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A Secular Trend in Age at Menarche in Japan

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  • 日本における初潮年齢の推移
  • ニホン ニ オケル ショチョウ ネンレイ ノ スイイ

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Abstract

In Japan, the decline in the age at menarche after the Second World War has been repeatedly reported, but the observed period in the reports has not been long enough to evaluate the secular trend of it. More than a hundred reports of age at menarche of Japanese have been published from the year of 1886. More than half of these populations in the reports consisted of students or young workers, some of whom had not attained menarche at the survey, and the menarcheal ages were represented by the arithmetric mean for the menarche attained girls. Thus, ages at menarche of these reports have biases toward younger menarcheal ages which depend on the proportion of non-menstruating girls. The authors aimed to correct these biased menarcheal ages on the assumption that (1) menarcheal ages of a population distributes normally when all of the girls are menstruating; (2) when some girls are not menstruating, the distribution is censored sample of normal distribution. After eliminating these biases of historical sources, the trend of the menarcheal age in Japan (from the late 19th century to the present) was analysed.

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