Menarche

From Wikipedia, the free encyclopedia

Menarche (IPA: /ˈmɛˌnɑɹ.ki/) is the first menstrual period, or first menstrual bleeding. From both social and medical perspectives it is often considered the central event of female puberty, as it signals the possibility of fertility. Timing of menarche is influenced by both genetic and environmental factors, especially nutritional status. The average age of menarche has declined over the last century but the magnitude of the decline and the factors responsible remain subjects of contention. The average age of menarche in the United States is about 12 years and 6 months. To maintain a regular menstrual cycle, fat content of the body must be 22% or greater.

Contents

[edit] Physiologic aspects

[edit] Menarche as part of puberty

Menarche is the culmination of a series of physiological and anatomic processes of puberty, described in more detail in that article:

  • Attainment of a sufficient body mass (typically 17% body fat PMID 3117838).
  • Disinhibition of the GnRH pulse generator in the arcuate nucleus of the hypothalamus
  • Secretion of estrogen by the ovaries begins in response to pituitary hormones.
  • Over an interval of about 2 to 3 years, estrogen stimulates growth of the uterus (as well as height growth, breast growth, widening of the pelvis, and increased regional adipose tissue).
  • Estrogen stimulates growth and vascularity of the endometrium, the lining of the uterus.
  • Fluctuations of hormone levels can result in changes of adequacy of blood supply to parts of the endometrium.
  • Death of some of the endometrial tissue from these hormone or blood supply fluctuations leads to deciduation, a sloughing of part of the lining with some bleeding from the vagina.

A specific hormonal signal for menarche is not known; menarche as a discrete event is thought to be the relatively chance result of the gradual thickening of the endometrium induced by rising but fluctuating pubertal estrogen.

The menstruum, or "flow", consists of a combination of fresh and clotted blood with endometrial tissue. The initial flow of menarche may be brighter red than mature menstrual flow. It is often scanty in amount and may be very brief, even a single instance of "spotting". Like other menses, menarche may be accompanied by cramping.

[edit] Menarche and fertility

In most girls, menarche does not signal that ovulation has occurred. Studies of American girls suggest that the average interval between menarche and ovulation is several months. Irregular, anovulatory menses commonly occur for 1-2 years or more before regular ovulation is established.

Regular ovulation is usually indicated by predictable and consistent intervals between menses, predictable and consistent durations of menses, and predictable and consistent patterns of flow (e.g., heaviness or cramping). Continuing ovulation typically requires a body fat content of at least 22%. An anthropological term for this state of potential fertility is nubility.

On the other hand, not every girl follows the typical pattern and some girls have ovulated before the first menstruation. Although unlikely, it is possible for a girl engaging in coitus shortly before her menarche would occur to conceive and become pregnant, delaying her menarche until after the birth. This would be a rare exception to the widely held assumption that a woman cannot become pregnant until after menarche.

[edit] Influences on timing of menarche

Menarche usually occurs about two years after the first changes of breast development (thelarche), typically when the body fat content has exceeded 17%. A wide range of genetic, nutritional, social, and other environmental factors, can influence the timing of puberty, and many research studies have used menarche as a retrievable index of timing of puberty.

[edit] Effects of stress and social environment on timing of menarche

Some of the least understood environmental influences on timing of puberty are social and psychological. Nearly all of the research on these effects has concerned girls, partly because female puberty requires greater physiologic resources and partly because it involves a unique event (menarche) that makes survey research into female puberty much simpler than male. In most of these studies menarche was specifically examined, assuming it to be a valid "proxy" for the more general process of puberty. In comparison with the effects of genetics, nutrition, and general health, social influences are small, shifting timing by a few months rather than years. The most important part of a child's psychosocial environment is the family.

Some of the aspects of family structure and function reported to be independently associated with earlier menarche:

  • The increased incidence of childhood obesity (reaching a body weight of 100 lbs. appears to be an important "trigger" for the very young reaching menarche--both estrogen and progesterone are fat-soluble hormones).
  • Absence of father from the home from early childhood
  • Presence of stepfather or other genetically unrelated males in the home from early childhood
  • Prolonged childhood sexual abuse
  • High conflict family relationships
  • Being adopted from an underdeveloped country into an affluent home
  • Living in an urban environment

Some of the aspects of family structure and function reported to be independently associated with later puberty:

  • Larger family size
  • Warmer, closer or more positive relationship with biological father
  • Warmer, more supportive, low stress family environment
  • Having a number of older sisters

Other research has focused on the effect of childhood stress on timing of puberty, especially female. Stress is a vague term and studies have examined conditions ranging from family tensions or conflict to wartime refugee status with threat to physical survival. The more dire social conditions have been found to be associated with delay of maturation, an effect that may be compounded by dietary inadequacy. There is more uncertainty and mixed evidence as to whether milder degrees of stress or early-life undernutrition can accelerate puberty in girls as would be predicted by life history theory and demonstrated in many other mammals.

Our understanding of these environmental effects is incomplete and the following observations and cautions are relevant:

  • Mechanisms of these social effects are unknown, though a variety of physiological processes, including pheromones, have been suggested based on animal research.
  • Most of these "effects" are statistical associations revealed by epidemiologic surveys. Statistical associations are not necessarily causal, and a variety of covariables and alternative explanations can be imagined. Effects of such small size can never be confirmed or refuted for any individual child.
  • Despite the small magnitude of effect, interpretations of the data are politically controversial because of the ease with which this type of research can be used for political advocacy. Accusations of bias based on political agenda sometimes accompany scientific criticism.

[edit] Changes over time in the average age of menarche

There were few systematic studies of timing of menarche before the latter half of the 20th century. Most older estimates of average timing of menarche were based on observation of a small homogeneous population not necessarily representative of the larger population, or based on recall by adult women, which is also susceptible to various forms of error. Most sources agree that average age of menarche in girls in modern societies has declined, though the reasons and the degree remain subjects of controversy. There have been claims of a 2 to 2.5 year decline from about 1900 to the 1960s, but the best North American surveys reported only a 2-3 month decline from the mid-1970s to the mid-1990s. This is commonly attributed to larger body size and earlier average attainment of sufficient body fat, but other factors such as environmental exposure to chemicals that mimic estrogen or the urbanization or "sexualization" of Western society have also been offered by some.

Less than 10% of US girls start to menstruate before 11 years, and 90% of all US girls are menstruating by 13.75 years of age, with a median age of 12.43 years. This age at menarche is not significantly different (0.34 years earlier) than that reported for US girls in 1973. Age at menarche for non-Hispanic black girls was significantly earlier than that of white girls at 10%, 25%, and 50% of those who had attained menarche, whereas Mexican American girls were only significantly earlier than the white girls at 25%. [1]

[edit] Cultural aspects of menarche

Menarche is celebrated in many cultures around the world as a time to recognise that a girl is moving into womanhood. It may be considered a rite of passage. Some traditions mark this event with gifts of red articles or a meal of special symbolic foods. You can see below at the Rites of Passage section for examples.

[edit] Medical aspects of menarche

When menarche occurs, it confirms that the girl has had a gradual estrogen-induced growth of the uterus, especially the endometrium, and that the "outflow tract" from the uterus, through the cervix to the vagina, is open.

In very rare instances, menarche may occur at an unusually early age, preceding thelarche and other signs of puberty. This is termed isolated premature menarche, but other causes of bleeding must be investigated and excluded. Growth is usually normal. [2] Isolated premature menarche is rarely the first manifestation of precocious puberty.

When menarche has failed to occur for more than 3 years after thelarche, or past 14.5 years of age, the delay is referred to as primary amenorrhea.

[edit] Rites of Passage

Some cultures have in past centuries have had rites of passage for a girl experiencing menarche.

  • In the U.S.A., rites of passage are rare since girls are taught to keep aspects of sexual development private. However, some families change that by giving the girl a card of congratulations, or even a candle lit ceremony.
  • The Navajo Indians had a celebration called kinaalda (kinn-all-duh). Girls run footraces to show strength. A cornmeal pudding is made for the tribe to taste. The girls who experience menarche wear special clothes and style their hair like the Navajo goddess "Changing Woman"
  • The Nootka Indians thought menarche was a time for a physical strength test. The girl is taken out to sea and left there. She is to swim back and when she returns to the shore of the village, she is cheered.
  • The Mescalero Apaches consider their menarche celebration the most important. Each year, an 8-day-long ceremony is celebrated in honor of each girl who began their period that year. The first 4 days include of feasting and dancing. Boy singers recount the history of the tribe each evening. The other four days are a private celebration where girls have a private ceremony, reflecting on passing into womanhood.
  • In Australia, the Aborigines treated a girl to "love magic". The women teach her of the female powers and the physical changes marking womanhood.
  • When a Japanese girl gets her period, a big party is thrown. Family and friends are invited. However, they are not told of why they are celebrating until a decorated pear, candied apple, or red colored rice and beans are brought to the girl on a tray.
  • The Ulithi (oo-lith-ee) tribe of Micronesia call a girl's menarche kufar (koo-faar). She goes to a menstrual house where the women bathe her and recite spells. The girl will go back to the menstrual house when her next period comes.
  • In Nigeria, the Tiv tribe really marks a girl. Fours lines are cut in her abdomen, being thought to make her a woman and more fertile.
  • Sri Lanka notes the time and day. An astrologer is contacted, whom studies the star's alignment at the noted moment. This is done to predict the girl's future. Her house is prepared for a ritual bathing, where the girl is scrubbed all over by the women of the family. She then is dressed in white. Printed invites for a party are sent out, where the girl receives money and special gifts.

[edit] See also

[edit] References

  1. ^ Chumlea WC, Schubert CM, Roche AF, Kulin HE, Lee PA, Himes JH, Sun SS (2003 Jan). "Age at menarche and racial comparisons in US girls., 2005. PMID 12509562". Lifespan Health Research Center, Department of Community Health, Wright State University, School of Medicine, Dayton, Ohio, USA.
  2. ^ R. Stanhope, C. Traggiai (2006). Isolated Menarche. Precocious Puberty (Complete, Partial). Retrieved on 2006-11-26.

[edit] External links

Retrieved from "http://en.wikipedia.org../../../m/e/n/Menarche.html"