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. 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%.
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 • Lack of exercise in childhood 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. == Society and culture ==