The menstrual cycle is under the control of the hormone system and is necessary for reproduction. It may be divided into several phases: menstruation, the follicular phase, and the luteal phase. Ovulation defines the transition from the follicular phase to the luteal phase. Counted from the first day of menstrual flow, the length of each phase varies from woman to woman and cycle to cycle. The average cycles length is 28 days. Hormonal contraception interferes with these normal hormonal changes with the aim of preventing reproduction.
Stimulated by gradually increasing amounts of estrogen in the follicular phase, the lining of the uterus thickens. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two become dominant (non-dominant follicles atrophy and die). The dominant follicle releases an ovum, or egg, in an event called ovulation. After ovulation, the remains of the dominant follicle in the ovary become a corpus luteum; this body has a primary function of producing large amounts of progesterone. Under the influence of progesterone, the endometrium (uterine lining) changes to prepare for potential implantation of an embryo to establish a pregnancy. If implantation does not occur within approximately two weeks, the corpus luteum will die, causing sharp drops in levels of both progesterone and estrogen. These hormone drops cause the uterus to shed its lining in a process termed menstruation.
In the menstrual cycle, changes occur in the female reproductive system as well as other systems (which lead to breast tenderness or mood changes, for example). A woman's first menstruation is termed menarche, and occurs typically somewhere around age 12. The end of the reproductive phase of a woman is called the menopause, which occurs somewhere between the ages of 45 and 55. Medical problems such as anemia and infertility can be due to disorders of the menstrual cycle.
The length of a woman's menstrual cycle will typically vary, with some shorter cycles and some longer cycles. A woman who experiences variations of less than eight days between her longest cycles and shortest cycles is considered to have regular menstrual cycles. It is unusual for a woman to experience cycle length variations of less than four days. Length variation between eight and 20 days is considered moderately irregular. Variation of 21 days or more between a woman's shortest and longest cycle lengths is considered very irregular (see cycle abnormalities).
|Name of phase|| Average start day|
assuming a 28-day cycle
|Average end day|
|follicular phase (also known as proliferative phase)||5||13|
|ovulation (not a phase, but an event dividing phases)||14||14|
|luteal phase (also known as secretory phase)||15||26|
|ischemic phase (some sources group this with secretory phase)||27||28|
Eumenorrhea denotes normal, regular menstruation that lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal). The menstrual fluid is largely a mixture of blood and tissue from the uterine lining (endometrium). The average blood loss during menstruation is 35 milliliters with 10–80 ml considered normal. (Because of this blood loss, women are more susceptible to iron deficiency than are men.) An enzyme called plasmin inhibits clotting in the menstrual fluid. Cramping in the abdomen, back, or upper thighs is common during the first few days of menstruation. When menstruation begins, symptoms of premenstrual syndrome (PMS) such as breast tenderness and irritability generally decrease. Many sanitary products are marketed to women for use during their menstruation.
Through the influence of a rise in follicle stimulating hormone (FSH) during the first days of the cycle, a few ovarian follicles are stimulated. These follicles, which were present at birth and have been developing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance. Under the influence of several hormones, all but one of these follicles will stop growing, while one dominant follicle in the ovary will continue to maturity. The follicle that reaches maturity is called a tertiary, or Graafian, follicle, and it forms the ovum.
As they mature, the follicles secrete increasing amounts of estradiol, an estrogen. The estrogens initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium. The estrogen also stimulates crypts in the cervix to produce fertile cervical mucus, which may be noticed by women practicing fertility awareness.
When the egg has nearly matured, the level of estradiol in the body has increased enough to trigger a sudden release of luteinizing hormone (LH) from the anterior pituitary gland. In the average cycle this LH surge starts around cycle day 12 and may last 48 hours. The release of LH matures the egg and weakens the wall of the follicle in the ovary, causing the fully developed follicle to release its secondary oocyte. The secondary oocyte promptly matures into an ootid and then becomes a mature ovum. The mature ovum has a diameter of about 0.2 mm.
Which of the two ovaries—left or right—ovulates appears essentially random; no known left/right co-ordination exists. Occasionally, both ovaries will release an egg; if both eggs are fertilized, the result is fraternal twins.
After being released from the ovary, the egg is swept into the fallopian tube by the fimbria, which is a fringe of tissue at the end of each fallopian tube. After about a day, an unfertilized egg will disintegrate or dissolve in the fallopian tube.
Fertilization by a spermatozoon, when it occurs, usually takes place well inside the fallopian tubes. An egg that is fertilized immediately begins the process of embryogenesis, or development. The developing embryo takes about three days to reach the uterus and another three days to implant into the endometrium. It has usually reached the blastocyst stage at the time of implantation.
In some women, ovulation features a characteristic pain called mittelschmerz (German term meaning middle pain). The sudden change in hormones at the time of ovulation sometimes also causes light mid-cycle blood flow.
After ovulation, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone and estrogens. The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. Consequently, the level of FSH and LH fall quickly over time, and the corpus luteum subsequently atrophies. Falling levels of progesterone trigger menstruation and the beginning of the next cycle. From the time of ovulation until progesterone withdrawal has caused menstruation to begin, this process typically takes about two weeks, with ten to sixteen days considered normal. For an individual woman, the follicular phase often varies in length from cycle to cycle; by contrast, the length of her luteal phase will be fairly consistent from cycle to cycle.
The loss of the corpus luteum can be prevented by fertilization of the egg; the resulting embryo produces human chorionic gonadotropin (hCG), which is very similar to LH and which can preserve the corpus luteum. Because the hormone is unique to the embryo, most pregnancy tests look for the presence of HCG.
Fertility awareness methods that rely on cycle length records alone are called calendar-based methods. Methods that require observation of one or more of the three primary fertility signs (basal body temperature, cervical mucus, and cervical position) are known as symptoms-based methods. Urine test kits are available that detect the LH surge that occurs 24 to 36 hours before ovulation; these are known as ovulation predictor kits (OPKs). Computerized devices that interpret basal body temperatures, urinary test results, or changes in saliva are called fertility monitors.
A woman's fertility is also affected by her age. As a woman's total egg supply is formed in fetal life, to be ovulated decades later, it has been suggested that this long lifetime may make the chromatin of eggs more vulnerable to division problems, breakage, and mutation than the chromatin of sperm, which are produced continuously during a man's reproductive life.
Mice have been used as an experimental system to investigate possible mechanisms by which levels of sex steroid hormones might regulate nervous system function. During the part of the mouse estrous cycle when progesterone is highest, the level of nerve-cell GABA receptor subtype delta was high. Since these GABA receptors are inhibitory, nerve cells with more delta receptors are less likely to fire than cells with lower numbers of delta receptors. During the part of the mouse estrous cycle when estrogen levels are higher than progesterone levels, the number of delta receptors decrease, increasing nerve cell activity, in turn increasing anxiety and seizure susceptibility.
Among women living closely together, the onsets of menstruation may tend to synchronise somewhat. This McClintock effect was first described in 1971, and possibly explained by the action of pheromones in 1998. However, subsequent research has called this hypothesis into question.
Sudden heavy flows or amounts in excess of 80 ml are termed menorrhagia. Very little flow (less than 10ml) is called hypomenorrhea. Cycles with intervals of 21 days or fewer are polymenorrhea; frequent but irregular menstruation is known as metrorrhagia. The term for cycles with intervals exceeding 35 days is oligomenorrhea. Amenorrhea refers to a prolonged absence of menses (180 days or longer) during the reproductive years of a woman. For example, women with very low body fat, such as athletes, may cease to menstruate. Amenorrhea also occurs during pregnancy.
While most forms of birth control do not affect the menstrual cycle, hormonal contraceptives work by disrupting it. Progestogen negative feedback decreases the pulse frequency of gonadotropin-releasing hormone (GnRH) release by the hypothalamus, which decreases the release of follicle-stimulating hormone (FSH) and greatly decreases the release of luteinizing hormone (LH) by the anterior pituitary. Decreased levels of FSH inhibit follicular development, preventing an increase in estradiol levels. Progestogen negative feedback and the lack of estrogen positive feedback on LH release prevent a mid-cycle LH surge. Inhibition of follicular development and the absence of a LH surge prevent ovulation.
The degree of ovulation suppression in progestogen-only contraceptives depends on the progestogen activity and dose. Low dose progestogen-only contraceptives—traditional progestogen only pills, subdermal implants Norplant and Jadelle, and intrauterine system Mirena—inhibit ovulation in ~50% of cycles and rely mainly on other effects, such as thickening of cervical mucus, for their contraceptive effectiveness. Intermediate dose progestogen-only contraceptives—the progestogen-only pill Cerazette and the subdermal implant Implanon—allow some follicular development but more consistently inhibit ovulation in 97–99% of cycles. The same cervical mucus changes occur as with very low dose progestogens. High dose progestogen-only contraceptives—the injectables Depo-Provera and Noristerat—completely inhibit follicular development and ovulation.
Combined hormonal contraceptives include both an estrogen and a progestogen. Estrogen negative feedback on the anterior pituitary greatly decreases the release of FSH, which makes combined hormonal contraceptives more effective at inhibiting follicular development and preventing ovulation. Estrogen also reduces the incidence of irregular breakthrough bleeding. Several combined hormonal contraceptives—the pill, NuvaRing, and the contraceptive patch—are usually used in a way that causes regular withdrawal bleeding. In a normal cycle, menstruation occurs when estrogen and progesterone levels drop rapidly. Discontinuing use of combined oral contraceptives (a placebo week) has the same effect of causing the uterine lining to shed. If withdrawal bleeding is not desired, combined oral contraceptives may be taken continuously, although this increases the risk of breakthrough bleeding.
Some authors believe women in traditional societies without nightlighting ovulated with the full moon and menstruated with the new moon. A few studies in both humans and animals have found that artificial light at night does influence the menstrual cycle in humans and the estrus cycle in mice (cycles are more regular in the absence of artificial light at night), though none have demonstrated the synchronization of women's menstrual cycles with the lunar cycle. One author has suggested that sensitivity of women's cycles to nightlighting is caused by nutritional deficiencies of certain vitamins and minerals.
Other animals' menstrual cycles may be greatly different from lunar cycles: while the average cycle length in orangutans is the same as in humans—28 days—the average for chimpanzees is 35 days. Some take this as evidence that the average length of humans' cycle is most likely a coincidence.
A few mammals do not experience obvious, visible signs of fertility (concealed ovulation). In humans, while women can be taught to recognize their own level of fertility (fertility awareness), whether men can detect fertility in women is debated; recent studies have given conflicting results. Orangutans also lack visible signs of impending ovulation. Also, it has been said that the extended estrus period of the bonobo (reproductive-age females are in heat for 75% of their menstrual cycle) has a similar effect to the lack of a "heat" in human females.