Menstruation is the periodic shedding of blood, endometrial tissue and cellular material from the uterus through the vagina when pregnancy has not occurred and reproductive hormone levels fall at the end of the preceding cycle. It is a normal physiological process associated with the reproductive years, although cycle characteristics vary substantially between individuals.
Menstrual bleeding results primarily from withdrawal of progesterone and oestrogen after regression of the corpus luteum, leading to changes in the endometrial vasculature, inflammatory signalling and tissue breakdown. Menstruation is therefore not simply “old blood” leaving the body; menstrual fluid contains blood as well as endometrial tissue, cervical mucus and vaginal secretions. The timing and amount of bleeding are influenced by endocrine function, uterine anatomy, medications, pregnancy status and general health.
The menstrual cycle is conventionally described from the first day of bleeding, which is designated cycle day one. During the preceding cycle, oestrogen produced by developing ovarian follicles stimulates proliferation of the endometrium. After Ovulation, progesterone produced by the corpus luteum changes the endometrium into a secretory state. If implantation and pregnancy do not occur, corpus-luteum function declines and progesterone and oestrogen concentrations fall.
This hormonal withdrawal contributes to constriction and subsequent disruption of specialised endometrial blood vessels, local inflammatory responses and shedding of the functional layer of the endometrium. The basal layer remains and provides tissue from which the endometrium can regenerate during the following cycle. Menstruation is consequently part of a coordinated endocrine and tissue-remodelling process rather than a random loss of blood.
Typical menstrual cycles vary in length, and cycle length can change with age, reproductive stage, stress, illness and other factors. Bleeding often lasts several days, with the heaviest flow commonly occurring during the earlier part of the episode, but normal variation is broad. Menstrual blood does not usually clot extensively because the endometrium contains local fibrinolytic activity that helps prevent excessive coagulation, although clots can occur, particularly when flow is heavy enough for blood to accumulate before leaving the uterus.
Menstrual pain, or dysmenorrhoea, is commonly related to increased prostaglandin activity causing uterine contractions and inflammatory effects. Some people experience little or no pain, while others have substantial symptoms. Severe pain is not automatically a normal feature and can sometimes indicate an underlying condition requiring assessment.
Menstruation changes throughout the reproductive lifespan. The first menstrual period is called menarche and usually occurs during puberty after maturation of the hypothalamic-pituitary-ovarian axis. Early cycles can be irregular because endocrine regulation is still developing, and anovulatory cycles are relatively common during the initial years after menarche. Later in life, the menopausal transition is characterised by increasing variability in cycle length, ovulation and bleeding patterns before menstruation eventually ceases.
Menopause is defined retrospectively after twelve consecutive months without menstruation when there is no other explanation. Pregnancy and lactation can also alter or suppress menstrual bleeding. Hormonal contraception, intrauterine devices and certain medications can change the amount, frequency or presence of bleeding without necessarily indicating disease.
Menstruation has important clinical value because changes in bleeding patterns can provide clues about reproductive and systemic health. Abnormally heavy bleeding can lead to iron-deficiency anaemia and may arise from structural uterine abnormalities, hormonal disturbances, bleeding disorders, medications or other causes. Bleeding between periods, after sexual intercourse or after menopause has distinct clinical implications and should not simply be assumed to represent ordinary menstruation.
Absence of menstruation, known as amenorrhoea, can result from pregnancy, endocrine disorders, low energy availability, ovarian dysfunction, medications and other causes. Very irregular cycles may indicate anovulation or conditions such as Polycystic Ovary Syndrome. Menstrual history is therefore a routine component of reproductive assessment and can provide information about endocrine function, fertility, medication effects and possible disease.
Menstrual products and menstrual practices vary across cultures and circumstances. Common products include disposable pads, tampons, menstrual cups and reusable absorbent underwear, each with different practical characteristics. Product choice depends on comfort, bleeding pattern, accessibility, cost, environmental considerations and personal preference. Basic hygiene is important, but menstruation itself is not dirty or inherently harmful. Prolonged use of some products beyond recommended intervals can increase risks, including toxic shock syndrome with tampon use, although this complication is rare.
Access to safe menstrual products, sanitation, privacy and accurate information is an important aspect of health and social participation. Difficulties obtaining appropriate products can contribute to school or workplace absence and can disproportionately affect people experiencing poverty, homelessness or other forms of limited access to resources.
Menstruation has historically been surrounded by cultural beliefs, taboos and misconceptions. The idea that menstrual blood is inherently toxic or contaminating has no scientific basis, while cultural restrictions around menstruation have varied widely between societies and historical periods. Medical understanding has also changed from early explanations based on humoral concepts to modern knowledge of reproductive endocrinology and endometrial physiology. Menstruation is not evidence that an egg has been released in every cycle, because bleeding can occur after anovulatory hormonal changes. It is also not necessary for a person to menstruate regularly to be healthy in every circumstance, particularly when hormonal contraception intentionally suppresses bleeding. Conversely, unexpected changes in bleeding should not automatically be dismissed as normal variation.
Menstruation is closely connected with Menstrual Health because health involves more than the biological presence of bleeding. A person may menstruate regularly yet experience debilitating pain, excessive bleeding or barriers to obtaining appropriate products and care. Conversely, absence of bleeding may be an expected effect of pregnancy, lactation, hormonal treatment or menopause rather than a disease. Understanding Menstruation therefore requires separating normal physiological variation from clinically significant abnormalities while recognising individual circumstances. Menstrual experiences can also affect sleep, exercise, education, work and social participation. Accurate terminology helps distinguish menstruation itself from the broader menstrual cycle, from symptoms associated with the premenstrual phase, and from Menstrual Disorders that produce abnormal bleeding or other clinically significant problems.
