Menstrual cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that makes pregnancy possible.
Triggered by falling progesterone levels, menstruation (a "period", in common parlance) is the cyclical shedding of the lining, and is a sign that pregnancy has not occurred.
The menstrual cycle can cause some women to experience premenstrual syndrome with symptoms that may include tender breasts, and tiredness.
After puberty, GnRH is released in large pulses, and the frequency and magnitude of these determine how much FSH and LH are produced by the pituitary.
[7][8] Between menarche and menopause the ovaries regularly alternate between luteal and follicular phases during the monthly menstrual cycle.
[9] Stimulated by gradually increasing amounts of estrogen in the follicular phase, discharges of blood flow stop and the uterine lining thickens.
About mid-cycle, some 10–12 hours after the increase in luteinizing hormone, known as the LH surge,[4] the dominant follicle releases an oocyte, in an event called ovulation.
[10] After ovulation, the oocyte lives for 24 hours or less without fertilization,[11] while the remains of the dominant follicle in the ovary become a corpus luteum – a body with the primary function of producing large amounts of the hormone progesterone.
[12][a] Under the influence of progesterone, the uterine lining changes to prepare for potential implantation of an embryo to establish a pregnancy.
At the same time the granulosa cells surrounding the maturing follicle develop receptors that bind FSH, and in response start secreting androstenedione, which is converted to estrogen by the enzyme aromatase.
This surge of FSH and LH usually occurs one to two days before ovulation and is responsible for stimulating the rupture of the antral follicle and release of the oocyte.
[24] Called ovulation, this occurs when a mature egg is released from the ovarian follicles into the pelvic cavity and enters the fallopian tube, about 10–12 hours after the peak in LH surge.
During the first few months of pregnancy, the corpus luteum continues to secrete progesterone and estrogens at slightly higher levels than those at ovulation.
During the luteal phase, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone.
[41] Menstruation (also called menstrual bleeding, menses or a period) is the first and most evident phase of the uterine cycle and first occurs at puberty.
[44][45] Menstruation is initiated each month by falling levels of estrogen and progesterone and the release of prostaglandins,[20] which constrict the spiral arteries.
[48] The flow of blood normally serves as a sign that a woman has not become pregnant, but this cannot be taken as certainty, as several factors can cause bleeding during pregnancy.
[24] Women who experience menorrhagia (heavy menstrual bleeding) are more susceptible to iron deficiency than the average person.
[2] As estrogen levels increase, cells in the cervix produce a type of cervical mucus[53] that has a higher pH and is less viscous than usual, rendering it more friendly to sperm.
During the secretory phase, the corpus luteum produces progesterone, which plays a vital role in making the endometrium receptive to the implantation of a blastocyst (a fertilized egg, which has begun to grow).
[58] In early pregnancy, progesterone also increases blood flow and reduces the contractility of the smooth muscle in the uterus[22] and raises basal body temperature.
[15] The other third lack ovulation or have a short luteal phase (less than ten days[60]) in which progesterone production is insufficient for normal physiology and fertility.
[64] Anovulatory cycles or short luteal phases are normal when women are under stress or athletes increasing the intensity of training.
[60] Although a normal and natural process,[65] some women experience premenstrual syndrome with symptoms that may include acne, tender breasts, and tiredness.
[72] These issues can significantly affect a woman's health and quality of life and timely interventions can improve the lives of these women.
[73] There are common culturally communicated misbeliefs that the menstrual cycle affects women's moods, causes depression or irritability, or that menstruation is a painful, shameful or unclean experience.
Much of the research is weak, but there appears to be a very small increase in mood fluctuations during the luteal and menstrual phases, and a corresponding decrease during the rest of the cycle.
[77] Improvements in spatial reasoning ability during the menstruation phase of the cycle are probably caused by decreases in levels of estrogen and progesterone.
[20] Even when normal, the changes in hormone levels during the menstrual cycle can increase the incidence of disorders such as autoimmune diseases,[81] which might be caused by estrogen enhancement of the immune system.
[84] Sequential dosing and discontinuation of the COCP can mimic the uterine cycle and produce bleeding that resembles a period.
