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Age of pubertal onset is declining among girls. According to a study published online February 10 in JAMA Pediatrics, researchers have found that pubertal onset, as measured by breast development in healthy girls worldwide, has decreased by 0.24 years or roughly three months per decade between 1977 and 2013. Researcher Alexander S. Busch said this is the first study using thelarche (initial breast gland development) as a clinical marker for pubertal onset. While most studies use age of menarche to assess pubertal onset, menarche is a later pubertal sign and may not accurately reflect changes over time. The study findings provide updated baseline clinical parameters for diagnostic purposes, possibly altering the currently accepted definition of early pubertal onset. The researchers posited that increases in obesity rates and exposure to "endocrine-disrupting chemicals" may be contributing factors to the changes reported, and recommended areas of further investigation.