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Menstrual cycle

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Menstrual cycle

The menstrual cycle is the set of recurring physiological changes in a female's body that are under the control of the reproductive hormone system and necessary for reproduction. In women, menstrual cycles occur typically on a monthly basis between puberty and menopause. Besides humans, only other great apes exhibit menstrual cycles, in contrast to the estrus cycle of most mammalian species.

During the menstrual cycle, the sexually mature female body releases one egg (or occasionally two, which might result in dizygotic, or non-identical, twins) at the time of ovulation. The lining of the uterus, the endometrium, builds up in a synchronised fashion. After ovulation, this lining changes to prepare for potential implantation of the fertilised egg to establish a pregnancy. If fertilisation and pregnancy do not ensue, the uterus sheds the lining and a new menstrual cycle begins. The process of the shedding of the lining is called menstruation. Menstruation 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.

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 can occur any time between the ages of 8 and 16.[1] 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, and 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 ovulation did not 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 normal 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 four phases:

Menstruation

Menstruation lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal[2]) and involves the loss of about 50 millilitres of blood (including shed lining). An enzyme called plasmin — contained in the endometrium — inhibits 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 fertilised, the embryo will implant itself within this hospitable flesh.

Ovulation

This ovary is 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 fertilisation. 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 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 unfertilised egg will eventually disintegrate or dissolve in the uterus. Scientific investigations like [3] have indicated that the olfactory acuity or the sense of smell is greatest during ovulation in women.

Luteal phase

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 half- to one degree Fahrenheit (one-quarter to one-half degree Celcius), thus women who record their temperature on a daily basis will notice that they have entered the luteal phase. If fertilisation of an egg has occurred, it will travel as an early embryo through the 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.


Menstrual symptoms

In many women, various unpleasant symptoms caused by the involved hormones and by cramping of the uterus can precede or accompany menstruation. More severe symptoms may include significant menstrual pain (dysmenorrhea), abdominal pain, migraine headaches, depression and irritability. Some women encounter premenstrual stress syndrome (PMS or premenstrual syndrome), a cyclic clinical entity. The list of symptoms experienced varies from person to person. Furthermore, within an individual, the severity of the symptoms may vary 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 can survive for 3 to 4 days (possibly up to 7 days) inside a woman, so 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. Researchers first described this phenomenon in 1971, and explained it by the action of pheromones in 1998 (Stern and McClintock 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. Two sex hormones play a role in the control of the menstrual cycle: estradiol and progesterone. While 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. 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. 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.

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.

During times when progesterone is highest, nerve-cell receptor subtype delta was high. Nerve cells with more delta receptors are less likely to fire than cells with lower numbers of delta receptors. When estrogen levels are higher, the number of delta receptors decrease, increasing nerve cell activity, in turn increasing anxiety and seizure susceptibility. (Maguire et al., 2005)

Hidden ovulation

Unlike other species, ovulation remains hidden in humans. 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. In this context, evidence suggests that women's preferences for men may change during their most fertile days; that is, before and shortly after ovulation. During this time, they may prefer different male scents, more masculine faces, and social presence in males considered as partners. (Gangestad 2004; debated) Women, especially young teens, have been noted to dress more provocatively, to say that they feel sexier, to flirt more, and to be more likely to initiate sexual activity around the time of ovulation than they did at other points in their menstrual cycle. Most of this, especially in younger women, appears to be subconscious.

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 conceptuses 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. 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.

Cycle abnormalities

Frequency

The "normal menstrual cycle" occurs every 28 days ± 7 days. Doctors term cycles with intervals of 21 days or fewer as polymenorrhea and, on the other hand, call cycles with intervals exceeding 35 days 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 ("is on her period") for three to seven days out of each month.

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.

The 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 placebo 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 remove the pill's pregnancy protection.) 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.

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 (The synodical lunar month, the period between two new moons, lasts about 29 and a half days).

Many women, after a period of not being exposed to artificial nighttime lighting, find their menstrual cycles begin to occur in rhythm with the lunar cycle.

Menstrual products

While some women allow their menses to flow freely or learn to recognise when their menses will flow, most women prefer to use some artifical means to absorb or catch their menses to prevent soiling their clothes. There are a number of different methods used:

  • Sanitary towels, sanitary napkins, or pads - Rectangular pieces of material worn in the panties to absorb menstrual flow, often with "wings," pieces that fold around the panties, or a sticky backing to hold the pad in place. 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. Disposable synthetic pads are made of wood pulp or synthetic products, usually with a plastic lining and bleached.
  • Tampons - Disposable wads of treated rayon/cotton blends or all-cotton fleece, usually bleached, that are inserted into the vagina to absorb menstrual flow. Some women also make their own tampons from rolled up cotton strips.
  • Menstrual cups - A firm, flexible cup- or bell-shaped device worn inside the vagina to catch menstrual flow. Reusable versions include rubber or silcone cups (like the Keeper, Divacup, and Mooncup) or the birth control cervical cap known as the diaphragm. Disposable versions come in soft plastic cups (like Insteads brand).
  • Sea sponges - Reuseable soft sponges from plant-like animals that grow on the ocean floor, worn internally to absorb blood.
  • Padettes - Disposable wads of treated rayon/cotton blend fleece that are placed within the inner labia to absorb menstrual flow.
  • Padded panties - Reuseable cloth (usually cotton) underwear with extra absorbent layers sewn in to absorb flow.
  • Blanket, towel, or "bleeding blankie" - Large reuseable piece of cloth, most often used at night, placed between legs to absorb menstrual flow.

Pharmaceutical companies also provide products — commonly Non-steroidal anti-inflammatory drugs (NSAIDs) — to relieve menstrual cramps.

Debate

Much debate centers around which menstrual products to use. The main debate can be summarized as one between the convenience, availability, and general knowledge of disposables versus the environmental, monetary, and potential health benefits of reuseables. A secondary aspect of this is commercial responsibility. Disposable menstrual products compose a large and powerful industry in the West, with a near monopoly on advertising, supermarket shelves, and menstrual education, leading many people to believe that these corporate products are their only options. Many people object to the negative portrayal of menstruation in advertising as shameful, unnatural, stinking, and hindering. In contrast, the reuseable menstrual products industry is composed mostly of small, independent, and woman-owned, woman-positive businesses. Finally, some believe that the disposable menstrual products industry is imperialist, forcing or coercing women of other cultures to leave their resueable, inexpensive or free menstrual products to become consumers of disposables.

A summary of the main issues of debate:

Environmental waste
Disposable tampons, pads, and cups and their packaging generate tons of bulky waste per year, much of which is not biodegradeable.
Cost
Many disposable have a cheaper upfront cost than reuseables, but over time (a period of a few months), this cost is recouped many times over from savings on reuseables. Many reuseables can also be made for free from old clothes or scraps of cloth.
Health concerns
1. Bleaching - Many women object to the chlorine bleaching of disposable menstrual products, which leaves trace amounts of dioxin, a carcinogen, in them. 2. Scents and deoderizers - Chemical scents and deoderizers can cause rashes, irritation, and allergic reactions. They can upset the pH balance of the vagina and cause yeast infections.
Health concerns specific to tampons
Toxic Shock Syndrome is caused by Staphylococcus bacteria, which thrive the environment found in tampon fibers. TSS is not a staph infection. It is caused when the bacteria go into reproductive overdive and overwhelm the body in a matter of hours (long before an infection could ever set in). Toxic Shock Syndrome can, and does, cause death. TSS can be avoided by using the least absorbent tampon possible for one's flow, and changing tampons at least every 8 hours, or by avoiding tampons altogether. This may apply to sea sponges also, though no cases of TSS with sea sponge use have been reported.

Culture and menstruation

Mysticism

Mystics have sometimes elaborated "equivalencies", analogising the waxing and waning of the moon with influences on human menstruation. In this spiritual, moon goddess, or astrological context some women call menstruation their "moontime". Some ancient views also regarded menstruation as a cleansing of the body: compare bloodletting as a major medical treatment of pre-modern times.

Religion

Some religions consider women "unclean" during menstruation.

Islam on menstruation

The Islamic world considers a woman "not in a state to have intercourse" during menstruation. A verse from the Holy Qur'an (with parenthesised interpolations by Dr. Muhammed Muhsin Khan) affirms this:

"They ask you concerning menstruation. Say: that is an Adha (a harmful thing for a husband to have sexual intercourse with his wife while she is having her menses), therefore keep away from women during menses and go not unto them till they have purified (from menses and have taken a bath). And when they have purified themselves, then go in unto them as Allâh has ordained for you. Truly, Allâh loves those who turn unto Him in repentance and loves those who purify themselves (by taking a bath and cleaning and washing thoroughly their private parts, bodies, for their prayers, etc.)." (Al-Baqarah 2:222)

See an Islamic review on the subject.

Judaism on menstruation

Further information: Niddah

A ritual exclusion applies to a woman while menstruating and for about a week thereafter, until she immerses herself in a mikvah (ritual bath).

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 in some way. 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 menstural flow, however due to it's small amount (and personal cleanliness in cats) it passes largely unoticed pet owners.

References

Notes

  1. ^ "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).
  2. ^ Ibid., "What is a typical menstrual period like?" (accessed June 11, 2005).
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