Introduction
The female human repro ductive system comprises the hyp Othalamic- pituitary-ovarian (HPO) axis and the reproductive tract (fallopian tubes, uterus, cervix, and vagina).
The principal functions of this system are to produce an ovum, enable its fertilization and implantation, and allow growth and safe expulsion of the fetus into the external world.
The menstrual cycle is critical for facilitation of the initial steps of this raison d’etre of the female reproductive system.Why women menstruate
Menstruation is not a ubiquitous process. All female placental mammals have a uterine lining that is receptive at fertile time points, but menstruation is predominantly limited to primates, several species of bat, and the elephant shrew. In species where there is no outward menstruation, oestrus cycles are followed by ‘covert menstruation' in which the endometrium is completely reabsorbed. The benefits of one system over another are not fully understood. Initially menstruation was thought to be an evolutionary defence against sperm- carried pathogens (1) or that the energy expenditure of maintaining the endometrium outweighed shedding and rebuilding (2). However, the current hypothesis is that decidualization is a prerequisite for menstruation. Decidualization is the process of conversion of endometrial stromal cells into specialized decidual cells that have the capacity to nourish an embryo. Decidualization occurs in menstruating women prior to fertilization; in contrast, non-menstruating mammals decidualize only at the point of implantation. Decidualization is thought to confer evolutionary benefits though facilitating placental invasion of healthy embryos, enabling an element of embryo selection by the uterus in menstruating species (3).
Why menstrual problems are a modern problem
Menstruation has traditionally been viewed as a negative process.
Aristotle considered it a sign of female inferiority (4) and Pliny the Elder said of menstruating women ‘hardly can there be found a thing more monstrous than is that flux and course of theirs' (5). Behavioural dogma are enshrined in elements of Judaism, Christianity, Islam, and Hinduism and only in 2005 was the practice of banishing a woman to the cow shed during her menses banned in Nepal (6). However, the impact of menstruation and menstrual disorders has in some respects increased rather than decreased despite emancipation, decreasing taboos, and improved access to sanitary ware. This is in part because of the increased frequency of menstruation, secondary to earlier menarche, decreased parity (in part as a result of access to contraception), and decreased lactational amenorrhoea. Our ancestors menstruated approximately 40 times (7), but in modern industrialized nations a women can expect to have approximately 450 periods in her lifetime (8). While decreased family size has significant obstetric benefits, the disadvantage is that menstrual disorders have become a relatively modern phenomenon for women and their clinicians. Assessment and evidence-based treatment of these common clinical complaints are underpinned by an understanding of menstrual physiology.