|
|
|
|
Menarche
|
| |
|
| |
Menarche (American English: ; British English: ) is the first menstrual period, or first menstrual bleeding in the females of 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. 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 12.5 years and 13.06 ± 0.10 years in Iceland.
Discussion
Ask a question about 'Menarche' Start a new discussion about 'Menarche' Answer questions from other users |
Encyclopedia
Menarche (American English: ; British English: ) is the first menstrual period, or first menstrual bleeding in the females of 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. 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 12.5 years and 13.06 ± 0.10 years in Iceland.
Physiology
Menarche as part of puberty
Menarche is the culmination of a series of physiological and anatomic processes of puberty:
- 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 oestrogen by the ovaries in response to pituitary hormones.
- Over an interval of about 2 to 3 years, oestrogen stimulates growth of the uterus (as well as height growth, breast growth, widening of the pelvis, and increased regional adipose tissue).
- Oestrogen 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 blood flow 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 oestrogen.
The menstruum, or "flow," consists of a combination of fresh and clotted blood with endometrial tissue. The initial flow of menarche is usually 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 abdominal cramping.
Menarche and fertility
In most girls, menarche does not signal that ovulation has occurred. In postmenarchal girls, about 80% of the cycles were anovulatory in the first year after menarche, 50% in the third and 10% in the sixth year.
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 ovulate before the first menstruation. Although unlikely, it is possible for a girl who has engaged in sexual intercourse shortly before her menarche to conceive and become pregnant, which would delay her menarche until after the birth. This goes against the widely held assumption that a woman cannot become pregnant until after menarche.
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 physiological 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 or 45 kg appears to be an important "trigger" for the very young reaching menarche--both oestrogen and progesterone molecules are derived from cholesterol).
- Absence of father from the home from early childhood
- High-conflict family relationships
Some of the aspects of family structure and function reported to be independently associated with later menarche:
- 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.
The 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 secondary variables and alternative explanations can be possibly intervening. 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.
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 the 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 oestrogen or the urbanization and sexualization of Western society have also been considered as contributing factors.
Less than 10% of U.S. girls start to menstruate before 11 years of age, 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 U.S. 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%.
Cultural aspects of menarche
Menarche is celebrated in many cultures around the world as a rite of passage, a time to recognize that a girl is moving into womanhood.
Rites of passage
Some cultures have in past centuries have had rites of passage for a girl experiencing menarche.
Africa
- In the Tiv tribe in Nigeria, four lines were cut in the girl's abdomen, the practice of which was thought to make her a woman and more fertile.
Asia, Australia, and Pacific Islands
- In Australia, the Aborigines treat a girl to "love magic." The women teach her of the female powers and the physical changes marking womanhood.
- When a Japanese girl has her first period, the family celebrates by eating red-colored rice and beans.
- 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 then returns to the menstrual house when her next period comes.
- Sri Lanka notes the time and day. An astrologer is contacted, who studies the star's alignment at the noted moment, in order to predict the girl's future. Her house is prepared for a ritual bathing, where the girl is scrubbed all over her body by the women of the family; she then is dressed in white. Printed invitations for a party are sent out, during which the girl receives money and special gifts.
- In rural India, a girl who has reached puberty is given a ceremonial bath, decked with ornate jewels and garments, and the girl's kith and kin are all invited for a ceremony, in which it is announced that the girl has come of age and that celebrations follow. The tradition is quite nonexistent in urban India, although some families still hold on to this tradition.
- In Nepal, Kumaris are young girls worshiped as goddesses by Hindus and some Buddhists, at the onset of menarche it is believed the goddess spirit vacates her body and they are returned to ordinary life after a series of rituals.
North America
- In the United States, rites of passage are rare, since girls are usually taught to keep aspects of sexual development private. Some families celebrate by giving the girl a card of congratulations, or even a candlelight ceremony.
- The Navajo Indians have 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 believe menarche to be a time for a physical strength test; the girl is taken out to sea and left alone. She is to swim back and is cheered upon returning to the shore of the village.
- 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 her period earlier that year. The first four days include feasting and dancing. Boy singers recount the history of the tribe each evening. The other four days are a private celebration during which girls have a private ceremony, reflecting on their passing into womanhood.
Medical aspects of menarche
When menarche occurs, it confirms that the girl has had a gradual oestrogen-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. 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 beyond 16 years of age, the delay is referred to as primary amenorrhea.
See also
Further reading
- Quinlan, R. (2003). Father-Absence, Parental Care & Female Reproductive Development. Evolution & Human Behavior. 24(6): 376-390.
External links
- Mark Hanson, P. Gluckman. , Trends in Endocrinology & Metabolism, January 2006.
- Discusses some of the social influences
|
| |
|
|
