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Menstrual cycle

2007 Schools Wikipedia Selection. Related subjects: Evolution and
reproduction

   Menstrual cycle
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   Menstrual cycle

   The menstrual cycle is a recurring cycle of physiological changes in
   the females of some animal species that is associated with reproductive
   fertility.

   This article concentrates on the menstrual cycle as it occurs in human
   beings.

Overview

   Only human beings and great apes experience a true menstrual cycle.
   Most placental mammals experience estrus instead. The menstrual cycle
   is under the control of the reproductive hormone system and is
   necessary for reproduction. In women, menstrual cycles occur typically
   on a monthly basis between puberty and menopause.

   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). Follicles begin developing, and within a few days one
   "matures" into an ovum or egg. The ovary then releases this egg, (or
   occasionally two, which might result in dizygotic, or
   fraternal/non-identical, twins) at the time of ovulation. ( Identical
   twins form from a single egg, as a result of the zygote splitting.) The
   lining of the uterus, the endometrium, peaks shortly there afterward in
   a synchronised fashion. After ovulation, this lining changes to prepare
   for potential conception and implantation of the fertilized egg to
   establish a pregnancy. The hormone progesterone rises after ovulation,
   and peaks shortly thereafter.

   If fertilization and pregnancy do not ensue, the uterus sheds the
   lining, culminating with menstruation, which marks the low point for
   estrogen activity. This manifests itself to the outer world in the form
   of the menses (also menstruum): essentially part of the endometrium and
   blood products that pass out of the body through the vagina. Although
   this is commonly referred to as blood, it differs in composition from
   venous blood. Menstrual cycles are counted from the beginning of
   menstruation, because this is an outside sign that corresponds closely
   with the hormonal cycle. Menses, or bleeding and other menstrual signs
   may end at different points in the new cycle.

   Common usage refers to menstruation and menses as a period. This
   bleeding serves as a sign that a woman has not become pregnant.
   (However, this cannot be taken as certainty, as sometimes there is some
   bleeding in early pregnancy.) During the reproductive years, failure to
   menstruate may provide the first indication to a woman that she may
   have become pregnant. A woman might say that her "period is late" when
   an expected menstruation has not started and she might have become
   pregnant.

   Menstruation forms a normal part of a natural cyclic process occurring
   in healthy women between puberty and the end of the reproductive years.
   The onset of menstruation, known as menarche, occurs at an average age
   of 12, but is normal anywhere between 8 and 16. Factors such as
   heredity, diet and overall health can accelerate or delay menarche.

   The condition precocious puberty has caused menstruation to occur in
   girls as young as 8 months old. Some women experience their first
   period in their late teens. The last period, menopause, usually occurs
   between the ages of 45 and 55. Deviations from this pattern deserve
   medical attention. Amenorrhea refers to a prolonged absence of menses
   during the reproductive years of a woman for reasons other than
   pregnancy. For example, women with very low body fat, such as athletes,
   may cease to menstruate. The presence of menstruation does not prove
   that ovulation took place; women who do not ovulate may have menstrual
   cycles. Those anovulatory cycles tend to take place less regularly and
   show greater variation in cycle length. In addition, the absence of
   menstruation also does not prove that fertilization did take place,
   because hormone disruptions in non-pregnant women can suppress bleeding
   on occasion.

The normal menstrual cycle in humans

   Women show considerable variation in the lengths of their menstrual
   cycles, and the length of the menstrual cycle differs in different
   animals (see below).

   While cycle length may vary, 28 days is generally taken as
   representative of the average ovulatory cycle in women. Convention uses
   the onset of menstrual bleeding to mark the beginning of the cycle, so
   the first day of bleeding is called "Cycle Day one".

   One can divide the menstrual cycle into several phases. Although the
   length of each phase varies widely from woman to woman and cycle to
   cycle, most references use the following average numbers:
   Name of phase                                                 Days
   menstrual phase                                               1-4
   follicular phase (also known as proliferative phase)          4-14
   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

   More detail on these phases are provided below:

Menstruation

   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 average blood loss during menstruation is 35 millilitres
   with 10-80 mL considered normal; many women also notice shed
   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, also referred to as dysmenorrhea,
   during this time. A vast industry has grown to provide sanitary
   products to help women to manage their menses.

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 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. In a signal cascade kicked off
   by luteinizing hormone (LH), the follicles secrete estradiol, a steroid
   that acts to inhibit pituitary secretion of FSH. With diminished FSH
   supply comes a slowing in growth that eventually leads to follicle
   death, known as atresia. The largest follicle secretes inhibin that
   serves as a finishing blow to less competent follicles by further
   suppressing FSH. This dominant follicle continues growing, forms a
   bulge near the surface of the ovary, and soon becomes competent to
   ovulate.

   The follicles also secrete estrogens (of which estradiol is a member).
   Estrogens initiate the formation of a new layer of endometrium in the
   uterus, histologically identified as the proliferative endometrium. If
   fertilized, the embryo will implant itself within this hospitable
   flesh.

Ovulation

   An ovary about to release an egg.
   Enlarge
   An ovary about to release an egg.

   When the follicle 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
   Fallopian tube needs to capture the egg and provide the site for
   fertilization. A characteristic clear and stringy mucus exhibiting
   spinnbarkeit develops at the cervix, ready to accept sperm from
   intercourse. In some women, ovulation features a characteristic pain
   called Mittelschmerz (German term meaning 'middle pain') which lasts
   for several hours. The sudden change in hormones at the time of
   ovulation also causes light mid-cycle bleeding for some women. Many
   women perceive the vaginal and cervical mucus changes at ovulation,
   particularly if they are monitoring themselves for signs of fertility.
   An unfertilized egg will eventually disintegrate or dissolve in the
   uterus. Scientific investigations have indicated that the olfactory
   acuity or the sense of smell is greatest during ovulation in women.

Luteal phase

   The corpus luteum is the solid body formed in the ovaries after the egg
   has been released from 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 one-half to one degree Fahrenheit
   (one-quarter to one-half degree Celsius), thus women who record their
   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.

Physical experience of menstruation

   In many women, various intense sensations brought about by the involved
   hormones and by cramping of the uterus can precede or accompany
   menstruation. Stronger sensations may include significant menstrual
   pain ( dysmenorrhea), abdominal pain, migraine headaches, depression
   and emotional sensitivity, and changes in sex drive. Diarrhea or loose
   stools are common for the first few days of a woman's period. Breast
   discomfort caused by premenstrual water retention is very common. The
   sensations experienced vary from woman to woman and from cycle to
   cycle.

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. Some women have a luteal phase of
   10 days, others of 16 days (the average is 14 days), but for each
   individual woman, this length will remain constant. Sperm survive
   inside a woman for 3 days on average, with survival time up to five
   days considered normal. A pregnancy resulting from sperm life of eight
   days has been documented. The most fertile period (the time with the
   highest likelihood of sexual intercourse leading to pregnancy) 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.

   People who have heard about the menstrual cycle and ovulation may
   commonly and mistakenly assume, for contraceptive purposes, that
   menstrual cycles always take a regular 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 bleeding
   event counts as a menstruation, and this can mislead people in their
   calculation of the fertile window.

   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).

   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 conclusion into
   question.

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:
   estradiol and progesterone:
     * Estrogen 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.

Hypothalamus and pituitary

   These sex hormones come under the influence of the pituitary gland, and
   both FSH and LH play necessary roles:
     * 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.

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 progresterone declines and estrogen increases.

   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.

Hidden ovulation

   Unlike almost all other species, the external physical changes of a
   human female near ovulation are very subtle. A woman may sense her own
   ovulation while it may remain indiscernible to others; this is
   considered to have sociobiological significance. In contrast, other
   species often signal receptivity through heat. The great apes are the
   only other mammals to have hidden ovulation.

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
   eggs 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 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, 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. This possibility is
   supported by the observation that fetuses and infants of older mothers
   have higher rates of chromosome abnormalities than those of older
   fathers.

The anovulatory menstrual cycle

   Not all menstruations result from an ovulatory menstrual cycle (
   Anovulatory cycle - literally 'an-' absence of 'ovulation'). In some
   women, follicular development may start but not complete, nevertheless
   estrogens will form and will stimulate the uterine lining. Sooner or
   later the uterus will shed this lining. As no ovulation and no
   progesterone involvement occurs, doctors call this type of bleeding an
   estrogen breakthrough bleeding, and cannot always predict its duration
   or frequency. Anovulatory bleeding commonly occurs prior to menopause
   (premenopause) or in women with polycystic ovary syndrome. Infrequent
   or irregular ovulation is called oligoovulation.

Cycle abnormalities

Frequency

   The "normal menstrual cycle" occurs every 28 days ± 7 days.

   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).

Flow

   The normal menstrual flow amounts to 50 mL ± 30 mL. It follows a
   "crescendo-decrescendo" pattern; that is, it starts at a moderate
   level, increases somewhat, and then slowly tapers. Sudden heavy flows
   or amounts in excess of 80 mL (hypermenorrhea or menorrhagia) may stem
   from hormonal disturbance, uterine abnormalities, including uterine
   leiomyoma or cancer, and other causes. Doctors call the opposite
   phenomenon, of bleeding very little, hypomenorrhea.

Duration

   The typical woman bleeds for three to seven days at the beginning of
   each menstrual cycle.

   Prolonged bleeding (metrorrhagia, also meno-metrorrhagia) no longer
   shows a clear interval pattern. Dysfunctional uterine bleeding refers
   to hormonally caused bleeding abnormalities, typically anovulation. All
   these 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 bleeding.

Contraception and menstruation

Hormonal options

   Estrogens and progesterone-like hormones make up the main active
   ingredients of birth control pills. Typically they tend to mimic a
   menstrual cycle in appearance, but to suppress the critical event of
   the ovulatory cycle, namely ovulation. Normally, a woman takes hormone
   pills for 21 days, followed by 7 days of non-functional sugar 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. (Two main versions of the pill
   exist: monophasic and triphasic. With triphasic pills, skipping of the
   placebos and continuing with the next month's dose can make a woman
   more likely to experience spotting or breakthrough bleeding.) In 2003
   the United States Food and Drug Administration (FDA) approved low-dose
   monophasic birth control pills which induce withdrawal bleedings only
   every 3 months. Yet another version of the pill is the
   recently-released Loestrin Fe, which has only a four-day placebo "week"
   (the placebos are actually iron supplements intended to replenish iron
   lost via bleeding), the other three placebos being replaced with active
   hormone pills; this system is intended to help shorten the duration of
   a period.

   Other types of hormonal birth control which affect menstruation include
   the vaginal Nuvaring and the transdermal patch (like the standard pill
   pack: used for three weeks, then given a one-week break for bleeding)
   and the shot (which can eliminate bleeding for several months at a
   time).

Effects on menstruation

   All such methods are designed to regulate monthly bleeding. 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
   factors such as cramping, heavy bleeding, 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

   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. Tubal sterilization alone will not affect
   menstruation, though the ablation option often performed at the same
   time will cease menstruation. Hysterectomy will, of course, completely
   stop menstruation as it entails the removal of the uterus (and
   sometimes related organs). 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.

Etymology and the lunar month

   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.

   A 1975 book by Louise Lacey documented the experience of herself and 27
   of her friends, who found that when they removed all artificial
   nightlighting their menstrual cycles began to occur in rhythm with the
   lunar cycle. She dubbed the technique Lunaception. Later 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 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.

   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.

Menstrual products

   While some women allow their menses to flow freely, most women prefer
   to use some external means to absorb or catch their menses to prevent
   soiling their clothes. There are a number of different methods
   available.

   Disposable items:
     * Sanitary napkins (Sanitary towels) or pads - Somewhat rectangular
       pieces of material worn in the underpants to absorb menstrual flow,
       often with "wings," pieces that fold around the panties, and/or an
       adhesive backing to hold the pad in place. Disposable synthetic
       pads are made of wood pulp or synthetic products, usually with a
       plastic lining and bleached. Some sanitary napkins, particularly
       older styles, are held in place by a belt-like apparatus, instead
       of adhesive or wings.
     * Tampons - Disposable wads of treated rayon/cotton blends or
       all-cotton fleece, usually bleached, that are inserted into the
       vagina to absorb menstrual flow.
     * Padettes - Disposable wads of treated rayon/cotton blend fleece
       that are placed within the inner labia to absorb menstrual flow.
     * Disposable menstrual cups -- A firm, flexible cup-shaped device
       worn inside the vagina to catch menstrual flow. Disposable cups are
       made of soft plastic (like Instead).

   Reusable items:
     * Reusable cloth pads are made of cotton (often organic), terrycloth,
       or flannel, and may be handsewn (from material or reused old
       clothes and towels) or storebought.
     * Menstrual cups - A firm, flexible bell-shaped device worn inside
       the vagina to catch menstrual flow. Reusable versions include
       rubber or silcone cups (like the Keeper, DivaCup, Lunette, and
       Mooncup).
     * Sea sponges - Natural sponges, worn internally like a tampon to
       absorb menstrual flow.
     * Padded panties - Reuseable cloth (usually cotton) underwear with
       extra absorbent layers sewn in to absorb flow. (like Lunapads)
     * Blanket, towel - Large reuseable piece of cloth, most often used at
       night, placed between legs to absorb menstrual flow.

   In addition to products to contain the menstrual flow, pharmaceutical
   companies likewise provide products — commonly non-steroidal
   anti-inflammatory drugs (NSAIDs) — to relieve menstrual cramps. Some
   herbs, such as dong quai, raspberry leaf and crampbark, are also
   claimed to relieve menstrual pain.

Culture and menstruation

   Many religions have menstruation-related traditions. These may be bans
   on certain actions during menstruation (such as intercourse in orthodox
   Judaism and Islam), or rituals to be performed at the end of each
   menses (such as the mikvah in Judaism and the ghusl in Islam).

Menstruation in other mammals

   A regular menstrual cycle as described here only occurs in the great
   apes. Menstrual cycles vary in length from an average of 29 days in
   orangutans to an average of 37 days in chimpanzees.

   Females of other mammalian species go through certain episodes called "
   estrus" or "heat" in each breeding season. During these times,
   ovulation occurs and females become receptive to mating, a fact
   advertised to males principally in some way presumably mediated by
   pheromones. If no fertilisation takes place, the uterus reabsorbs the
   endometrium: no menstrual bleeding occurs. Significant differences
   exist between the estrus and the menstrual cycle. Some animals, such as
   domestic cats and dogs do produce a very short and mild menstrual flow.
   However, due to its small amount (and personal cleanliness in cats) it
   passes pet owners largely unnoticed.
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