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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 uterus through 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.
Eumenorrhea expresses 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 women 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.
All female 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 re-absorbed.
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.
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.
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, changes in sex drive and nausea. Breast swelling and discomfort caused by premenstrual water retention or hormone fluctuation is very common. Binge eating occurs in a minority of menstruating women. This may be due to fluctuation in beta-endorphin levels. More severe symptoms may be classified as premenstrual dysphoric disorder (PMDD). The sensations experienced vary from woman to woman and from cycle to cycle.
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.
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. Dysfunctional uterine bleeding typically occurs in premenopausal women who do not ovulate normally (i.e. are anovulatory). 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.
Most women use something to absorb or catch their menses. There are a number of different methods available.
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 undergarment 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.
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