Menarche 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 years and 5 months.Relative Weight and Race Influence Average Age at Menarche: Results From Two Nationally Representative Surveys of US Girls Studied 25 Years Apart - Anderson et al. 111 (4): 844 - Pediatrics
Menarche is the culmination of a series of physiological and anatomic processes of puberty:
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.
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 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.
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:
Some of the aspects of family structure and function reported to be independently associated with later menarche:
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:
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/or 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%.
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.
Some cultures have in past centuries have had rites of passage for a girl experiencing menarche.
Asia, Australia, and Pacific Islands
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.
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