Even before the meaning of the critical weight for an individual girl was analyzed, the idea that menarche is associated with a critical weight for a population explained simply many observations associated with early or late menarche. Observations of earlier menarche are associated with attaining the critical weight more quickly. The most important example is the secular (long-term) trend to an earlier menarche of approximately 3 or 4 months per decade in Europe in the past 100 years (Figure 2). The explanation is that children become larger sooner; therefore, girls on average reach 46 or 47 kg, the mean weight at menarche of US and many European populations, more quickly. Theoretically, the secular trend should end when the weight of children of successive cohorts remains the same because of the attainment of maximum nutrition and child care; this has happened in the United States.
Conversely, a late menarche is associated with body weight growth that is slower prenatally, post-natally, or both so that the average critical weight is reached at a later age: Malnutrition delays menarche, twins have later menarche than do singletons of the same population, and high altitude delays menarche.
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