Menarche

The timing of menarche is influenced by female biology, as well as genetic, environmental factors, and nutritional factors. The mean age of menarche has declined over the last century, but the magnitude of the decline and the factors responsible remain subjects of contention. The worldwide average age of menarche is very difficult to estimate accurately, and it varies significantly by geographical region, race, ethnicity and other characteristics, and occurs mostly during a span of ages from 8 to 16, with a small percentage of girls having menarche by age 10, and the vast majority having it by the time they were 14. There is a later age of onset in Asian populations compared to the West, but it too is changing with time. For example a Korean study in 2011 showed an overall average age of 12.7, with around 20% before age 12, and more than 90% by age 14. A Chinese study from 2014 published in Acta Paediatrica showed similar results (overall average of age 12.8 in 2005 down to age 12.3 in 2014) and a similar trend in time, but also similar findings about ethnic, cultural, and environmental effects. The average age of menarche was about 12.7 years in Canada in 2001, and 12.9 in the United Kingdom. A study of girls in Istanbul, Turkey, in 2011 found the median age at menarche to be 12.7 years. In the United States, an analysis of 10,590 women aged 15–44 taken from the 2013–2017 round of the CDC's National Survey of Family Growth found a median age of 11.9 years (down from 12.1 in 1995), with a mean of 12.5 years (down from 12.6).

Physiology

Puberty

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

• Attainment of a sufficient body fat percentage (typically around 17% of total body mass).
• Disinhibition of the GnRH pulse generator in the arcuate nucleus of the hypothalamus.
• Secretion of estrogen by the ovaries in response to pituitary hormones.
• Over an interval of about 2 to 3 years, estrogen stimulates growth of the uterus and breasts, as well as an increase in height, 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 off of part of the lining with some blood, which together flows from the vagina, i.e. menstrual flow.

Menarche tends to be painless and occurs without warning.

The menstruum, or flow, consists of a combination of fresh and clotted blood with endometrial tissue. Flow may be scanty in amount and might be as little as a single instance of "spotting". Like other menses, menarche may be accompanied by lower abdominal cramps.

Relation to fertility

For most girls, menarche does not mean that ovulation has occurred. In post-menarchal girls, about 80% of the cycles are 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, and predictable and consistent patterns of flow (e.g., heaviness or cramping). Continuing ovulation typically requires a body fat percentage of at least 22%.

Not every girl follows the typical pattern. Some girls ovulate prior to their 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 (delaying her menarche until after the end of her pregnancy, if she carries to full term).

Younger age of menarche is not correlated with a younger age of first sexual intercourse.

Onset

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.

When experiencing menarche, the blood flow (colloquially described as having one's "period") can vary from a slow and spotty discharge to a consistent blood flow for 3–7 days. The color of the blood ranges from bright red to brown in color; this is normal. Periods may be light or heavy.

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 vaginal bleeding must be investigated and excluded.

When menarche has failed to occur for more than three years after thelarche, or beyond 15 years of age, the delay is referred to as primary amenorrhea. Imperforate hymen is one cause of primary amenorrhea.

Timing

Chronic illness

Certain systemic or chronic illness can delay menarche, such as diabetes mellitus type 1, cystic fibrosis, asthma, inflammatory diseases, and untreated celiac disease, among others. Sometimes, lab tests do not return determinative results, so that underlying pathologies are not identified and the girl is diagnosed with constitutional growth delay.

Conditions and disease states

Studies have been conducted to observe the association of the timing of menarche with various conditions and diseases. Some studies have shown that there may be an association between early or late-age menarche and cardiovascular disease, although the mechanism of the association is not well understood. A systematic review has concluded that early onset of menarche is a risk factor for insulin resistance and breast cancer risk.

There is conflicting evidence regarding the association between obesity and timing of menarche; a meta-analysis and systematic review has determined that more studies must be conducted to make any definitive conclusions about this association.

Effects of stress and social environment

Some of the aspects of family structure and function reported to be independently associated with earlier menarche [antenatal and early childhood]

• Being non-white (in the UK)
• Having experienced pre-eclampsia in the womb
• Being a singleton, i.e. not a twin or triplet
• Low birthweight
• Not having been breast-fed
• Exposure to smoking
• High-conflict family relationships
• Lack of exercise in childhood

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 is compounded by inadequate diet and nutrition. There is mixed evidence if milder degrees of stress can accelerate puberty in girls as would be predicted by life history theory and demonstrated in non-human mammals.

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

• Most of these "effects" are statistical associations revealed by epidemiologic surveys. Statistical associations are not necessarily causal, and secondary variables and alternative explanations can be possible instead. Effects of small size studies can never be confirmed or refuted for any individual child.
• Despite the small magnitude of effect, interpretations of the studies are politically controversial because this type of research is often be used for political advocacy. Accusations of bias based on political agenda sometimes accompany scientific criticism.
• Correlation does not imply causation. While correlation can be objectively measured, causation is statistically inferred. For example, some suggest that childhood stress is caused by precocious puberty recognized later, rather than being the cause of it.

Changes in time of average age

There were few systematic studies of timing of menarche before the second half of the 20th century. Most older estimates of average onset of menarche were based on observation of a small, homogeneous, non-representative sample of the larger population, or based on recall by adult women, which is susceptible to 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 study.

From the sixth to the 15th centuries in Europe, most women reached menarche at about 14, between the ages of 12 and 15. The average age of menarche dropped from 14–15 years in the early 20th century to 12–13 years in the present, but girls in the 19th century had a later age of menarche (16 to 18 years) compared to girls in earlier centuries. A large North American survey reported a 2–3 month decline from the mid-1970s to the mid-1990s. A 2011 study found that each 1 kg/m² increase in childhood body-mass index (BMI) can be expected to result in a 6.5% higher absolute risk of early menarche (before age 12 years). This is called the secular trend.

In 2002, fewer than 10% of US girls started to menstruate before 11 years of age, and 90% of all US girls were menstruating by 13.75 years of age, with a median age of 12.43 years. This age at menarche is not much different (0.3 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, whereas non-white Mexican-American girls were only slightly earlier than white girls.