menstruation, periodic flow of blood and cells from the lining of the uterus in humans and most other primates, occurring about every 28 days in women. Menstruation commences at puberty (usually between age 10 and 17). The onset of menstruation, called menarche, signals the body's coming readiness for childbearing. It continues, unless interrupted by pregnancy until menopause (around age 50).

There have been many myths and taboos associated with menstruation. Some cultures isolated women or thought the menstrual flow "unclean" or a "curse." More recent taboos against exercise or sexual intercourse during menstruation are slowly lifting. Some scientists have asked why menstruation occurs at all—why the uterine lining does not remain in place, regenerating itself as other parts of the body (such as the skin and digestive tract) do. One theory is that menstruation is a defense against microbes that enter the uterus with incoming sperm.

The Menstrual Cycle

In the first phase of each cycle, the lining, or endometrium, of the uterus undergoes rapid proliferation of cells and venous channels in preparation for pregnancy. Midway through the cycle an ovum (egg) is released from an ovary. If, while passing through the fallopian tube the ovum is fertilized by a sperm, implantation in the uterus occurs and the thickened lining helps support the pregnancy. When the ovum is not fertilized, this tissue and blood are shed. The proliferation of the uterine wall then begins once more in expectation of the next release of an ovum, and if conception does not take place, it sloughs off again. The process continues monthly until pregnancy occurs or until ovulation ceases at menopause.

The natural rhythm of the menstrual cycle may be broken or temporarily halted by hormonal imbalance, malnutrition, illness, or emotional disturbance (see amenorrhea). Menstruation is controlled by the hypothalamus and the pituitary gland and hormones, such as estrogen, which prepares the lining of the uterus, and progesterone, which helps maintain a pregnancy.

Dysmenorrhea and Premenstrual Syndrome

Many women experience painful menstruation, or dysmenorrhea. The uterine contractions that result in the cramps experienced by these women appear to be caused by hormones called prostaglandins that are produced in the second half of the cycle. Oral contraceptives and other drugs that reduce the production of prostaglandins are sometimes used in treatment. Other women experience symptoms such as behavioral changes, breast tenderness, and fatigue during the week immediately preceding menstruation, a condition referred to as premenstrual syndrome, or PMS.

Periodic discharge from the vagina of blood, secretions, and shed mucous lining of the uterus (endometrium). The endometrium prepares to receive a fertilized egg by thickening and producing secretions. If the egg released by the ovary is not fertilized, the endometrium breaks down and is expelled by contractions of the uterus. The first menstruation (menarche) occurs after other changes of puberty, usually at 11–13 years of age, apparently triggered by the passing of a weight threshold. Bleeding may be irregular or heavy at first. In adult women, menstrual periods begin at an average interval of 28 days and last about five days; some variation among women and in the same woman is normal. Uterine contractions are felt as cramps. The amount of blood lost is usually less than 1.7 oz (50 ml). Menstruation ends with menopause. Menstrual disorders include dysmenorrhea (painful menstruation) and amenorrhea (no bleeding), heavy or light bleeding, and uterine bleeding. Seealso premenstrual syndrome.

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See also "Mensuration", a term sometimes used to describe Measurement, particularly in the context of forestry.

Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive age females of certain mammal species. Overt menstruation (where there is bleeding from the vagina) is found primarily in humans and close evolutionary relatives such as chimpanzees. The females of other placental mammal species have estrous cycles, in which the endometrium is reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. Many zoologists regard this as different from a "true" menstrual cycle.

Overview

Eumenorrhea denotes normal, regular menstruation that lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal). The average blood loss during menstruation is 35 millilitres with 10-80 mL considered normal; many females also notice shedding of the endometrium lining that appears as tissue mixed with the blood. (Sometimes this is erroneously thought to indicate an early-term miscarriage of an embryo.) An enzyme called plasmin — contained in the endometrium — tends to inhibit the blood from clotting. Because of this blood loss, premenopausal women have higher dietary requirements for iron to prevent iron deficiency. Many women experience uterine cramps, also referred to as dysmenorrhea, during this time, caused largely by the contractions of the uterine muscle as it expels the endometrial blood from the woman's body. A vast industry has grown to provide drugs to aid in these cramps, as well as sanitary products to help manage menses.

As part of the menstrual cycle

Menstruation is the most visible phase of the menstrual cycle. Menstrual cycles are counted from the first day of menstrual bleeding, because the onset of menstruation corresponds closely with the hormonal cycle.

During pregnancy and for some time after childbirth, menstruation is normally suspended; this state is known as amenorrhoea, i.e. absence of the menstrual cycle. If menstruation has not resumed, fertility is low during lactation. The average length of postpartum amenorrhoea is longer when certain breastfeeding practices are followed; this may be done intentionally as birth control (lactational amenorrhea method).

Evolution

All placental mammals have a uterine lining that builds up when the animal is fertile, but is dismantled (menstruated) when the animal is infertile. Some anthropologists have questioned the energy cost of rebuilding the endometrium every fertility cycle. However, anthropologist Beverly Strassmann has proposed that the energy savings of not having to continuously maintain the uterine lining more than offsets energy cost of having to rebuild the lining in the next fertility cycle, even in species such as humans where much of the lining is lost through bleeding (overt menstruation) rather than reabsorbed (covert menstruation). However, even in humans, much of it is reabsorbed.

Many have questioned the evolution of overt menstruation in humans and related species, speculating on what advantage there could be to losing blood associated with dismantling the endometrium rather than absorbing it, as most mammals do. The ancient writer Hippocrates considered that menstruation was intended to cleanse the body of "evil humours", and modern evolutionary biologist Margie Profet contends that the primary function of menstruation is to remove sperm-borne pathogens from the uterus. In support of this hypothesis, she has pointed to the relatively high levels of macrophages in menstrual blood.

Beginning in 1971, some research suggested that menstrual cycles of co-habiting human females became synchronized. A few anthropologists hypothesized that in hunter-gatherer societies, males would go on hunting journeys whilst the females of the tribe were menstruating, speculating that the females would not have been as receptive to sexual relations while menstruating. However, there is currently significant dispute as to whether menstrual synchrony exists.

Humans do, in fact, reabsorb about two-thirds of the endometrium each cycle. Strassmann asserts that overt menstruation occurs not because it is beneficial in itself. Rather, the fetal development of these species requires a more developed endometrium, one which is too thick to completely reabsorb. Strassman correlates species that have overt menstruation to those that have a large uterus relative to the adult female body size.

Culture and menstruation

Common usage refers to menstruation and menses as a period. Aside from its biological purpose, this bleeding serves as a sign that a woman has not become pregnant. (However, this cannot be taken as certainty, as sometimes there is some bleeding in early pregnancy, and some women have irregular cycles.) During the reproductive years, failure to menstruate may provide the first indication to a woman that she may have become pregnant. A woman might say that her "period is late" when an expected menstruation has not started and she might have become pregnant.

Many religions have menstruation-related traditions. These may be bans on certain actions during menstruation (such as intercourse in orthodox Judaism and Islam), or rituals to be performed at the end of each menses (such as the mikvah in Judaism and the ghusl in Islam). Some traditional societies sequester females in residences ("menstrual huts") that are reserved for that exclusive purpose until the end of their menstrual period.

Since the late 1960s, some women have chosen to prevent menstruation with long-acting hormonal birth control. Injections such as depo-provera became available in the 1960s, progestogen implants such as Norplant in the 1980s and extended cycle combined oral contraceptive pills in the early 2000s.

Characteristics

Physical experience

In many women, various intense sensations brought about by the involved hormones and by cramping of the uterus can precede or accompany menstruation. Stronger sensations may include significant menstrual pain (dysmenorrhea), abdominal pain, migraine headaches, depression, emotional sensitivity, feeling bloated, and changes in sex drive. Breast discomfort caused by premenstrual water retention or hormone fluctuation is very common. The sensations experienced vary from woman to woman and from cycle to cycle.

Emotional reactions

Women may experience emotional disturbances associated with menstruation. These range from the irritability popularly associated with Premenstrual Syndrome (PMS), to tiredness, or "weepiness" (i.e. tears of emotional closeness). A similar range of emotional effects and mood swings is associated with pregnancy.

Flow

The normal menstrual flow follows a "crescendo-decrescendo" pattern; that is, it starts at a moderate level, increases somewhat, and then slowly tapers. Sudden heavy flows or amounts in excess of 80 mL (hypermenorrhea or menorrhagia) may stem from hormonal disturbance, uterine abnormalities, including uterine leiomyoma or cancer, and other causes. Doctors call the opposite phenomenon, of bleeding very little, hypomenorrhea.

Duration

The typical woman bleeds for two to seven days at the beginning of each menstrual cycle. Prolonged bleeding (metrorrhagia, also meno-metrorrhagia) no longer shows a clear interval pattern. Dysfunctional uterine bleeding is hormonally caused bleeding abnormalities, typically anovulation. All these bleeding abnormalities need medical attention; they may indicate hormone imbalances, uterine fibroids, or other problems. As pregnant patients may bleed, a pregnancy test forms part of the evaluation of abnormal bleeding.

Menstrual products

Most women use something to absorb or catch their menses. There are a number of different methods available.

Disposable items:

• Sanitary napkins (Sanitary towels) or pads — Somewhat rectangular pieces of material worn in the underwear to absorb menstrual flow, often with "wings," pieces that fold around the panties, and/or an adhesive backing to hold the pad in place. Disposable pads may contain wood pulp or gel products, usually with a plastic lining and bleached. Some sanitary napkins, particularly older styles, are held in place by a belt-like apparatus, instead of adhesive or wings.
• Tampons — Disposable cylinders of treated rayon/cotton blends or all-cotton fleece, usually bleached, that are inserted into the vagina to absorb menstrual flow.
• Padettes — Disposable wads of treated rayon/cotton blend fleece that are placed within the inner labia to absorb menstrual flow.
• Disposable menstrual cups — A firm, flexible cup-shaped device worn inside the vagina to catch menstrual flow. Disposable cups are made of soft plastic.

Reusable items:

• Reusable cloth pads are made of cotton (often organic), terrycloth, or flannel, and may be handsewn (from material or reused old clothes and towels) or storebought.
• Menstrual cups — A firm, flexible bell-shaped device worn inside the vagina to catch menstrual flow. Reusable versions include rubber or silicone cups.
• Sea sponges — Natural sponges, worn internally like a tampon to absorb menstrual flow.
• Padded panties — Reusable cloth (usually cotton) underwear with extra absorbent layers sewn in to absorb flow.
• Blanket, towel — (also known as a draw sheet) — large reusable piece of cloth, most often used at night, placed between legs to absorb menstrual flow.

In addition to products to contain the menstrual flow, pharmaceutical companies likewise provide products — commonly non-steroidal anti-inflammatory drugs (NSAIDs) — to relieve menstrual cramps. Some herbs, such as dong quai, raspberry leaf and crampbark, are also claimed to relieve menstrual pain; however there is no documented scientific evidence to prove this.

Further reading

• Museum of Menstruation
• Keeper.com Interview with Director of Museum of Menstruation
• Pill, Katherine, (Illustration by Alexandra Westrich), Greening Your Monthly Flow: Reusable menstrual products, F Newsmagazine, School of the Art Institute of Chicago, April 4th, 2008

Footnotes