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 occurs most frequently when a young woman reaches 17% body fat (PMID 3117838). The average age of menarche in the United States is about 12 years and 8 months, which is over two and a half years earlier than it was in 1900. To maintain a regular menstrual cycle, fat content of the body must be 22% or greater.

Menarche as part of puberty
Menarche is the culmination of a series of physiological and anatomic processes of puberty:
 * 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".

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). 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 to conceive and 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, just 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: Some of the aspects of family structure and function reported to be independently associated with later puberty:
 * 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
 * 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.

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 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.

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.