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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.

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