Secular trends are marked changes in physical development that have occurred over generations, resulting from environmental changes such as improvement in health and nutrition.
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The pattern of a younger age of onset for sexual maturation in adolescent is the secular trend in puberty. Adolescence is the period of development from childhood to adulthood that occurs after the start of puberty. This indicates a trend toward younger onset of puberty in children than in their forebears.
Puberty is starting earlier for boys and girls now than it did a century ago, which is a secular tendency in development. The beginning of puberty is still getting earlier, especially for females, although the end of puberty (when girls start getting their period, for example), has largely levelled off.
Ellie is therefore not unique. Today's girls frequently begin the process of developing breasts far earlier than their ancestors did, which is frequently thought of as the beginning of puberty.
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