Menarche

Menarche 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. Menarche is the most commonly remembered milestone of puberty for most women. Menarche can occur within a wide range of ages, and the timing is influenced by both genetic and environmental factors. The average age of menarche in the United Statesis about 12 years and 8 months, which is a few months earlier than fifty years ago.

Physiologic aspects

Menarche as part of puberty

Menarche is the culmination of a series of physiological and anatomic processes of puberty:

• Secretion of estrogenby the ovariesbegins in response to pituitaryhormones.
• Over an interval of about 2 to 3 years, estrogen stimulates growth of the uterus(as well as height growth, breastgrowth, widening of the pelvis, and increased regional adiposetissue).
• 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 "sluffing" 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".

Menarche and fertility

In most girls, menarche does not signal that ovulationhas 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). An anthropologicalterm 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 coitusshortly before her menarche to conceiveand become pregnant, delaying her menarche until after the birth. The widely held assumption that a woman cannot become pregnant until after menarche is usually but not always true.

Influences on timing of menarche

Menarche usually occurs about two years after the first changes of breast development (thelarche).

Over the centuries, menarche has occurred younger, jsut as people have grown taller, due to improved diet and health.

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 female menarche:

• Absence of father from the home frome early childhood
• Presence of stepfather or other genetically unrelated males in the home from early childhood
• Prolonged childhood sexual abuse
• High conflict famly relationships
• Being adopted from an underdeveloped country into a affluent home

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 environmnent

Other research has focused on the effect of childhood stresson 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 theoryand 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 epidemiologicsurveys. 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.

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.

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 cervixto the vagina, is open.

In very rare instances, menarche may occur at an unusually early age, preceding thelarcheand other signs of puberty. This is termed isolated premature menarche, but other causes of bleeding must be investigated and excluded.

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.

See also

• Puberty
• Delayed puberty
• Precocious puberty
• Thelarche
• Adrenarche
• Pubarche
• Gonadarche
• Menstrual cycle

External links

• Research shows how evolution explains age of puberty, ScienceDaily, December 1, 2005.
• Mark Hanson, P. Gluckman. Evolution, development and timing of puberty, Trends in Endocrinology & Metabolism, January 2006.pl:Menarche

de:Menarche zh:初潮

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Menarche Wikipedia article Menarche.