Menstruation

# Overview
Menstruation is a phase of the menstrual cycle in which the uterine lining (endometrium) is shed. Menstrual cycles occur exclusively in humans and other great apes. The females of other mammalian species experience an estrus, in which the endometrium is reabsorbed by the animal at the end of its reproductive cycle.

# Characteristics
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. An enzyme called plasmin  contained in the endometrium  tends to inhibit the blood from clotting. Because of this blood loss, females have higher dietary requirements for iron than do males to prevent iron deficiency. Many females experience uterine cramps, also referred to as dysmenorrhea, during this time. 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.
The evolutionary impetus for menstruation remains somewhat unclear. Most mammals reabsorb the uterine lining during their oestral cycle.  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 theory, she has pointed to the relatively high levels of macrophages in menstrual blood. Anthropologist  Beverly Strassmann has posited that the energy savings of not having to continuously maintain the uterine lining more than offsets the blood loss of menstruation.  Currently, however, no single explanation of the evolutionary purpose of menstruation is accepted.
Beginning in 1971, some research suggested that menstrual cycles of co-habiting human females became synchronized.  Anthropologists such as Desmond Morris and Chris Knight 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.

# 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.

## 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
Many women experience emotional side-effects. These range from the irritability popularly associated with Premenstrual Syndrome, 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 refers to 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 synthetic pads are made of wood pulp or synthetic 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 - Reuseable cloth (usually cotton) underwear with extra absorbent layers sewn in to absorb flow. (like Lunapads)
- 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.