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

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 posited 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.