Recent Decline in Age at Breast Development: The Copenhagen Puberty Study.

This study investigates the secular trend of earlier puberty onset in European girls by comparing two cohorts from the same region, evaluated by the same research group over a 15-year interval. In the 1991-1993 cohort (n = 1100) and the 2006-2008 cohort (n = 995), girls aged 5.6 to 20.0 years underwent systematic assessments, including palpation for glandular breast tissue, height and weight measurements, and blood sampling for reproductive hormones (estradiol, luteinizing hormone, and follicle-stimulating hormone). The results revealed a significant decline in the mean estimated age of puberty onset, defined as Tanner breast stage 2+, from 10.88 years in the 1991 cohort to 9.86 years in the 2006 cohort, even after adjusting for BMI. While the age at menarche showed a smaller decrease (13.42 years in 1991 vs. 13.13 years in 2006), and hormone levels (FSH and LH) remained consistent across cohorts, estradiol levels were notably lower in 8- to 10-year-olds from the 2006 cohort. These findings indicate a marked trend toward earlier breast development that cannot be fully attributed to changes in BMI or reproductive hormones, pointing to potential unidentified environmental, genetic, or lifestyle factors driving this shift.