes in blood flow. The decreased blood flow and the muscle contractions contribute to the cramping pain that many women feel just before and at the time of menstrual bleeding. Menstrual bleeding starts when the blood flow to the endometrium is reestablished and the dead tissue is sloughed off and washed out of the uterus. This event signals the start of a new menstrual cycle.

Complications and Disorders

Many disorders involving menstruation exist. Toxic shock syndrome is a disease that, while not caused directly by menstruation, sometimes occurs during menstruation in women who use tampons to absorb the menstrual flow. The symptoms of toxic shock syndrome—fever, rash, a drop in blood pressure, diarrhea, vomiting, and fainting—are caused by toxins produced by the bacterium Staphylococcus aureus. This bacterium is normally present in limited numbers within the vagina, but the use of high-absorbency tampons is associated with a higher-than-normal bacterial growth and toxin production. Toxic shock syndrome requires immediate medical attention, since it may be fatal if left untreated. Women can reduce the risk of toxic shock syndrome by changing tampons often, using lower-absorbency types, and alternating the use of tampons and sanitary napkins.

Amenorrhea is defined as the absence of menstruation. It is usually, but not always, coincident with a lack of ovulation. Amenorrhea may be primary (the woman has never menstruated) or secondary (menstrual cycles that were once normal have stopped). The condition is usually associated with abnormal patterns of hormone secretion, but the problem in hormone secretion may itself be merely the symptom of some other underlying disorder. One of the most common situations leading to both primary and secondary amenorrhea is low body weight, caused by malnutrition, eating disorders, or sustained exercise. Body fat has two roles in reproduction: it provides energy needed for tissue growth and cell functions, and it contributes to circulating estrogen levels. Loss of body fat may create a situation in which the reproductive system ceases to function because of low estrogen levels and because of lack of needed energy. The result is seen as amenorrhea. Emotional or physical stress may also cause amenorrhea, because stress results in the release of hormones that interfere with the reproductive hormones. Ideally, amenorrhea is treated by removing its cause; for example, a special diet or a change in an exercise program can bring about an increase in body fat stores, or stress levels can be reduced through changes in lifestyle or with counseling. Ironically, sometimes birth control pills are prescribed for women with amenorrhea. The pills do not cure the amenorrhea, but they can counteract some of the long-term problems associated with it, such as changes in the endometrial lining and loss of bone density.

Dysmenorrhea refers to abnormally intense uterine pain associated with menstruation. It is estimated that 5 to 10 percent of women experience pain intense enough to interfere with their school or work schedules. Dysmenorrhea may be primary (occurring in women with no known disease) or secondary (caused by a disease condition such as a tumor or infection). Studies have shown uterine prostaglandin levels to be correlated with the degree of pain perceived in primary dysmenorrhea, and drugs that interfere with prostaglandins offer an effective treatment for this condition. These drugs include aspirin, acetaminophen, ibuprofen, and naproxen; some formulas are available without a doctor’s prescription, but the stronger drugs require one. Secondary dysmenorrhea is best managed by removing the underlying cause; if this is not possible, the antiprostaglandin drugs may be useful in controlling the pain.

Menorrhagia is excessive menstrual blood loss, usually defined as more than eighty milliliters of fluid lost per cycle. This condition can have serious health consequences because of the loss of red blood cells, which are essential for carrying oxygen to tissues. Women who have given birth to several children are more likely to suffer from menorrhagia, possibly because of enlargement of the uterine cavity and interference with the mechanisms that limit menstrual blood flow. Women who have diseases that interfere with blood clotting may also have menorrhagia. Although the menstrual discharge itself does not usually form clots after it leaves the uterus, clots do form within the uterine endometrium; these clots normally prevent excessive blood loss. Treatment for menorrhagia may begin with iron and vitamin supplements to induce increased red blood cell production, or transfusions may be used to replace the lost red blood cells. If this is unsuccessful, treatment with birth control pills, destruction of the endometrium by laser surgery, or a hysterectomy (surgical removal of the uterus) may be necessary.

Endometriosis is a condition in which endometrial cells from the uterus become misplaced within the abdominal cavity, adhering to and growing on the surface of internal organs. The outside of the uterus, the oviducts (fallopian tubes), the surface of the ovaries, and the outer surface of the intestines can all support the growth of endometrial tissue. Endometriosis is thought to arise during menstruation, when endometrial tissue enters the oviducts instead of being carried outward through the cervix and vagina. Through the oviducts, the endometrial tissue has access to the abdominal cavity. Since the misplaced endometrial tissue responds to hormones in the same way that the normal endometrium does, it undergoes cyclic changes in thickness and attempts to shed at the time of menstruation. Endometriosis results in intense pain during menstruation and can cause infertility because of interference with ovulation, ovum or sperm transport, or uterine function. Endometriosis is treated with birth control pills or with drugs that suppress menstrual cycles, or the endometrial tissue may be removed surgically.

Premenstrual syndrome (PMS) is a set of symptoms that occurs in some women in the week before the start of menstruation, with the symptoms disappearing once menstruation begins. Researchers and physicians who study PMS have struggled to devise a standard definition for the disorder, but the list of possible symptoms is lengthy and varies from woman to woman and even within one woman from month to month. The possible symptoms include both psychological and physical changes: irritability, nervous tension, anxiety, moodiness, depression, lethargy, insomnia, confusion, crying, food cravings, fatigue, weight gain, swelling and bloating, breast tenderness, backache, headache, dizziness, muscle stiffness, and abdominal cramps. A diagnosis of PMS requires that the symptoms show a clear relation to the timing of menstruation and that they recur during most menstrual cycles. Researchers estimate that 3 to 5 percent of women have PMS symptoms that are so severe that they are incapacitating, but that milder symptoms occur in about 50 percent of all women.

Because of the variability in symptoms between women, some researchers believe that there are several subtypes of PMS, each with its own cluster of symptoms. It is possible that each subtype has a unique cause. Suggested causes of PMS include an imbalance in the ratio of estrogen to progesterone following ovulation; changes in the hormones that control salt and water balance (the renin-angiotensin-aldosterone system); increased levels of prolactin (a hormone that acts on the breast); changes in amounts of brain chemicals; altered functioning of the biological clock that determines daily rhythms; poor diet or sensitivity to certain foods; and psychological factors such as attitude toward menstruation, stresses of family or professional life, and underlying personality disorders. Studies evaluating these theories have yielded contradictory results, so that no one cause of PMS has yet been found. Current treatments for PMS include dietary therapy, hormone administration, and psychological counseling, but no treatment has been found effective in all PMS patients.

An interesting phenomenon associated with menstruation is menstrual synchrony, also known as the “dormitory effect.” Among women who live together, menstrual cycles gradually become synchronized, so that the women begin to menstruate within a few days of one another. Researchers have found that this phenomenon probably occurs because of
pheromones, chemical signals that are produced by an individual and that have an effect on another individual. Pheromones act on the brain through the sense of smell, even though there may not be an odor that is consciously perceived.

Perspective and Prospects

Early beliefs about menstruation were based on folk magic and superstition rather than on scientific evidence. Even today, some cultures persist in believing that menstruating women possess deleterious powers: that the presence of a menstruating woman can cause crops to fail, farm animals to die, or beer, bread, jam, and other foods to be spoiled. Some people believe that these incidents will occur even if the menstruating woman has no evil intention. Because of the possibility of these events, some cultures prohibit menstruating women from interacting with others. In the most rigorous example of such a taboo, some societies require that menstruating women live in special huts for the duration of the bleeding period.

Folk beliefs about menstruating women have been bolstered by religious views of menstruating women as “unclean” and in need of purification. In Orthodox Judaism, there are detailed proscriptions to be observed by a menstruating woman, including the avoidance of sexual intercourse. Seven days after her menstrual flow has stopped, the Orthodox Jewish woman undergoes a ritual purification, after which she may resume sexual relations with her husband. Early Christians absorbed the Jewish belief in the uncleanliness of a menstruating woman and prohibited her from entering church or receiving the sacraments. These injunctions were lifted by the seventh century, but the view of women as spiritually and bodily impure persists in some Christian groups to this day.

Many couples abstain from intercourse during the woman’s menstrual period. There is no medical justification for this behavior; in fact, research has demonstrated that intercourse can alleviate menstrual cramping, at least temporarily. Still, surveys have shown that a majority of both men and women think that it is wrong for a woman to have intercourse while menstruating.

There are also persistent beliefs that women’s physical and mental abilities suffer during menstruation. In fact, this was the predominant medical opinion up through the nineteenth and early twentieth centuries. Medical writings from this time are filled with injunctions for women to rest and to refrain from exercise and intellectual strain while menstruating. It was a common belief that education could actually cause physical harm to women. Some men used this advice as justification for excluding women from equal opportunities in education and employment. Starting in the late nineteenth century, however, scientific studies clearly demonstrated that education has no harmful effects and that there is no diminution of intellectual or physical performance during menstruation. Nevertheless, the latter finding has been one that the general population finds difficult to accept.

The latest view of menstruation is that, far from being harmful, menstrual bleeding is directly beneficial to a woman’s health. Margie Profet, an evolutionary biologist at the University of California, theorized that menstruation evolved as a means of periodically removing disease-causing bacteria and viruses from the woman’s uterus. These organisms might enter the uterus along with sperm after sexual activity. In Profet’s view, the energetic cost of replacing the blood and tissue lost through menstruation is more than outweighed by the protective benefits of menstruation. Her theory implies that treatments that suppress menstruation, as birth control drugs sometimes do, are not always advantageous.

The suppression of menstruation through extended or continuous cycling with combined hormonal contraception has recently been reexamined for various benefits, including increased contraceptive efficacy. Some clinicians and consumers have embraced this concept, which can be done with any continuous (no placebo or no-pill interval) use of a monophasic combined oral contraceptive pill, the Ortho Evra patch, or NuvaRing. New formulations of combined oral contraceptives include Seasonale and Seasonique, both of which result in menstrual bleeding every three months, and Lybrel, which eliminates cycles for one year. Other formulations have shortened the one week pill-free interval, resulting in shorter and lighter menses.

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