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.

[edit] 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 thereafter 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.

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.^[1]

The condition precocious puberty has caused menstruation to occur in girls as young as 8 months old.^[2] 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.

[edit] 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:

More detail on these phases are  provided below:

[edit] Menstruation

Main article: Menstruation

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.

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).^[3] The average blood loss during menstruation is  35 millilitres with 10-80 mL considered normal;^[4] 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, also referred to as dysmenorrhea, during this time. A vast industry has grown to provide sanitary products to help women to manage their menses.

[edit] 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 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.

[edit] Ovulation

Main article: Ovulation

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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.^[5]

[edit] Luteal phase

Main article: 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.

[edit] 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.^[6][7][8] 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.^[9] However, subsequent research has called this conclusion into question.

[edit] 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.

[edit] 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.

[ and  pituitary">edit] 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.

[edit] 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.^[10]

[edit] Hidden ovulation

Main article: Concealed 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.

[ an  egg-bank">edit] 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.

[edit] 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.

[edit] Cycle abnormalities

Main article: Menstrual disorder

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).

[ and  menstruation">edit] Contraception and  menstruation

[edit] Hormonal options

Main article: Birth control pill

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  and  Drug Administration">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).

[edit] 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.

[ and  flexibility">edit] 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.

[edit] 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.

[ and  the lunar month">edit] 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.^[11] Later studies in both humans^[12] and  animals^[13] 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.^[14]

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.^[15][16]

[edit] Menstrual cycles 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.

[edit] References

[edit] Notes

[edit] External links

• Museum of Menstruation and  Women's Health
• Leslie Botha-Williams, Women's Health Educator: A Woman's Guide to Understanding Her Hormone Cycle
• An Islamic answer for the question "What is  the wisdom behind the prohibition on having intercourse with one's wife when she is  menstruating or  bleeding following childbirth?"
• Menstrual Suppression With Birth Control Pills
• On The Rag: Everything you need to know about your fertility cycles and  menstruation...period - Article on the menstrual cycle by notable sex activist and  educator Heather Corinna
• Menopause and  Menstrual Cycles - WomensHealthChannel
• Patient Queries - Menstrual Cycle Queries related to Menstrual Cycle answered by Obstetricians & Gynaecologists

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