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Encyclopedia > Menstrual cycle
Menstrual cycle
Menstrual cycle

In the female reproductive system, the menstrual cycle is a recurring cycle of physiologic changes that occurs in reproductive-age females. Overt menstruation (where there is blood-flow from the vagina) occurs primarily in humans and close evolutionary relatives such as chimpanzees.[1] The females of other species of placental mammal have estrous cycles, in which the endometrium is completely reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. This article concentrates on the menstrual cycle as it occurs in human beings. Image File history File links Size of this preview: 424 × 600 pixelsFull resolution (794 × 1123 pixel, file size: 92 KB, MIME type: image/png) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Menstruation Menstrual cycle ... Image File history File links Size of this preview: 424 × 600 pixelsFull resolution (794 × 1123 pixel, file size: 92 KB, MIME type: image/png) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Menstruation Menstrual cycle ... The human females reproductive system. ... This article or section does not cite any references or sources. ... For other uses, see Female (disambiguation). ... The vagina, (from Latin, literally sheath or scabbard ) is the tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. ... Type species Simia troglodytes Blumenbach, 1775 distribution of Species Pan troglodytes Pan paniscus Chimpanzee, often shortened to chimp, is the common name for the two extant species of apes in the genus Pan. ... Orders[1] Bobolestes Eomaia Maelestes Montanalestes Murtoilestes Prokennalestes Placentalia Superorder Xenarthra: Cingulata (Armadillos) Pilosa (Sloths, True Anteaters) Superorder Afrotheria: Afrosoricida (Tenrecs, etc. ... The estrous cycle (also oestrous cycle; originally derived from Latin oestrus) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females. ...


The menstrual cycle is under the control of the hormone system and is necessary for reproduction. Menstrual cycles are counted from the first day of menstrual flow, because the onset of menstruation corresponds closely with the hormonal cycle. The menstrual cycle may be divided into several phases, and the length of each phase varies from woman to woman and cycle to cycle. Average values are shown below: For other uses, see Hormone (disambiguation). ... For other uses, see Reproduction (disambiguation) Reproduction is the biological process by which new individual organisms are produced. ...

Name of phase Days
menstrual phase 1–4
follicular phase (also known as proliferative phase) 5–13
ovulation (not a phase, but an event dividing phases) 14
luteal phase (also known as secretory phase) 15–26
ischemic phase (some sources group this with secretory phase) 27–28

During the follicular phase the lining of the uterus thickens, stimulated by gradually increasing amounts of estrogen. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two follicles become dominant (non-dominant follicles atrophy and die). The dominant follicle releases an ovum or egg in an event called ovulation. (An egg that is fertilized by a spermatozoon will become a zygote, taking one to two weeks to travel down the fallopian tubes to the uterus. If the egg is not fertilized within about a day of ovulation, it will die and be absorbed by the woman's body.[2]) 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. The endometrium is the inner membrane of the mammalian uterus. ... This article is about female reproductive anatomy. ... Estriol. ... // For ovary as part of plants see ovary (plants) An ovary is an egg-producing reproductive organ found in female organisms. ... A human ovum Sperm cells attempting to fertilize an ovum An ovum (plural ova) is a haploid female reproductive cell or gamete. ... Ovulation is the process in the menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or casually, an egg) that participates in reproduction. ... A spermatozoon or spermatozoan ( spermatozoa), from the ancient Greek σπέρμα (seed) and (living being) and more commonly known as a sperm cell, is the haploid cell that is the male gamete. ... This article is about fertilisation in animals and plants. ... The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges (singular salpinx) are two very fine tubes leading from the ovaries of female mammals into the uterus. ... The corpus luteum (Latin for yellow body) is a small, temporary endocrine structure in animals. ... Progesterone is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy (supports gestation) and embryogenesis of humans and other species. ... Implantation is a phenomenon in prenatal development, i. ... This article is about human pregnancy in biological females. ...

Contents

Terminology

A woman's first menstruation is termed menarche, and is one of the later stages of puberty in girls. The average age of menarche in humans is 12 years, but is normal anywhere between ages 8 and 16. Factors such as heredity, diet and overall health can accelerate or delay menarche.[3] The cessation of menstrual cycles at the end of a woman's reproductive life is termed menopause. The average age of menopause in women is 51 years, with anywhere between 40 and 58 being common. Menopause before age 35 is considered premature. The age of menopause is largely a result of genetics; however, illnesses, certain surgeries, or medical treatments may cause menopause to occur earlier.[4] Menarche (IPA: ) is the first menstrual period, or first menstrual bleeding in the females of human beings. ... Puberty refers to the process of physical changes by which a childs body becomes an adult body capable of reproduction. ... The word menopause literally means the permanent physiological, or natural, cessation of menstrual cycles, from the Greek roots meno (month) and pausis (a pause, a cessation). ...


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).[5]


Phases of the menstrual cycle

Menstruation

Main article: Menstruation

Menstruation is also called menstrual bleeding, menses, a period or catamenia. The flow of menses normally serves as a sign that a woman has not become pregnant. (However, this cannot be taken as certainty, as sometimes there is some flow of blood in early pregnancy.) During the reproductive years, failure to menstruate may provide the first indication to a woman that she may have become pregnant. Not to be confused with Mensuration. ... This article is about human pregnancy in biological females. ... Diverse women. ...


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).[6] The average blood loss during menstruation is 35 millilitres with 10–80 ml considered normal;[7] many women also notice shedding of the endometrium lining that appears as tissue mixed with the blood. An enzyme called plasmin — contained in the endometrium — tends to inhibit the blood from clotting. Because of this blood loss, women have higher dietary requirements for iron than do males to prevent iron deficiency. Many women experience uterine cramps during this time (severe cramps or other symptoms are called dysmenorrhea), as well as other premenstrual syndrome symptoms. A vast industry of sanitary products has grown to help women during their menstruation. The endometrium is the inner membrane of the mammalian uterus. ... Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... Plasmin is an important degrading enzyme (EC 3. ... Coagulation is the thickening or congealing of any liquid into solid clots. ... General Name, symbol, number iron, Fe, 26 Chemical series transition metals Group, period, block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ... For a more specific and detailed discussion of anemia caused by iron deficiency, see the Wikipedia article iron deficiency anemia. ... This article is about female reproductive anatomy. ... Dysmenorrhea (or dysmenorrhoea) is a medical condition characterized by severe uterine pain during menstruation. ... PMS links to this article. ... Not to be confused with Mensuration. ...


Follicular phase

Main article: Follicular phase

Through the influence of a rise in follicle stimulating hormone (FSH), five to seven tertiary-stage ovarian follicles are recruited for entry into the next menstrual cycle. These follicles, that have been growing 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 undergo atresia, while one (or occasionally two) dominant follicles will continue to maturity. As they mature, the follicles secrete increasing amounts of estradiol, an estrogen. Menstrual cycle The follicular phase (or proliferative phase) is the phase of the estrous cycle, (or, in humans and great apes, the menstrual cycle) during which follicles in the ovary mature. ... Follicle stimulating hormone (FSH) is a hormone produced by gonadotropes in the anterior pituitary gland. ... Ovarian follicles or Graafian follicles (after Regnier de Graaf) are the roughly spherical cell aggregations in the ovary containing an ovum and from which the egg is released during ovulation. ... In biology, folliculogenesis refers to the maturation of the ovarian follicle, a densely-packed shell of somatic cells that contains an immature oocyte. ... Estradiol (17β-estradiol) (also oestradiol) is a sex hormone. ... Estriol. ...


The estrogens that follicles secrete 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. Estriol. ... The endometrium is the inner membrane of the mammalian uterus. ... Crypts are anatomical structures that are narrow but deep invaginations into a larger structure. ... The cervix (from Latin neck) is the lower, narrow portion of the uterus where it joins with the top end of the vagina. ... Fertility Awareness (FA) is the practice of observing one or more of a woman’s primary fertility signs to determine the fertile and infertile phases of her menstrual cycle. ...


Ovulation

Main article: Ovulation
An ovary about to release an egg.
An ovary about to release an egg.

When the egg has matured, it secretes enough estradiol to trigger the acute release of luteinizing hormone (LH). 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. This process leads to ovulation: the release of the now mature ovum, the largest cell of the body (with a diameter of about 0.5 mm). Which of the two ovaries — left or right — ovulates appears essentially random; no known left/right co-ordination exists. The egg is swept into the fallopian tube by the fimbria - a fringe of tissue at the end of each fallopian tube. If fertilization occurs, it will happen in the fallopian tube. Ovulation is the process in the menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or casually, an egg) that participates in reproduction. ... Download high resolution version (688x973, 276 KB)Impending Ovulation This is an ovary shortly before an egg is released. ... Download high resolution version (688x973, 276 KB)Impending Ovulation This is an ovary shortly before an egg is released. ... Luteinizing hormone (LH) is a hormone synthesized and secreted by gonadotropes in the anterior lobe of the pituitary gland. ... Ovulation is the process in the menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or casually, an egg) that participates in reproduction. ... A human ovum Sperm cells attempting to fertilize an ovum An ovum (plural ova) is a haploid female reproductive cell or gamete. ... The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges (singular salpinx) are two very fine tubes leading from the ovaries of female mammals into the uterus. ... Uterus and uterine tubes In the female reproductive system, the fimbria is a fringe of tissue near the ovary leading to the Fallopian tube. ...


In some women, ovulation features a characteristic pain called mittelschmerz (German term meaning 'middle pain') which may last a few hours. The sudden change in hormones at the time of ovulation also causes light mid-cycle blood flow from the vagina of some women. An unfertilized egg will eventually disintegrate or dissolve. Mittelschmerz (German: middle pain) is a medical term for ovulation pain. Some women have mittelschmerz regularly and can time their ovulation by it, but many never experience it. ...


Luteal phase

Main article: Luteal phase

The corpus luteum is the solid body formed in the ovaries after the egg has been released into the fallopian tube which continues to grow and divide for a while. After ovulation, the residual follicle transforms into the corpus luteum under the support of the pituitary hormones. This corpus luteum will produce progesterone in addition to estrogens for approximately the next 2 weeks. Progesterone plays a vital role in converting the proliferative endometrium into a secretory lining receptive for implantation and supportive of the early pregnancy. It raises the body temperature by 0.25 °C to 0.5 °C (0.5 °F to 1.0 °F), thus women who record their basal body temperature on a daily basis will notice that they have entered the luteal phase. If fertilization of an egg has occurred, it will travel as an early blastocyst through the fallopian tube to the uterine cavity and implant itself 6 to 12 days after ovulation. Shortly after implantation, the growing embryo will signal its existence to the maternal system. One very early signal consists of human chorionic gonadotropin (hCG), a hormone that pregnancy tests can measure. This signal has an important role in maintaining the corpus luteum and enabling it to continue to produce progesterone. In the absence of a pregnancy and without hCG, the corpus luteum demises and inhibin and progesterone levels fall. This will set the stage for the next cycle. Progesterone withdrawal leads to menstrual shedding (progesterone withdrawal bleeding), and falling inhibin levels allow FSH levels to rise to raise a new crop of follicles. The luteal phase is the latter phase of the estrous cycle in animals. ... The corpus luteum (Latin for yellow body) is a small, temporary endocrine structure in animals. ... Progesterone is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy (supports gestation) and embryogenesis of humans and other species. ... Implantation is a phenomenon in prenatal development, i. ... This article is about human pregnancy in biological females. ... Basal body temperature is the body temperature measured immediately after awakening and before any physical activity has been undertaken. ... The blastocyst is an early stage of the human (or any other mammal) development early in pregnancy. ... Human chorionic gonadotropin (hCG) is a peptide hormone produced in pregnancy, that is made by the embryo soon after conception and later by the syncytiotrophoblast (part of the placenta). ... A modern pregnancy test A pregnancy test is a test to determine whether or not a woman is pregnant. ... Inhibin is a peptide that is an inhibitor of FSH synthesis and secretion and participates in the regulation of the menstrual cycle. ... Progestagens (also spelled progestogens or gestagens) are hormones similar in effect to progesterone, the only natural progestagen. ...


The fertile window

The length of the follicular phase — and consequently the length of the menstrual cycle — may vary widely. The luteal phase, however, almost always takes the same number of days for each woman: Some women have a luteal phase of 10 days, others 16 days, while the average is 14 days. Normal sperm life inside a woman ranges from 1-5 days, though a pregnancy resulting from sperm life of 8 days has been documented.[8][9][10] The most fertile period (the time with the highest likelihood of pregnancy resulting from sexual intercourse) covers the time from some 5 days before ovulation until 1–2 days after ovulation. In an average 28 day cycle with a 14-day luteal phase, this corresponds to the second and the beginning of the third week of the cycle. Fertility awareness methods of birth control attempt to determine the precise time of ovulation in order to find the relatively fertile and the relatively infertile days in the cycle. A spermatozoon or spermatozoan ( spermatozoa), from the ancient Greek σπέρμα (seed) and (living being) and more commonly known as a sperm cell, is the haploid cell that is the male gamete. ... It has been suggested that Duration of sexual intercourse be merged into this article or section. ... Fertility Awareness (FA) is the practice of observing one or more of a woman’s primary fertility signs to determine the fertile and infertile phases of her menstrual cycle. ... For other uses, see Birth control (disambiguation). ...


People who have heard about the menstrual cycle and ovulation often mistakenly assume, for contraceptive purposes, that menstrual cycles regularly take 28 days, and that ovulation always occurs 14 days after beginning of the menses. This assumption may lead to unintended pregnancies. Note too that not every event of blood flow counts as a menstruation, and this can mislead people in their calculation of the fertile window. Birth control is the practice of preventing or reducing the probability of pregnancy without abstaining from sexual intercourse; the term is also sometimes used to include abortion, the ending of an unwanted pregnancy, or abstinence. ...


If a woman wants to conceive, the most fertile time occurs between 19 and 10 days prior to the expected menses. Many women use ovulation detection kits that detect the presence of the LH surge in the urine to indicate the most fertile time. Other ovulation detection systems rely on observation of one or more of the three primary fertility signs (basal body temperature, cervical fluid, and cervical position). Basal body temperature is the body temperature measured immediately after awakening and before any physical activity has been undertaken. ...


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.[11] However, subsequent research has called this conclusion into question.[12] The McClintock effect (also known as Menstrual Synchrony) is the observed phenomenon that the menstrual cycles of women who live together (such as in prisons, convents, bordellos, dormitories, etc. ... Fanning honeybee exposes Nasonov gland (white-at tip of abdomen) releasing pheromone to entice swarm into an empty hive A pheromone (from Greek φέρω phero to bear + ‘ορμόνη hormone) is a chemical that triggers a natural behavioral response in another member of the same species. ...


Hormonal control

Extreme intricacies regulate the menstrual cycle. For many years, researchers have argued over which regulatory system has ultimate control: the hypothalamus, the pituitary, or the ovary with its growing follicle; but all three systems have to interact. In any scenario, the growing follicle has a critical role: it matures the lining, provides the appropriate feedback to the hypothalamus and pituitary, and modifies the mucus changes at the cervix.


Gonadal

Two sex hormones play a role in the control of the menstrual cycle: estrogen and progesterone: Sex hormones are hormones that affect the reproductive system. ... Estriol. ... Progesterone is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy (supports gestation) and embryogenesis of humans and other species. ...

  • Estradiol peaks twice, during follicular growth and during the luteal phase.
  • Progesterone remains virtually absent prior to ovulation, but becomes critical in the luteal phase and during pregnancy. Many tests for ovulation check for the presence of progesterone.

After ovulation the corpus luteum — which develops from the burst follicle and remains in the ovary — secretes both estradiol and progesterone. Only if pregnancy occurs do hormones appear in order to suspend the menstrual cycle, while production of estradiol and progesterone continues. Abnormal hormonal regulation leads to disturbance in the menstrual cycle. The corpus luteum (Latin for yellow body) is a small, temporary endocrine structure in animals. ...


Hypothalamus and pituitary

These sex hormones come under the influence of the pituitary gland, and both FSH and LH play necessary roles: | Latin = hypophysis, glandula pituitaria | GraySubject = 275 | GrayPage = 1275 | Image = Gray1180. ...

  • FSH stimulates immature follicles in the ovaries to grow.
  • LH triggers ovulation.

The gonadotropin-releasing hormone of the hypothalamus controls the pituitary, yet both the pituitary and the hypothalamus receive feedback from the follicle. Ovarian follicles or Graafian follicles (after Regnier de Graaf) are the roughly spherical cell aggregations in the ovary containing an ovum and from which the egg is released during ovulation. ... Gonadotropin-releasing hormone 1 (GNRH1 also called LHRH) is a peptide hormone responsible for the release of FSH and LH from the anterior pituitary. ... The hypothalamus links the nervous system to the endocrine system via the pituitary gland (hypophysis). ...


Cyclic effects upon nervous system

Some women with neurological conditions experience increased activity of their conditions at about the same time every month. 80 percent of women with epilepsy have more seizures than usual in the phase of their cycle when progesterone declines and estrogen increases. This is called "catamenial epilepsy". Neurology is a branch of medicine dealing with disorders of the nervous system. ...


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.[13] This article is about cells in the nervous system. ... The GABAA receptor is one of the three ligand-gated ion channels responsible for mediating the effects of Gamma-AminoButyric Acid (GABA), the major inhibitory neurotransmitter in the brain. ... Inhibitory Postsynaptic Potential is commonly abbreviated to Impulses are transmitted from neuron to neuron by the release of a chemical transmitter across synaptic clefts from the synaptic vesicles along the axon to the postsynaptic receptors of another neuron. ...


Hidden ovulation

Main article: Concealed ovulation

Unlike almost all other species, the external physical changes of a human female near ovulation are very subtle. In contrast, other species often signal receptivity through heat, swellings, and/or changes in color in the genital area. Humans are the only mammal to lack obvious, visible manifestations of ovulation, although some argue that the extended estrus period of the bonobo (reproductive-age females are in heat for 75% of their menstrual cycle)[14] has a similar effect to the lack of a "heat" in human females.[15] Human and bonobo females have concealed ovulation or hidden estrus. ... Estrus (also spelled œstrus) or heat in female mammals is the period of greatest female sexual responsiveness usually coinciding with ovulation. ... For other uses, see Bonobo (disambiguation). ...


While women can be taught to recognize their own level of fertility (fertility awareness), whether men can detect fertility in women is highly debated. On one hand, several small studies have found that fertile women (compared to women in infertile portions of the menstrual cycle or on hormonal contraception) appear more attractive.[16][17] On the other hand, two small studies of monogamous couples found that women initiated sex significantly more frequently when fertile, but male-initiated sex occurred at a constant rate in all phases of the menstrual cycle.[18] Fertility Awareness (FA) is the practice of observing one or more of a woman’s primary fertility signs to determine the fertile and infertile phases of her menstrual cycle. ...


The ovary as an egg-bank

Evidence suggests that eggs are formed from germ cells early in fetal life. The number is reduced to an estimated 400,000 to 450,000 immature ova residing in each ovary at puberty. The menstrual cycle, as a biologic event, allows for ovulation of one egg typically each month. Thus over her reproductive lifetime a woman will ovulate approximately 400 to 450 times. All the other eggs dissolve by a process called atresia. As a woman's total egg supply is formed in fetal life,[19] 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. Gametes (in Greek: γαμέτες) —also known as sex cells, germ cells, or spores—are the specialized cells that come together during fertilization (conception) in organisms that reproduce sexually. ... Atresia is a condition in which a body orifice or passage in the body is abnormally closed or absent. ... Chromatin is the complex of DNA and protein found inside the nuclei of eukaryotic cells. ...


Cycle abnormalities

Main article: Menstrual disorder

A menstrual disorder is an irregular condition in a womans menstrual cycle. ...

Anovulation

Apparently normal menstrual flow can occur without ovulation preceding it (anovulatory cycle - "an-" meaning "absence of" +ovulation). In some women, follicular development may start but not be completed; nevertheless, estrogens will form and will stimulate the uterine lining. Anovulatory flow resulting from a very thick endometrium caused by prolonged, continued high estrogen levels is called estrogen breakthrough bleeding. Anovulatory bleeding triggered by a sudden drop in estrogen levels is called estrogen withdrawal bleeding.[20] The anovulatory cycle is a menstrual cycle characterized by varying degrees of menstrual intervals and the absence of ovulation and a luteal phase. ...


Anovulatory bleeding may occur on a regular basis, but more commonly happens with irregular frequency. Anovulatory flow commonly occurs prior to menopause (premenopause) or in women with polycystic ovary syndrome. The word menopause literally means the permanent physiological, or natural, cessation of menstrual cycles, from the Greek roots meno (month) and pausis (a pause, a cessation). ... Polycystic Ovary Syndrome (abbreviated PCOS or POS), also known clinically as Stein-Leventhal syndrome, is an endocrine disorder that affects approximately 10% of all women . ...


Infrequent or irregular ovulation is called oligoovulation.


Flow

Sudden heavy flows or amounts in excess of 80 ml (hypermenorrhea or menorrhagia) are not normal. Menorrhagia is an abnormally heavy and prolonged menstrual period at regular intervals. ...


Very little flow (less than 10ml) is called hypomenorrhea.


Prolonged flow (metrorrhagia, also meno-metrorrhagia) no longer shows a clear interval pattern. Dysfunctional uterine bleeding refers to hormonally caused flow abnormalities, typically anovulation. Dysfunctional Uterine Bleeding (DUB) is the most common cause of functional abnormal uterine bleeding, which is abnormal genital tract bleeding based in the uterus and found in the absence of demonstrable organic pathology. ...


All bleeding abnormalities need medical attention; they may indicate hormone imbalances, uterine fibroids, or other problems. As pregnant patients may bleed, a pregnancy test forms part of the evaluation of abnormal flow. A modern pregnancy test A pregnancy test is a test to determine whether or not a woman is pregnant. ...


Cycle length

The medical term for cycles with intervals of 21 days or fewer is polymenorrhea and, on the other hand, the term for cycles with intervals exceeding 35 days is oligomenorrhea (or amenorrhea if intervals exceed 180 days). Polymenorrhea ... Oligomenorrhea is the medical term for infrequent or light menstrual periods in women of child-bearing age. ... Amenorrhoea (BE) or amenorrhea (AmE) is the absence of a menstrual period in a woman of reproductive age. ...


Amenorrhea refers to a prolonged absence of menses 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. Amenorrhoea (BE) or amenorrhea (AmE) is the absence of a menstrual period in a woman of reproductive age. ... In biochemistry, fat is a generic term for a class of lipids. ... A sportsperson (British and American English) or athlete (principally American English) is any person who participates regularly in a sport. ...


Early menarche

The condition precocious puberty has caused menstruation to occur in girls as young as eight months old.[21] Precocious puberty means early puberty. ...


Contraception and menstruation

Hormonal options

Estrogens and progesterone-like hormones make up the main active ingredients of hormonal birth control methods such as the pill. Typically they cause regular monthly flow that roughly mimics a menstrual cycle in appearance, but suppresses ovulation. With most pills, a woman takes hormone pills for 21 days, followed by 7 days of non-functional placebo pills or no pills at all, then the cycle starts again. During the 7 placebo days, a withdrawal bleeding occurs; this differs from ordinary menstruation, and skipping the placebos and continuing with the next batch of hormone pills may suppress it. (There are two main versions of the pill: monophasic and triphasic. With triphasic pills, skipping placebos and continuing with the next month's dose can make a woman more likely to experience spotting or breakthrough bleeding.) In 2003, the U.S. Food and Drug Administration (FDA) approved low-dose monophasic birth control pills that induce withdrawal bleeding every 3 months. Yet another version of the pill is the Loestrin Fe, which has only a four-day placebo "week" (the placebos are actually iron supplements intended to replenish iron lost by uterine shedding); the other three placebos are replaced with active hormone pills. This system is intended to help shorten periods. Mircette contains several days of estrogen-only pills in addition to the usual combination estrogen/progestin pills, in the case of women who may have problems with low estrogen during the placebo days with other pills. Hormonal contraception refers to birth control methods that act on the hormonal system. ... The Pill redirects here. ... Breakthrough bleeding is bleeding between regular menstrual cycles. ... FDA redirects here. ... Oral contraceptives come in a variety of formulations. ...


Other types of hormonal birth control which affect menstruation include the vaginal Nuvaring and the transdermal patch (like the standard pill pack, active hormones are given for three weeks, followed by a one-week break to allow blood flow) and the injection (which can eliminate all flow as long as the injections are taken every twelve weeks, although spotting is a common side effect). - This is a copy of manufacturers copyrighted patient information leaflet, rather than an encylopedic entry - please edit. ... A contraceptive patch is a transdermal patch applied to the skin that releases synthetic estrogen and progestin hormones to prevent pregnancy. ... Depot medroxyprogesterone acetate (DMPA) is a progestogen-only hormonal contraceptive birth control drug which is injected every 3 months. ...


Effects on menstruation

All such methods are designed to regulate the monthly blood flow. Because of this, they are often chosen by females who wish to regulate the frequency and length of their period, often for basic convenience and especially when such factors are irregular and problematic on their own. Hormonal contraception has also been shown to improve menstrual symptoms such as cramping, heavy flow, and other bothersome physical and emotional issues related to periods.


Control and flexibility

Hormonal methods which are controlled by the user day-to-day, including pills, the ring, and the patch, need not always be used according to the standard cycle/calendar. Their use can be rescheduled and altered in various ways to postpone or skip periods when desired for reasons of convenience (e.g., traveling or scheduled gynecological exams), personal enjoyment (such as expected sexual encounters or events like a wedding or dance), or health (including very painful periods or sensitivity to hormone fluctuations). Similarly, abrupt cessation of use can induce a breakthrough period mid-cycle.


Other contraceptive methods

Most IUDs are not designed to affect menstruation or breakthrough bleeding, but may exacerbate cramps or the heaviness of the flow due to their placement within the uterus. The Mirena IUD releases a small continuous dosage of a progesterone-like hormone, which can sometimes cause menstruation to cease. Tubal sterilization alone will not affect menstruation, though the ablation option often performed at the same time will cause menstruation to cease. Hysterectomy will, of course, completely stop menstruation as it entails the removal of the uterus (and sometimes the ovaries, fallopian tubes, and cervix). Fertility awareness methods do not affect the period in and of themselves, but involve careful observation of various kinds, of which the timing of the period is an essential factor. An intrauterine device (intra meaning within, and uterine meaning of the uterus) is a birth control device also known as an IUD or a coil( this colloquialism is based on the coil-shaped design of early IUDs). ... Tubal ligation (informally known as getting ones tubes tied) is a permanent form of female sterilization, in which the fallopian tubes are severed and sealed or pinched shut, in order to prevent fertilization. ... Endometrial ablation is a medical procedure that is used to remove (ablate) or destroy the endometrial lining of a womans uterus. ... This article or section is in need of attention from an expert on the subject. ... This article is about female reproductive anatomy. ... Fertility Awareness (FA) is the practice of observing one or more of a woman’s primary fertility signs to determine the fertile and infertile phases of her menstrual cycle. ...


Menstruation and the moon

See also: Culture and menstruation

Traditional sources agree that the menstrual cycle is linked to the cycle of the moon.[citation needed] These sources generally indicate that women menstruate at the time of the new moon, and ovulate at the full moon. Although scientific evidence for this has been weak, the problem may be that most women today live in urban environments where the moon is no longer a significant contributor to nocturnal light. The fact that women who work on night shifts, where they are exposed to strong light at night, often experience menstrual irregularities, is just one example of how rhythms of light and darkness do influence hormonal physiology, including the menstrual cycle.[22] The relationship between culture and menstruation is expressed in many ways. ...


The word "menstruation" is etymologically related to moon. The terms "menstruation" and "menses" come from the Latin mensis (month), which in turn relates to the Greek mene (moon) and to the roots of the English words month and moon — reflecting the fact that the moon also takes close to 28 days to revolve around the Earth (actually 27.32 days). The synodical lunar month, the period between two new moons (or full moons), is 29.53 days long. For other uses, see Latins and Latin (disambiguation). ... This article is about Earths moon. ... In lunar calendars, a lunar month is the time between two successive similar syzygies (new moons or full moons). ...


A 1975 book by Louise Lacey documented the experience of herself and 27 of her friends, who found that when they removed all artificial night lighting their menstrual cycles began to occur in rhythm with the lunar cycle. She dubbed the technique Lunaception.[23] Later studies in both humans[24] and animals[25] 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 duplicated 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.[26] Lunaception is a form of birth control that depends upon the woman practicing it to align her menstrual cycle with the phases of the moon. ...


Some have suggested that the fact that other animals' menstrual cycles appear to be greatly different from lunar cycles is evidence that the average length of humans' cycle is most likely a coincidence.[27][28]


Menstrual cycles in other mammals

Females of most species advertise ovulation to males with visual cues and behavioral cues, pheromones, or both (humans are a species that does not). This period of advertised fertility is known as estrus or heat. However, in animals with menstrual cycles, females can be sexually active at any time in their cycle, even when they are not in heat. Great apes' cycles vary in length from an average of 29 days in orangutans to an average of 37 days in chimpanzees. Fanning honeybee exposes Nasonov gland (white-at tip of abdomen) releasing pheromone to entice swarm into an empty hive A pheromone is any chemical produced by a living organism that transmits a message to other members of the same species. ... Human and bonobo females have concealed ovulation or hidden estrus. ... This article is about the primate. ... Type species Simia troglodytes Blumenbach, 1775 distribution of Species Pan troglodytes Pan paniscus Chimpanzee, often shortened to chimp, is the common name for the two extant species of apes in the genus Pan. ...


By contrast, in species that have estrous cycles rather than menstrual cycles, females are only receptive to copulation while they are in heat (dolphins are an exception). The other significant difference is that in an estrous cycle, if no fertilization takes place, the uterus reabsorbs the endometrium: no menstrual blood flow occurs. Some animals, such as domestic cats and dogs, experience small amounts of bleeding while in heat. This phase of the estrous cycle corresponds most closely to the follicular phase of the menstrual cycle and should not be confused with menstruation. The estrous cycle (also oestrous cycle; originally derived from Latin oestrus) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females. ... For other uses, see Dolphin (disambiguation). ... Binomial name Felis catus Linnaeus, 1758 Synonyms Felis lybica invalid junior synonym The cat (or domestic cat, house cat) is a small carnivorous mammal. ... Trinomial name Canis lupus familiaris The dog (Canis lupus familiaris) is a domestic subspecies of the wolf, a mammal of the Canidae family of the order Carnivora. ...


References

  • Gangestad SW, Simpson JA, Cousins AJ, Garver-Apgar CE, Christensen PN (2004). "Women's preferences for male behavioral displays change across the menstrual cycle". Psychol Sci 15 (3): 203-7. doi:10.1111/j.0956-7976.2004.01503010.x. PMID 15016293. 

A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

Notes

  1. ^ Strassmann BI (1996). "The evolution of endometrial cycles and menstruation". Q Rev Biol 71 (2): 181–220. doi:10.1086/419369. PMID 8693059. 
  2. ^ Ovulation : American Pregnancy Association
  3. ^ "At what age does a girl get her first period?," from Menstruation and the Menstrual Cycle, National Women's Health Information Center (accessed June 11, 2005)
  4. ^ Shuman, Tracy (February 2006). Your Guide to Menopause. WebMD. Retrieved on 2006-12-16.
  5. ^ Kippley, John; Sheila Kippley (1996). The Art of Natural Family Planning, 4th Edition, Cincinnati, OH: The Couple to Couple League, p. 92. ISBN 0-926412-13-2. 
  6. ^ >The National Women's Health Information Center (November 2002). What is a typical menstrual period like?. U.S. Department of Health and Human Services. Retrieved on 2005-06-11.
  7. ^ David L Healy (2004-11-24). Menorrhagia Heavy Periods - Current Issues. Monash University.
  8. ^ Ball M (1976). "A prospective field trial of the "ovulation method" of avoiding conception". Eur J Obstet Gynecol Reprod Biol 6 (2): 63-6. doi:10.1016/0028-2243(76)90004-6. PMID 985763. 
  9. ^ Dr Evelyn Billings & Ann Westmore (2005). Trials of The Billings Ovulation Method. Retrieved on 2005-11-03.
  10. ^ Sinha G, Sinha A (1993). "A field trial of Billings' ovulation method for spacing and limitation of birth". J Indian Med Assoc 91 (10): 255-6. PMID 8308307. 
  11. ^ Stern K, McClintock MK (1998). "Regulation of ovulation by human pheromones". Nature 392 (6672): 177-9. PMID 9515961. 
  12. ^ Adams, Cecil (2002-12-20). Does menstrual synchrony really exist?. The Straight Dope. The Chicao Reader. Retrieved on 2007-01-10.
  13. ^ Maguire, J.L. . . . and I. Mody. (2005). "Ovarian cycle-linked changes in GABAA receptors mediating tonic inhibition alter seizure susceptibility and anxiety". Nature Neuroscience 8 (June): 797–804. 
  14. ^ Lanting, Frans; Waal, F. B. M. de (1997). Bonobo: the forgotten ape. Berkeley: University of California Press, p.107. ISBN 0-520-20535-9. Retrieved on 2007-09-05. 
  15. ^ Stanford, Craig B. (March-April 2000). "The Brutal Ape vs. the Sexy Ape? The Make-Love-Not-War Ape". American Scientist 88 (2): 110. doi:10.1511/2000.2.110. Retrieved on 2007-09-05. 
  16. ^ Petrie, M. (August 2004). "Female facial attractiveness increases during the fertile phase of the menstrual cycle". Proc.R.Soc.Lond.B (Suppl.) 271: S270-S272. doi:10.1098/rsbl.2004.0174. 
  17. ^ Geoffrey Miller, Joshua M. Tybur and Brent D. Jordan (June 2007). "Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus?". Evolution and Human Behavior 28 (6): 375. doi:10.1016/j.evolhumbehav.2007.06.002. Retrieved on 2008-01-21. 
  18. ^ Susan B. Bullivant, Sarah A. Sellergren, Kathleen Stern, et al (February 2004). "Women's sexual experience during the menstrual cycle: identification of the sexual phase by noninvasive measurement of luteinizing hormone". Journal of Sex Research 41 (1): 82-93. PMID 15216427. 
  19. ^ Krock, Lexi (October 2001). Fertility Throughout Life. 18 Ways to Make a Baby. NOVA Online. Retrieved on 2006-12-24. Haines, Cynthiac (January 2006). Your Guide to the Female Reproductive System. The Cleveland Clinic Women's Health Center. WebMD. Retrieved on 2006-12-24.
  20. ^ Weschler, Toni (2002). Taking Charge of Your Fertility, Revised Edition, New York: HarperCollins, p.107. ISBN 0-06-093764-5. 
  21. ^ Mikkelson, Barbara; David P. Mikkelson (2004-07-21). Youngest Mother. Snopes.com. Retrieved on 2006-07-02.
  22. ^ Cohen S. "Melatonin, menstruation and the moon" Townsend Letter for Doctors and Patients, Feb-March, 2005.[1]
  23. ^ Lacey, Louise. Lunaception: A Feminine Odyssey into Fertility and Contraception. New York: Coward, McCann & Geoghegan, 1975.
  24. ^ Singer, Katie. "Fertility Awareness, Food, and Night-Lighting". Wise Traditions in Food, Farming and the Healing Arts, Spring 2004. See section on Night-Lighting.
  25. ^ Harder, Ben. Bright nights kindle cancers in mice. Science News, Week of Aug. 28, 2004; Vol. 166, No. 9 , p. 141.
  26. ^ Shannon, Marilyn. Fertility, Cycles, and Nutrition. Cincinnati, Ohio: The Couple to Couple League International, 2001. pp. 71–72.
  27. ^ As cited by Adams, Cecil, "What's the link between the moon and menstruation?" (accessed 6 June 2006): Abell, George O.; Barry Singer (1983). Science and the Paranormal: Probing the Existence of the Supernatural. Scribner Book Company. ISBN 0-684-17820-6. 
  28. ^ Cutler WB. Lunar and menstrual phase locking. Am J Obstet Gynecol. 1980 Aug 1;137(7):834-9. PMID: 7405975.
    Friedmann E. Menstrual and lunar cycles. Am J Obstet Gynecol. 1981 Jun 1;140(3):350. PMID: 7246643
    Law SP. The regulation of menstrual cycle and its relationship to the moon. Acta Obstet Gynecol Scand. 1986;65(1):45-8. PMID: 3716780
    Zimecki M. The lunar cycle: effects on human and animal behavior and physiology. Postepy Hig Med Dosw (Online). 2006;60:1–7. Review. PMID: 16407788

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External links

Human Physiology is the science of the mechanical, physical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. ... Endocrinology is a branch of medicine dealing with disorders of the endocrine system and its specific secretions called hormones. ... Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ... A pictorial illustration of the human female reproductive system. ... The male reproductive system is a series of organs located outside of the body and around the pelvic region of a male. ... The human females reproductive system. ... The estrous cycle (also oestrous cycle; originally derived from Latin oestrus) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females. ... Not to be confused with Mensuration. ... Menstrual cycle The follicular phase (or proliferative phase) is the phase of the estrous cycle, (or, in humans and great apes, the menstrual cycle) during which follicles in the ovary mature. ... Ovulation is the process in the menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or casually, an egg) that participates in reproduction. ... The luteal phase is the latter phase of the estrous cycle in animals. ... Gametogenesis is the creation of gametes by meiotic division of gametocytes into various gametes. ... Cross section of the epithelium of a seminiferous tubule showing various stages of spermatocyte development Spermatogenesis is the process by which male spermatogonia develop into mature spermatozoa. ... A Spermatogonium (plural: spermatogonia) is an intermediary male gametogonium (a kind of germ cell) in the production of spermatozoa. ... Spermatogenesis refers to the creation, or genesis, of sperm cells, which occurs in the male gonads or testes. ... The term spermatid refers to the haploid male germ cell that results from secondary spermatocyte division. ... For other uses, see Sperm (disambiguation). ... Oogenesis or rarely oögenesis is the creation of an ovum (egg cell). ... An oogonium is a female gametogonium. ... An oocyte or ovocyte is a female gametocyte or germ cell involved in reproduction. ... An ootid is a female gametid, as opposed to a male spermatid. ... A human ovum Sperm cells attempting to fertilize an ovum An ovum (plural ova) is a haploid female reproductive cell or gamete. ... A germ cell is part of the germline and is involved in the reproduction of organisms. ... A gamete (from Ancient Greek γαμετης; translated gamete = wife, gametes = husband) is a cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. ... This article is about sexual practices (i. ... It has been suggested that Duration of sexual intercourse be merged into this article or section. ... This article is about human physiological erection. ... Ejaculation is the ejecting of semen from the penis, and is usually accompanied by orgasm. ... An orgasm (sexual climax) is the conclusion of the plateau phase of the sexual response cycle, and may be experienced by both males and females. ... Insemination is the introduction of semen into the genital tract of a female. ... This article is about fertilisation in animals and plants. ... Fertility is the natural capability of giving life. ... Masturbation is the manual excitation of the sexual organs, most often to the point of orgasm. ... This article is about human pregnancy in biological females. ... This article does not cite any references or sources. ... This article is about prenatal development in humans. ... Female (left) and male Common Pheasant, illustrating the dramatic difference in both color and size, between the sexes Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. ... This article is about the development of sexual dimorphisms in humans. ... Puberty refers to the process of physical changes by which a childs body becomes an adult body capable of reproduction. ... Menarche (IPA: ) is the first menstrual period, or first menstrual bleeding in the females of human beings. ... Adrenarche refers to a stage of maturation of the cortex of the human adrenal glands. ... Graph showing increased risk of Down Syndrome over time The maternal age effect describes the exponentially increasing risks of chromosomal abnormalities as a prospective mother ages. ... The paternal age effect describes the influence that a fathers age has on the chances of conferring a genetic defect to his offspring. ... The word menopause literally means the permanent physiological, or natural, cessation of menstrual cycles, from the Greek roots meno (month) and pausis (a pause, a cessation). ... Andropause is a medical phenomenon, similar to the female menopause, that can affect men between the ages of 40 and 55. ... A human ovum Sperm cells attempting to fertilize an ovum An ovum (plural ova) is a haploid female reproductive cell or gamete. ... Oviparous animals are animals that lay eggs, with little or no other embryonic development within the mother. ... Ovoviviparous animals develop within eggs that remain within the mothers body up until they hatch or are about to hatch. ... Poa alpina, a grass which shows vivipary: the seeds germinate while still attached to the mother plant. ... Reproductive endocrinology (RE) is a medical subspecialty that addresses hormonal functioning as it pertains to reproduction. ... The hypothalamic-pituitary-gonadal axis (also HPTA) is a way of referring to the combined effects of the hypothalamus, pituitary gland, and gonads as if these individual endocrine glands were a single entity. ... Andrology (from the Greek andros, man) is the medical specialty that deals with male health, particularly relating to the problems of the male reproductive system and urological problems that are unique to men. ... For other uses, see Hormone (disambiguation). ... Not to be confused with Mensuration. ... Menstrual cycle The follicular phase (or proliferative phase) is the phase of the estrous cycle, (or, in humans and great apes, the menstrual cycle) during which follicles in the ovary mature. ... Ovulation is the process in the menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or casually, an egg) that participates in reproduction. ... The luteal phase is the latter phase of the estrous cycle in animals. ... Menarche (IPA: ) is the first menstrual period, or first menstrual bleeding in the females of human beings. ... The word menopause literally means the permanent physiological, or natural, cessation of menstrual cycles, from the Greek roots meno (month) and pausis (a pause, a cessation). ... Basal body temperature is the body temperature measured immediately after awakening and before any physical activity has been undertaken. ... The cervix (from Latin neck) is the lower, narrow portion of the uterus where it joins with the top end of the vagina. ... Mittelschmerz (German: middle pain) is a medical term for ovulation pain. Some women have mittelschmerz regularly and can time their ovulation by it, but many never experience it. ... Fertility Awareness (FA) is the practice of observing one or more of a woman’s primary fertility signs to determine the fertile and infertile phases of her menstrual cycle. ... The Billings ovulation method (BOM) is a method which women use to monitor their fertility, by identifying when they are fertile and when they are infertile during each menstrual cycle. ... The Creighton Model teaches women to observe certain biological signs to monitor their own gynaecological health, and to identify times of fertility and infertility. ... Extended cycle combined oral contraceptive pills are COCPs packaged to reduce or eliminate the withdrawal bleeding that occurs once every 28 days in traditionally packaged COCPs. ... Natural family planning (NFP), sometimes described as periodic abstinence, is a form of birth control that involves recognizing the natural signs in a womans fertility. ... A menstrual disorder is an irregular condition in a womans menstrual cycle. ... Amenorrhoea (BE), amenorrhea (AmE), or amenorrhœa, is the absence of a menstrual period in a woman of reproductive age. ... In medicine, anovulation is absence of ovulation when it would be normally expected (in a post-menarchal, premenopausal woman). ... Dysmenorrhea (or dysmenorrhoea) is a medical condition characterized by severe uterine pain during menstruation. ... Menometrorrhagia is a condition in which prolonged or excessive uterine bleeding occurs irregularly and more frequently than normal. ... Menorrhagia is an abnormally heavy and prolonged menstrual period at regular intervals. ... Metrorrhagia refers to vaginal bleeding that among premenopausal women that is not synchronized with their menstrual period. ... Oligomenorrhea is the medical term for infrequent or light menstrual periods in women of child-bearing age. ... In biology, folliculogenesis refers to the maturation of the ovarian follicle, a densely-packed shell of somatic cells that contains an immature oocyte. ... The McClintock effect (also known as Menstrual Synchrony) is the observed phenomenon that the menstrual cycles of women who live together (such as in prisons, convents, bordellos, dormitories, etc. ... PMS links to this article. ...


  Results from FactBites:
 
Ovulation Calculator: Menstrual Cycle and Ovulation Prediction (711 words)
The menstrual cycle refers to the cyclical development and then shedding of the endometrium, the lining of the uterus.
A woman's fertile period during her menstrual cycle, on average, lasts about seven days: seven days before ovulation (the release of the egg), the day of ovulation, and the day after ovulation.
Though the length and regularity of a menstrual cycle may differ, the average duration of a complete menstrual cycle is 28 days (though healthy cycles can run from 21-36 days).
menstrual cycle: Definition and Much More from Answers.com (5762 words)
During the menstrual cycle, an ovum is released from one of the ovaries (the release is called ovulation), and the uterus develops an inner lining enriched with blood to prepare it for the possible implantation of a zygote.
During the menstrual cycle, the sexually mature female body builds up the lining of the uterus with gradually increasing amounts of oestrogen, and when this hormone reaches a critical level, estradiol is produced, and shortly thereafterward there is the stimulation of the ovaries with Follicle Stimulating Hormone (FSH), and luteinizing hormone (LH).
Menstrual cycles vary in length from an average of 29 days in orangutans to an average of 37 days in chimpanzees.
  More results at FactBites »

 
 

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