Menstrual cycle
Overview
The menstrual cycle is the scientific term for the physiological
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 changes that can occur in fertile women for the purpose of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

. This article focuses on the human menstrual cycle.

The menstrual cycle, under the control of the endocrine system
Endocrine system
In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...

, is necessary for reproduction. It is commonly divided into three phases: the follicular phase
Follicular phase
The follicular phase is the phase of the estrous cycle, during which follicles in the ovary mature. It ends with ovulation. The main hormone controlling this stage is estradiol....

, ovulation
Ovulation
Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...

, and the luteal phase
Luteal phase
The luteal phase is the latter phase of the menstrual cycle or the estrous cycle . It begins with the formation of the corpus luteum and ends in either pregnancy or luteolysis...

; although some sources use a different set of phases: menstruation
Menstruation
Menstruation is the shedding of the uterine lining . It occurs on a regular basis in sexually reproductive-age females of certain mammal species. This article focuses on human menstruation.-Overview:...

, proliferative phase, and secretory phase.
Encyclopedia
The menstrual cycle is the scientific term for the physiological
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 changes that can occur in fertile women for the purpose of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

. This article focuses on the human menstrual cycle.

The menstrual cycle, under the control of the endocrine system
Endocrine system
In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...

, is necessary for reproduction. It is commonly divided into three phases: the follicular phase
Follicular phase
The follicular phase is the phase of the estrous cycle, during which follicles in the ovary mature. It ends with ovulation. The main hormone controlling this stage is estradiol....

, ovulation
Ovulation
Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...

, and the luteal phase
Luteal phase
The luteal phase is the latter phase of the menstrual cycle or the estrous cycle . It begins with the formation of the corpus luteum and ends in either pregnancy or luteolysis...

; although some sources use a different set of phases: menstruation
Menstruation
Menstruation is the shedding of the uterine lining . It occurs on a regular basis in sexually reproductive-age females of certain mammal species. This article focuses on human menstruation.-Overview:...

, proliferative phase, and secretory phase. Menstrual cycles are counted from the first day of menstrual bleeding. Hormonal contraception
Hormonal contraception
Hormonal contraception refers to birth control methods that act on the endocrine system. Almost all methods are composed of steroid hormones, although in India one selective estrogen receptor modulator is marketed as a contraceptive. The original hormonal method—the combined oral contraceptive...

 interferes with the normal hormonal changes with the aim of preventing reproduction.

Stimulated by gradually increasing amounts of estrogen
Estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

 in the follicular phase, discharges of blood (menses) slow then stop, and the lining
Endometrium
-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...

 of the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

 thickens. Follicles in the ovary
Ovary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

 begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two become dominant (non-dominant follicles atrophy and die). Approximately mid-cycle, 24–36 hours after the Luteinizing Hormone (LH) surges, the dominant follicle releases an ovum, or egg
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

 in an event called ovulation. After ovulation, the egg only lives for 24 hours or less without fertilization while the remains of the dominant follicle in the ovary become a corpus luteum
Corpus luteum
The corpus luteum is a temporary endocrine structure in mammals, involved in production of relatively high levels of progesterone and moderate levels of estradiol and inhibin A...

; this body has a primary function of producing large amounts of progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...

. Under the influence of progesterone, the endometrium (uterine lining) changes to prepare for potential implantation of an embryo to establish a pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...

. If implantation does not occur within approximately two weeks, the corpus luteum will involute, causing sharp drops in levels of both progesterone and estrogen. These hormone drops cause the uterus to shed its lining and egg in a process termed menstruation.

In the menstrual cycle, changes occur in the female reproductive system as well as other systems (which lead to breast tenderness or mood
Mood (psychology)
A mood is a relatively long lasting emotional state. Moods differ from emotions in that they are less specific, less intense, and less likely to be triggered by a particular stimulus or event....

 changes, for example). A woman's first menstruation is termed menarche
Menarche
Menarche is the first menstrual cycle, or first menstrual bleeding, in female human beings. From both social and medical perspectives it is often considered the central event of female puberty, as it signals the possibility of fertility....

, and occurs typically around age 12-13. The average age of menarche is about 12.5 years in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, 12.72 in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, 12.9 in the UK and 13.06 ± 0.10 years in Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

. The end of a woman's reproductive phase is called the menopause
Menopause
Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...

, which commonly occurs somewhere between the ages of 45 and 55.

Terminology

The menarche
Menarche
Menarche is the first menstrual cycle, or first menstrual bleeding, in female human beings. From both social and medical perspectives it is often considered the central event of female puberty, as it signals the possibility of fertility....

 is one of the later stages of puberty
Puberty
Puberty is the process of physical changes by which a child's body matures into an adult body capable of reproduction, as initiated by hormonal signals from the brain to the gonads; the ovaries in a girl, the testes in a boy...

 in girls.
The average age of menarche in humans is 12–13 years, but is normal anywhere between ages 8 and 16. Factors such as heredity, diet and overall health can accelerate or delay menarche. The cessation of menstrual cycles at the end of a woman's reproductive period is termed menopause
Menopause
Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...

. The average age of menopause in women is 52 years in industrialised countries such as the UK, with anywhere between 45 and 55 being common. Menopause before age 45 is considered premature in industrialised countries. The age of menopause is largely a result of genetics; however, illnesses, certain surgeries, or medical treatments may cause menopause to occur earlier.

The length of a woman's menstrual cycle will typically vary, with some shorter cycles and some longer cycles. A woman who experiences variations of less than eight days between her longest cycles and shortest cycles is considered to have regular menstrual cycles. It is unusual for a woman to experience cycle length variations of less than four days. Length variation between eight and 20 days is considered as moderately irregular cycles. Variation of 21 days or more between a woman's shortest and longest cycle lengths is considered very irregular.

Phases

The menstrual cycle can be divided into several different phases. The average length of each phase is shown below, the first three are related to changes in the lining of the uterus whereas the final three are related to processes occurring in the ovary:
Name of phase Average start day
assuming a 28-day cycle
Average end day
>-
| Menstrual phase (Menstruation)
1 >-
| Proliferative phase (Some sources include Menstruation in this phase)
5 >-
| Ovulatory phase (Ovulation)
13 >-
| Luteal phase (also known as Secretory phase)
16 >-
| Ischemic phase
27 >-
| Follicular phase
1 >-

Menstruation

Menstruation is also called menstrual bleeding, menses, catamenia or a period. The flow of menses normally serves as a sign that a woman has not become pregnant
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...

. (However, this cannot be taken as certainty, as a number of factors can cause bleeding during pregnancy; some factors are specific to early pregnancy
First trimester bleeding
In obstetrics, first trimester bleeding, is bleeding from the vagina during pregnancy in the first trimester .It is a common occurrence and estimated to occur in approximately 25% of all pregnancies....

, and some can cause heavy flow
Obstetrical hemorrhage
Obstetrical hemorrhage refers to heavy bleeding during pregnancy, labor, or the puerperium. Bleeding may be vaginal and external, or, less commonly but more dangerously, internal, into the abdominal cavity...

.)
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
Bleeding
Bleeding, technically known as hemorrhaging or haemorrhaging is the loss of blood or blood escape from the circulatory system...

 during menstruation is 35 milliliters with 10–80 ml considered normal. (Because of this blood loss, women are more susceptible to iron deficiency
Iron deficiency (medicine)
Iron deficiency is one of the most common of the nutritional deficiencies. Iron is present in all cells in the human body, and has several vital functions...

 than men are.) An enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 called plasmin
Plasmin
Plasmin is an important enzyme present in blood that degrades many blood plasma proteins, most notably, fibrin clots. The degradation of fibrin is termed fibrinolysis. In humans, the plasmin protein is encoded by the PLG gene.- Function :...

 inhibits clotting in the menstrual fluid.

Painful cramping in the abdomen, back, or upper thighs is common during the first few days of menstruation (most women experience some pain during menstruation). Severe uterine pain during menstruation is known as dysmenorrhea
Dysmenorrhea
Dysmenorrhea is a gynecological medical condition of pain during menstruation that interferes with daily activities, as defined by ACOG and others. Still, dysmenorrhea is often defined simply as menstrual pain, or at least menstrual pain that is excessive...

, and it is most common among adolescents and younger women (affecting about 67.2% of adolescent females). When menstruation begins, symptoms of premenstrual syndrome (PMS) such as breast tenderness
Mastalgia
Mastodynia, mastalgia or mammalgia are names for a medical symptom that means breast pain .Pain can range from minor discomfort to severely incapacitating pain...

 and irritability generally decrease. Many sanitary products are marketed to women for use during their menstruation.

Follicular phase

This phase is also called the proliferative phase because a hormone causes the lining of the uterus to grow, or proliferate, during this time.

Through the influence of a rise in follicle stimulating hormone (FSH) during the first days of the cycle, a few ovarian follicle
Ovarian follicle
Ovarian follicles are the basic units of female reproductive biology, each of which is composed of roughly spherical aggregations of cells found in the ovary. They contain a single oocyte . These structures are periodically initiated to grow and develop, culminating in ovulation of usually a single...

s are stimulated. These follicles, which were present at birth and have been developing for the better part of a year in a process known as folliculogenesis
Folliculogenesis
In biology, folliculogenesis is the maturation of the ovarian follicle, a densely-packed shell of somatic cells that contains an immature oocyte...

, compete with each other for dominance. Under the influence of several hormones, all but one of these follicles will stop growing, while one dominant follicle in the ovary will continue to maturity. The follicle that reaches maturity is called a tertiary, or Graafian, follicle, and it forms the ovum.

As they mature, the follicles secrete increasing amounts of estradiol
Estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

, an estrogen
Estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

. The estrogens initiate the formation of a new layer of endometrium
Endometrium
-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...

 in the uterus, histologically identified as the proliferative endometrium. The estrogen also stimulates crypts
Crypt (anatomy)
Crypts are anatomical structures that are narrow but deep invaginations into a larger structure.One common type of anatomical crypt is the Crypts of Lieberkühn. However, it is not the only type: some types of tonsils also have crypts...

 in the cervix
Cervix
The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall...

 to produce fertile cervical mucus, which may be noticed by women practicing fertility awareness
Fertility awareness
Fertility awareness refers to a set of practices used to determine the fertile and infertile phases of a woman's menstrual cycle. Fertility awareness methods may be used to avoid pregnancy, to achieve pregnancy, or as a way to monitor gynecological health....

.

Ovulation

During the follicular phase, estradiol
Estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

 suppresses production of luteinizing hormone
Luteinizing hormone
Luteinizing hormone is a hormone produced by the anterior pituitary gland. In females, an acute rise of LH called the LH surge triggers ovulation and development of the corpus luteum. In males, where LH had also been called interstitial cell-stimulating hormone , it stimulates Leydig cell...

 (LH) from the anterior pituitary gland. When the egg has nearly matured, levels of estradiol
Estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

 reach a threshold above which they stimulate production of LH. These opposite responses of LH to estradiol may be enabled by the presence of two different estrogen receptors in the hypothalamus: estrogen receptor alpha
Estrogen receptor alpha
Estrogen receptor alpha , also known as NR3A1 , is a nuclear receptor that is activated by the sex hormone estrogen...

, which is responsible for the negative feedback estradiol-LH loop, and estrogen receptor beta
Estrogen receptor beta
Estrogen receptor beta , also known as NR3A2 , is a nuclear receptor which is activated by the sex hormone estrogen. In humans, ER-β is encoded by the ESR2 gene.- Function :...

, which is responsible for the positive estradiol-LH relationship. 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, causing the fully developed follicle to release its secondary oocyte. The secondary oocyte promptly matures into an ootid and then becomes a mature ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

. The mature ovum has a diameter of about 0.2 mm.

Which of the two ovaries—left or right—ovulates appears essentially random; no known left and right co-ordination exists. Occasionally, both ovaries will release an egg; if both eggs are fertilized, the result is fraternal twins.

After being released from the ovary and into the peritoneal space, the egg is swept into the fallopian tube
Fallopian tube
The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges are two very fine tubes lined with ciliated epithelia, leading from the ovaries of female mammals into the uterus, via the utero-tubal junction...

 by the fimbria
Fimbria (female reproductive system)
In the female reproductive system, the fimbria is a fringe of tissue around the ostium of the Fallopian tube, in the direction of the ovary....

, which is a fringe of tissue at the end of each fallopian tube. After about a day, an unfertilized egg will disintegrate or dissolve in the fallopian tube.

Fertilization by a spermatozoon
Spermatozoon
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

, when it occurs, usually takes place in the ampulla
Ampulla of uterine tube
The ampulla is the second portion of the uterine tube. It is an intermediate dilated portion, which curves over the ovary.It is the most common site of human fertilization.The word ampulla is from Latin for flask....

, the widest section of the fallopian tubes. A fertilized egg immediately begins the process of embryogenesis
Human embryogenesis
Human embryology is the study of human development during the first eight weeks from gametogenesis pre-conception through fertilization up to and including the 8th week after implantation of the zygote in the uterus...

, or development. The developing embryo takes about three days to reach the uterus and another three days to implant into the endometrium. It has usually reached the blastocyst
Blastocyst
The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...

 stage at the time of implantation.

In some women, ovulation features a characteristic pain called mittelschmerz
Mittelschmerz
Mittelschmerz is a medical term for "ovulation pain" or "midcycle pain". About 20% of women experience mittelschmerz, some every cycle, some intermittently.-Symptoms and diagnosis:...

(German term meaning middle pain). The sudden change in hormones at the time of ovulation sometimes also causes light mid-cycle blood flow.

Luteal phase

The luteal phase is also called the secretory phase. An important role is played by the corpus luteum
Corpus luteum
The corpus luteum is a temporary endocrine structure in mammals, involved in production of relatively high levels of progesterone and moderate levels of estradiol and inhibin A...

, the solid body formed in an ovary after the egg has been released from the ovary into the fallopian tube. This body continues to grow for some time after ovulation and produces significant amounts of hormones, particularly progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...

. Progesterone plays a vital role in making the endometrium
Endometrium
-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...

 receptive to implantation
Implantation
In humans, implantation is the beginning of pregnancy, in which the embryo Adheres to the wall of the uterus. At this stage of prenatal development, the embryo is a blastocyst...

 of the blastocyst
Blastocyst
The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...

 and supportive of the early pregnancy; it also has the side effect of raising the woman's basal body temperature
Basal body temperature
Basal body temperature is the lowest temperature attained by the body during rest . It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature...

. There is a noted secretion of prolactin towards the end of the secretory phase.

After ovulation, the pituitary hormones
Pituitary gland
In vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...

 FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone. The increased progesterone in the adrenals starts to induce the production of estrogen. The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. Consequently, the level of FSH and LH fall quickly over time, and the corpus luteum subsequently atrophies. Falling levels of progesterone trigger menstruation and the beginning of the next cycle. From the time of ovulation until progesterone withdrawal has caused menstruation to begin, the process typically takes about two weeks, with 14 days considered normal. For an individual woman, the follicular phase often varies in length from cycle to cycle; by contrast, the length of her luteal phase will be fairly consistent from cycle to cycle.

The loss of the corpus luteum can be prevented by fertilization of the egg; the resulting embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 produces human chorionic gonadotropin
Human chorionic gonadotropin
Human chorionic gonadotropin or human chorionic gonadotrophin is a glycoprotein hormone produced during pregnancy that is made by the developing embryo after conception and later by the syncytiotrophoblast .. Some tumors make this hormone; measured elevated levels when the patient is not...

 (hCG), which is very similar to LH and which can preserve the corpus luteum. Because the hormone is unique to the embryo, most pregnancy test
Pregnancy test
A pregnancy test attempts to determine whether or not a woman is pregnant.These markers are found in urine and blood, and pregnancy tests require sampling one of these substances. The first of these markers to be discovered, human chorionic gonadotropin , was discovered in 1930 to be produced by...

s look for the presence of hCG.

Length

Although many people believe the average menstrual cycle takes 28 days, a large study of more than 30,000 cycles from more than 2300 women showed that the mean cycle length was 29.1 with a standard deviation
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...

 of 7.5 days and a 95% prediction interval
Prediction interval
In statistical inference, specifically predictive inference, a prediction interval is an estimate of an interval in which future observations will fall, with a certain probability, given what has already been observed...

 of between 15 and 45 days. In that study, the subset of data with cycle lengths between 15 and 45 days had an average length of 28.1 days with a standard deviation of 4 days. A smaller study of 140 women performed in 2006 found a mean cycle length of 28.9 days.

The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 35 to 39. Subsequently, the variability increases slightly for women aged 40 to 44. Usually, length variation between eight and 20 days in a woman is considered as moderately irregular menstrual cycles. Variation of 21 days or more is considered very irregular.

It has long been thought that cycle length is associated with the moon. A 1979 study of 305 women found that approximately one-third of the subjects had lunar period cycles, i.e., a mean cycle length of 29.5 days plus or minus 1 day. Almost two-thirds of the subjects started their cycle in the brighter half of the lunar cycle, significantly more than would be expected by random distribution. Another study found a statistically significant number of menstruations occurred around the new moon.

Fertile window

The most fertile period (the time with the highest likelihood of pregnancy resulting from sexual intercourse
Sexual intercourse
Sexual intercourse, also known as copulation or coitus, commonly refers to the act in which a male's penis enters a female's vagina for the purposes of sexual pleasure or reproduction. The entities may be of opposite sexes, or they may be hermaphroditic, as is the case with snails...

 covers the time from some 5 days before until 1–2 days after ovulation. In a 28 day cycle with a 14-day luteal phase, this corresponds to the second and the beginning of the third week. A variety of methods have been developed to help individual women estimate the relatively fertile and the relatively infertile days in the cycle: these systems are called fertility awareness
Fertility awareness
Fertility awareness refers to a set of practices used to determine the fertile and infertile phases of a woman's menstrual cycle. Fertility awareness methods may be used to avoid pregnancy, to achieve pregnancy, or as a way to monitor gynecological health....

.

Fertility awareness methods that rely on cycle length records alone are called calendar-based methods. Methods that require observation of one or more of the three primary fertility signs (basal body temperature
Basal body temperature
Basal body temperature is the lowest temperature attained by the body during rest . It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature...

, cervical mucus, and cervical position) are known as symptoms-based methods. Urine test
Urinalysis
A urinalysis , also known as Routine and Microscopy , is an array of tests performed on urine, and one of the most common methods of medical diagnosis...

 kits are available that detect the LH surge that occurs 24 to 36 hours before ovulation; these are known as ovulation predictor kits (OPKs). Computerized devices that interpret basal body temperatures, urinary test results, or changes in saliva are called fertility monitors.

A woman's fertility is also affected by her age. 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
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...

 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. However, despite this hypothesis, a similar paternal age effect
Paternal age effect
The paternal age effect can refer to the statistical relationships of: a man's age to sperm and semen abnormalities; a man's age to his fertility; a man's age to adverse pregnancy outcomes in his female partner ; a father's age at the birth of his offspring on the probability of an adverse...

 has also been observed.

Effect on other systems

Some women with neurological conditions
Neurology
Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...

 experience increased activity of their conditions at about the same time during each menstrual cycle. For example, drops in estrogen levels have been known to trigger migraines (a neurological syndrome, migraines), especially when the woman who suffers migraines is also taking the birth control pill. Many women with epilepsy
Epilepsy
Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...

 have more seizures in a pattern linked to the menstrual cycle; this is called "catamenial epilepsy
Catamenial epilepsy
Catamenial epilepsy is a subtype of epilepsy, which is a chronic neurological condition characterized by recurrent seizures. Catamenial epilepsy is a subset of this population, which includes women of whom their seizure exacerbation is aligned with their menstrual cycle. Women with catamenial...

". Different patterns seem to exist (such as seizures coinciding with the time of menstruation, or coinciding with the time of ovulation), and the frequency with which they occur has not been firmly established. Using one particular definition, one group of scientists found that around one-third of women with intractable partial epilepsy have catamenial epilepsy. An effect of hormones has been proposed, in which progesterone declines and estrogen increases would trigger seizures. Recently, studies have shown that high doses of estrogen can cause or worsen seizures, whereas high doses of progestrone can act like an antiepileptic drug. Studies by medical journals have found that women experiencing menses are 1.68 times more likely to commit suicide
Suicide
Suicide is the act of intentionally causing one's own death. Suicide is often committed out of despair or attributed to some underlying mental disorder, such as depression, bipolar disorder, schizophrenia, alcoholism, or drug abuse...

.

Mice have been used as an experimental system to investigate possible mechanisms by which levels of sex steroid
Sex steroid
Sex steroids, also known as gonadal steroids, are steroid hormones that interact with vertebrate androgen or estrogen receptors. Their effects are mediated by slow genomic mechanisms through nuclear receptors as well as by fast nongenomic mechanisms through membrane-associated receptors and...

 hormones might regulate nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

 function. During the part of the mouse estrous cycle when progesterone is highest, the level of nerve-cell
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

 GABA receptor
GABA A receptor
The GABAA receptor is an ionotropic receptor and ligand-gated ion channel. Its endogenous ligand is γ-aminobutyric acid , the major inhibitory neurotransmitter in the central nervous system. Upon activation, the GABAA receptor selectively conducts Cl- through its pore, resulting in...

 subtype delta was high. Since these GABA receptors are inhibitory
Inhibitory postsynaptic potential
An inhibitory postsynaptic potential is a synaptic potential that decreases the chance that a future action potential will occur in a postsynaptic neuron or α-motoneuron...

, 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.

Estrogen levels may affect thyroid
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...

 behavior. For example, during the luteal phase (when estrogen levels are lower), the velocity of blood flow in the thyroid is lower than during the follicular phase (when estrogen levels are higher).

Among women living closely together, it was once thought that the onsets of menstruation tend to synchronize. This effect was first described in 1971, and possibly explained by the action of pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

s in 1998. Subsequent research has called this hypothesis into question.

Cycle abnormalities and disorders



Infrequent or irregular ovulation is called oligoovulation. The absence of ovulation is called anovulation
Anovulation
An anovulatory cycle is a menstrual cycle during which the ovaries do not release an oocyte. Therefore, ovulation does not take place. However, a woman who does not ovulate at each menstrual cycle is not necessarily going through menopause...

. Normal menstrual flow can occur without ovulation preceding it: an anovulatory cycle. In some cycles, follicular development may start but not be completed; nevertheless, estrogens will form and will stimulate the uterine lining. Anovulatory flow resulting from a very thick endometrium caused by prolonged, continued high estrogen levels is called estrogen breakthrough bleeding. Anovulatory bleeding triggered by a sudden drop in estrogen levels is called changes. Anovulatory cycles commonly occur before menopause
Menopause
Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...

 (perimenopause) and in women with polycystic ovary syndrome
Polycystic ovary syndrome
Polycystic ovary syndrome is one of the most common female endocrine disorders. PCOS is a complex, heterogeneous disorder of uncertain aetiology, but there is strong evidence that it can to a large degree be classified as a genetic disease....

.

Very little flow (less than 10 ml) is called hypomenorrhea
Hypomenorrhea
Hypomenorrhea or hypomenorrhoea, also known as short or scanty periods, is extremely light menstrual blood flow. It is the opposite of hypermenorrhea which is more properly called menorrhagia.-Overview:...

. Regular cycles with intervals of 21 days or fewer are polymenorrhea; frequent but irregular menstruation is known as metrorrhagia
Metrorrhagia
Metrorrhagia, also known as breakthrough bleeding or spotting, is uterine bleeding at irregular intervals, particularly between the expected menstrual periods.In some women, menstrual spotting between periods occurs as a normal and harmless part of ovulation...

. Sudden heavy flows or amounts greater than 80 ml are termed menorrhagia
Menorrhagia
Menorrhagia is an abnormally heavy and prolonged menstrual period at regular intervals. Causes may be due to abnormal blood clotting, disruption of normal hormonal regulation of periods or disorders of the endometrial lining of the uterus...

. Heavy menstruation that occurs frequently and irregularly is menometrorrhagia
Menometrorrhagia
Menometrorrhagia is a condition in which prolonged or excessive uterine bleeding occurs irregularly and more frequently than normal.-Causes:It can occur due to any of several causes, including hormonal imbalance, endometriosis, uterine fibroids, or cancer. It can lead to anemia in long-standing...

. The term for cycles with intervals exceeding 35 days is oligomenorrhea
Oligomenorrhea
Oligomenorrhea is infrequent menstruation. More strictly, it is menstrual periods occurring at intervals of greater than 35 days, with only four to nine periods in a year. Also, menstrual periods should have been regularly established previously before the development of infrequent flow...

. Amenorrhea refers to more than three to six months without menses (while not being pregnant) during a woman's reproductive years.

Hormonal contraception

While some forms of birth control
Birth control
Birth control is an umbrella term for several techniques and methods used to prevent fertilization or to interrupt pregnancy at various stages. Birth control techniques and methods include contraception , contragestion and abortion...

 do not affect the menstrual cycle, hormonal contraceptives work by disrupting it. Progestogen negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

 decreases the pulse frequency of gonadotropin-releasing hormone
Gonadotropin-releasing hormone
Gonadotropin-releasing hormone , also known as Luteinizing-hormone-releasing hormone and luliberin, is a tropic peptide hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the anterior pituitary. GnRH is synthesized and released from neurons within...

 (GnRH) release by the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

, which decreases the release of follicle-stimulating hormone
Follicle-stimulating hormone
Follicle-stimulating hormone is a hormone found in humans and other animals. It is synthesized and secreted by gonadotrophs of the anterior pituitary gland. FSH regulates the development, growth, pubertal maturation, and reproductive processes of the body. FSH and Luteinizing hormone act...

 (FSH) and luteinizing hormone
Luteinizing hormone
Luteinizing hormone is a hormone produced by the anterior pituitary gland. In females, an acute rise of LH called the LH surge triggers ovulation and development of the corpus luteum. In males, where LH had also been called interstitial cell-stimulating hormone , it stimulates Leydig cell...

 (LH) by the anterior pituitary
Anterior pituitary
A major organ of the endocrine system, the anterior pituitary, also called the adenohypophysis, is the glandular, anterior lobe of the pituitary gland...

. Decreased levels of FSH inhibit follicular development, preventing an increase in estradiol
Estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

 levels. Progestogen negative feedback and the lack of estrogen positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

 on LH release prevent a mid-cycle LH surge. Inhibition of follicular development and the absence of a LH surge prevent ovulation.

The degree of ovulation suppression in progestogen-only contraceptives depends on the progestogen activity and dose. Low dose progestogen-only contraceptives—traditional progestogen only pills, subdermal implant
Subdermal implant
A subdermal implant refers to a kind of body jewelry that is placed underneath the skin, therefore allowing the body to heal over the implant and creating a raised design. These kinds of implants fall under the broad category of body modification. Many people who have these implants use them in...

s Norplant
Norplant
Norplant is a form of birth control developed by the Population Council that was first approved in 1983 in Finland, where it was manufactured by Leiras Oy Pharmaceuticals...

 and Jadelle, and intrauterine system
IntraUterine System
The IUD with progestogen, intrauterine system , or IntraUterine Contraceptive is a long-acting reversible hormonal contraceptive device that is placed in the uterus. An IUS has a hormone cylinder that releases a progestin called levonorgestrel...

 Mirena—inhibit ovulation in ~50% of cycles and rely mainly on other effects, such as thickening of cervical mucus, for their contraceptive effectiveness. Intermediate dose progestogen-only contraceptives—the progestogen-only pill Cerazette and the subdermal implant Implanon
Implanon
Implanon, made by Organon International, is a single-rod long acting reversible hormonal contraceptive subdermal implant that is inserted just under the skin of a woman's upper arm. The 4 cm by 2 mm Implanon rod contains 68 milligrams of etonogestrel which is released over a three year...

—allow some follicular development but more consistently inhibit ovulation in 97–99% of cycles. The same cervical mucus changes occur as with very low dose progestogens. High dose progestogen-only contraceptives—the injectables Depo-Provera and Noristerat—completely inhibit follicular development and ovulation.

Combined hormonal contraceptives include both an estrogen and a progestogen. Estrogen negative feedback on the anterior pituitary greatly decreases the release of FSH, which makes combined hormonal contraceptives more effective at inhibiting follicular development and preventing ovulation. Estrogen also reduces the incidence of irregular breakthrough bleeding
Breakthrough bleeding
Breakthrough bleeding is an abnormal uterine bleeding that occurs between menstrual periods, especially due to irregular sloughing of the endometrium in women on combined oral contraceptives...

. Several combined hormonal contraceptives—the pill, NuvaRing
NuvaRing
NuvaRing is the trade name for a combined hormonal contraceptive vaginal ring manufactured by Merck that is available by prescription...

, and the contraceptive patch
Contraceptive patch
A contraceptive patch is a transdermal patch applied to the skin that releases synthetic estrogen and progestin hormones to prevent pregnancy. They have been shown to be as effective as the combined oral contraceptive pill with perfect use, and the patch may be more effective in typical use.The...

—are usually used in a way that causes regular withdrawal bleeding. In a normal cycle, menstruation
Menstruation
Menstruation is the shedding of the uterine lining . It occurs on a regular basis in sexually reproductive-age females of certain mammal species. This article focuses on human menstruation.-Overview:...

 occurs when estrogen and progesterone levels drop rapidly. Temporarily discontinuing use of combined hormonal contraceptives (a placebo week, not using patch or ring for a week) has a similar effect of causing the uterine lining to shed. If withdrawal bleeding is not desired, combined hormonal contraceptives may be taken continuously, although this increases the risk of breakthrough bleeding.

Lactational amenorrhea

Breastfeeding
Breastfeeding
Breastfeeding is the feeding of an infant or young child with breast milk directly from female human breasts rather than from a baby bottle or other container. Babies have a sucking reflex that enables them to suck and swallow milk. It is recommended that mothers breastfeed for six months or...

 causes negative feedback to occur on pulse secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). Depending on the strength of the negative feedback, breastfeeding women may experience complete suppression of follicular development, follicular development but no ovulation, or normal menstrual cycles may resume. Suppression of ovulation is more likely when suckling occurs more frequently. The production of prolactin
Prolactin
Prolactin also known as luteotropic hormone is a protein that in humans is encoded by the PRL gene.Prolactin is a peptide hormone discovered by Henry Friesen...

 in response to suckling is important to maintaining lactational amenorrhea. On average, women who are fully breastfeeding whose infants suckle frequently experience a return of menstruation at fourteen and a half months postpartum. There is a wide range of response between individual breastfeeding women, however, with some experiencing return of menstruation at two months and others remaining amenorrheic for up to 42 months postpartum.

Nightlighting and the moon

The word "menstruation" is etymologically related to "moon". The terms "menstruation" and "menses" are derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 mensis (month), which in turn relates to the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 mene (moon) and to the roots of the English words month and moon.

Some authors believe historically, women in traditional societies without nightlighting ovulated with the full moon
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...

 and menstruated with the new moon, and one author documents the controversial attempts to use the association to improve the Rhythm Method
Rhythm Method
Calendar-based methods are various methods of estimating a woman's likelihood of fertility, based on a record of the length of previous menstrual cycles. Various systems are known as the Knaus–Ogino Method, rhythm method, and Standard Days Method...

 of regulating conception.

A few studies in both humans and other 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). It has also been suggested that bright light exposure in the morning promotes more regular cycles. One author has suggested that sensitivity of women's cycles to nightlighting is caused by nutritional deficiencies
Illnesses related to poor nutrition
Nutritional diseases are diseases in humans that are directly or indirectly caused by a lack of essential nutrients in the diet. Nutritional diseases are commonly associated with chronic malnutrition. Additionally, conditions such as obesity from overeating can also cause, or contribute to, serious...

 of certain vitamins and minerals.

Some studies show a correlation between the human menstrual cycle and the lunar cycle, while a meta-analysis of studies from 1996 showed no correlation. Dogon villagers
Dogon people
The Dogon are an ethnic group living in the central plateau region of Mali, south of the Niger bend near the city of Bandiagara in the Mopti region. The population numbers between 400,000 and 800,000 The Dogon are best known for their religious traditions, their mask dances, wooden sculpture and...

did not have electric lighting and spent most nights outdoors, talking and sleeping; so they were an ideal population for detecting a lunar influence: none, however, was found.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK