Menstruation

Menstruation is the regular, cyclical discharge of blood and endometrial tissue through the vagina and represents the most visible phase of the menstrual cycle. The cycle is regulated by coordinated hormonal fluctuations and reflects the preparation of the uterus for potential pregnancy. When conception does not occur, falling progesterone levels trigger the breakdown of the endometrium, resulting in menstrual bleeding. Menstruation occurs from menarche in puberty until menopause, except during pregnancy and often during the early months of breastfeeding.

Characteristics, Timing, and Variability

The first menstrual period, known as menarche, typically occurs between the ages of 11 and 13, though onset between ages 8 and 15 is considered normal. The average age is generally slightly higher in developing countries and lower in more developed regions. After the onset of puberty, menstrual cycles occur with a typical length of 21–45 days in adolescents and 21–35 days in adults. Although the conventional average cycle is often cited as 28 days, large-scale analyses of menstrual tracking data indicate an average closer to 29.3 days.
Cycle variability is greatest in younger women under 25 and becomes more regular between ages 25 and 39, before increasing again in the years approaching menopause. Menopause, defined medically as twelve consecutive months without a period, usually occurs between ages 45 and 55. Premature menopause before 45 may be associated with illness, surgery, or other medical treatments. Irregular or absent periods (amenorrhoea) may also reflect hormonal disturbances, systemic illness, or stress.
Menstrual bleeding generally lasts between two and seven days. Lochia, a separate postpartum discharge, occurs after childbirth and is not considered menstruation.

Composition and Volume of Menstrual Fluid

Menstrual fluid consists of blood, cervical mucus, vaginal secretions, and shed fragments of the endometrium. The average monthly volume is typically between 30 and 80 millilitres. The fluid appears reddish-brown, slightly darker than venous blood. Approximately half of its content is blood, containing sodium, calcium, phosphate, chloride, and iron, although the amount of iron lost is usually small.
Clots or endometrial fragments may be observed; these are not usually pathological. An enzyme called plasmin, present in the endometrium, limits clot formation. Heavy menstrual bleeding can lead to iron-deficiency anaemia, though many women with iron deficiency may have unrelated underlying medical conditions requiring further investigation.

Hormonal Regulation and Physiological Effects

Fluctuations in progesterone, oestrogen, follicle-stimulating hormone, and luteinising hormone regulate the menstrual cycle. These hormonal changes affect not only the uterus but also other systems. Pelvic cramping (dysmenorrhoea) is caused by uterine muscle contractions and is common, though severe pain affecting daily activities occurs in a minority of women. Symptoms that arise before menstruation and significantly interfere with daily life are classified as premenstrual syndrome (PMS). Approximately 20–30% of women experience PMS, while 3–8% develop severe symptoms termed premenstrual dysphoric disorder.
Treatments for menstrual pain typically include non-steroidal anti-inflammatory drugs, which reduce prostaglandin-mediated uterine contractions. Some women do not respond to these medications and require alternative approaches such as heat therapy, simple analgesics, or hormonal contraceptives. Evidence for acupuncture is mixed.
Hormonal shifts can influence other health conditions. For example, some individuals experience migraine episodes associated with falling oestrogen levels. Women with epilepsy may find that seizure frequency follows a cyclical pattern linked to various phases of the menstrual cycle. Musculoskeletal research indicates that certain ligament injuries may occur more frequently in the pre-ovulatory phase.

Reproductive and Sexual Aspects

Menstruation marks the beginning of a new reproductive cycle. Ovulation usually occurs around the midpoint of the cycle, when fertility is at its peak. Oestrogen and androgen levels rise before and during ovulation, and some women experience increased sexual desire at this time. Although sexual interest is generally lower directly before and during menstruation, women may engage in sexual activity at any point in the cycle. There is no medical evidence against sexual intercourse during menstruation for healthy individuals.
Periods cease during pregnancy due to sustained progesterone levels and do not typically resume during the early months of breastfeeding, as lactation can suppress ovulation. With menopause, reproductive capacity ends permanently.

Menstrual Health and Associated Conditions

Most women—around 80%—do not experience symptoms severe enough to affect daily activities. However, menstrual irregularities and discomforts remain common concerns. Heavy bleeding, severe pain, irregular cycles, or prolonged absence of menstruation may indicate underlying conditions such as hormonal disorders, polycystic ovary syndrome, thyroid disease, or uterine abnormalities.
Understanding menstrual patterns and associated symptoms is crucial for reproductive health, early detection of medical issues, and overall wellbeing. The menstrual cycle reflects complex interactions between hormones, reproductive organs, and systemic physiological processes that continue to influence women’s health throughout their reproductive years.