- Overt menstruation, occurring in humans and great apes, in which some of the lining of the uterus is shed and expelled from the body;
- Covert menstruation, occurring in all other placental mammals, in which the endometrium is reabsorbed without being expelled.
Over the course of the menstrual cycle, the lining of the uterus builds up in preparation for implantation of a fertilized ovum. If no conception occurs, the thickened lining is sloughed off and flows out though the cervix and vaginal canal. The process of thickening then starts again for the next menstrual cycle. If conception occurs, hormonal changes occur in the body that prevent menstruation from occurring.
Menarche is the onset of menstruation in an adolescent. This average age of menarche is twelve, but there is much variation. Amenorrhea is an unusual absence of menstruation and should be cause for medical consultation.
Differing Views on the Purpose of Menstruation Through History
A number of hypotheses have been put forth regarding the purpose or benefit of menstruation:
- The ancient physician Hippocrates held that menstruation was the body's means of purging itself of bad humors, based on his belief that an excess of blood in the body led to health problems;
- Margie Profet argues that menstruation was the body's means of purging the uterus and reproductive system of bacterial infection, based on the fact that menstrual blood contains an unusually large amount of macrophages, or immune cells ;
- Beverly Strassmann argued that the purpose of menstruation is energy conservation -- that the endometrium is very energy-intensive, so it is more efficient to maintain it for only part of the time. "Endometrial economy," she argued, preserves the metabolic equivalent of six days worth of food for women.
- Elsimar M. Coutinho argues that menstruation acted as a sign that a woman had failed to conceive in the past, and as such is now "obsolete" and should be "cured" by modern medicine. 
Overt and covert menstruation
Both overt menstruation and covert menstruation, the majority of the endometrium is reabsorbed. This leaves us with the question: why do humans and great apes experience overt menstruation? Some speculate that the sophistication and size of the endometrium required to support a human and great ape fetus. It may simply be too large to be reabsorbed, and the excess material is expelled via overt menstruation.
Menstruation and the Bible
Under the Mosaic Law, a woman was to be put apart during menstruation:
And if a woman have an issue, [and] her issue in her flesh be blood, she shall be put apart seven days: and whosoever toucheth her shall be unclean until the even. And every thing that she lieth upon in her separation shall be unclean: every thing also that she sitteth upon shall be unclean. And whosoever toucheth her bed shall wash his clothes, and bathe [himself] in water, and be unclean until the even. And whosoever toucheth any thing that she sat upon shall wash his clothes, and bathe [himself] in water, and be unclean until the even. And if it [be] on [her] bed, or on any thing whereon she sitteth, when he toucheth it, he shall be unclean until the even. And if any man lie with her at all, and her flowers be upon him, he shall be unclean seven days; and all the bed whereon he lieth shall be unclean. Leviticus 15:19-24 (KJV)
Also, sex during menstruation was a forbidden form of sexual immorality:
And if a man shall lie with a woman having her sickness, and shall uncover her nakedness; he hath discovered her fountain, and she hath uncovered the fountain of her blood: and both of them shall be cut off from among their people. Leviticus 20:18 (KJV)
Max Neuberger, writing in his "History of Medicine" states:
|“||The commands concern prophylaxis and suppression of epidemics, suppression of venereal disease and prostitution, care of the skin, baths, food, housing and clothing, regulation of labour, sexual life, discipline of the people, etc. Many of these commands, such as Sabbath rest, circumcision, laws concerning food (interdiction of blood and pork), measures concerning menstruating and lying-in women, and those suffering from gonorrhea, isolation of lepers, and hygiene of the camp, are, in view of the conditions of the climate, surprisingly rational.||”|
An abstract for the L.L. Ben-Noun 2003 article in the European Journal of Obstetrics, Gynecology, and Reproductive Biology entitled "What is the biblical attitude towards personal hygiene during vaginal bleeding?" states the following:
|“||To analyze biblical passages associated with personal hygiene during vaginal bleeding. According to the Bible, a woman who is menstruating or who has pathological vaginal bleeding is unclean. Anybody who touches such a woman's bed or her personal things is also regarded as unclean and should therefore, wash carefully. Sexual relations are forbidden within 7 days from the beginning of menstruation and during pathological vaginal bleeding. Seven days after the cessation of vaginal bleeding, a woman is considered as clean, and therefore, sexual contacts are permitted. From a modern perspective sexual contacts during menses are associated with the development of chlamydial and gonococcal diseases, the risk of transmission of the human immunodeficiency virus infection, endometriosis and subsequent infertility.This report indicates that the roots of contemporary obstetric preventive medicine can be traced to Biblical times.||”|
Also, modern science has only very recently discovered that following such rules dramatically reduces the rate of illegitimacy. CreationWiki sees the biblical injunction about a man not sleeping with a woman who is menstruating as one of the many examples of Bible scientific foreknowledge.
- Menstruation and the Menstrual Cycle, from the US Dept. of Health and Human Services
- Strassmann, Beverly I. (1996) "The Evolution of Endometrial Cycles and Menstruation", Quarterly Review of Biology 71, 2(June): 181-221.
- Elsimar M. Coutinho, Is Menstruation Obsolete?
- Neuburger, Max. History of Medicine. Oxford University Press, 1910, Vol. I, p. 38.
- How Religion Promotes Confidence About Paternity