Menstrual cycle
 |
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.
Only
human beings and the
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
estrogen, 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 the onset of menarche.
["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)]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 ovulation did not take place, because hormone disruptions in non-pregnant women can suppress bleeding on occasion.
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 four phases:
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 — 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 fertilized, 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
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.
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. Breast discomfort caused by
premenstrual water retention is very common. The sensations experienced vary from woman to woman and from cycle to cycle.
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.
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.
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.
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.
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.
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.
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 ("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.
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.
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.
[Lacey, Louise. Lunaception: A Feminine Odyssey into Fertility and Contraception. New York: Coward, McCann & Geoghegan, 1975.] Later studies in both humans
[Singer, Katie. "[http://www.westonaprice.org/women/fertility.html Fertility Awareness,Food, and Night-Lighting]". Wise Traditions in Food, Farming and the Healing Arts, Spring 2004. See section on Night-Lighting.] and animals
[Harder, Ben. Bright nights kindle cancers in mice. Science News, Week of Aug. 28, 2004; Vol. 166, No. 9 , p. 141.] 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.
[Shannon, Marilyn. Fertility, Cycles, and Nutrition. Cincinnati, Ohio: The Couple to Couple League International, 2001. pp. 71-72.]Some have suggested that the fact that other animals' menstrual cycles appear to be greatly different from lunar cycles (see
Menstruation in other mammals below) is evidence that the average length of humans' cycle is most likely a coincidence.
[As cited by Adams, Cecil, "What's the link between the moon and menstruation?" (accessed 6 June 2006): ][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: 16407788While 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 methods:
*
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 methods:
* 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.
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. This leads many people to believe that these corporate products are their only options. Many people object to the negative portrayal of menstruation in advertising; menstruation is sometimes portrayed as shameful, unnatural, smelly, and hindering. In contrast, the reuseable menstrual products industry is composed mostly of small, independent, and woman-owned businesses. Finally, some believe that the disposable menstrual products industry is imperialist, forcing or coercing women of other cultures to leave their reuseable, inexpensive or free menstrual products to become consumers of disposables.
A summary of the main issues of debate:
* Environmental waste : Tampons, pads, disposable cups and their packaging generate tons of bulky waste per year, much of which is not biodegradable.
* Cost : Many disposables 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 other scraps of cloth.
* Health concerns:
*#
Bleaching - Many women object to the chlorine bleaching of disposable menstrual products, which leaves trace amounts of
dioxin, a
carcinogen, in their bodies.
*# Scents and deodorizers - Chemical scents and deodorizers can cause rashes, irritation, and allergic reactions. They can upset the pH balance of the vagina and cause yeast infections.
*# Specific to
tampons :
Toxic Shock Syndrome is indirectly caused by
Staphylococcus aureus, which can thrive in the environment found in tampon fibers. TSS is
very rare, with only approx. 40 cases per year in the
UK. Tampon-associated TSS is not caused by the staphylococci invading into tissues, but rather as a result of the release of a protein called
toxic shock syndrome toxin (TSST). TSST is absorbed into the body where it acts as a toxin. 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.
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).
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.
*
*
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*
Early 70s Educational FilmHygiene products*
Love Your Blood: An info-zine on menstrual products and their alternatives*
The rags: paraphernalia of menstruation*
Eight Myths About Washable Menstrual Pads Dispelled*
Tampaction and
The Bloodsisters Project- Menstrual activism against chlorine bleaching, excessive packaging, and negative attitudes toward menstruation in the West
