Early Postnatal Growth and Pubertal Timing: An Ultrasound-Based Study of Breast and Testicular Development.

Context: Rapid postnatal weight gain has been associated with earlier puberty in girls, while evidence in boys is less consistent. However, studies using objective pubertal markers remain limited. Objective: This work aimed to examine associations between postnatal growth during early infancy (0-0.5 years), late infancy (0.5-2 years), and early childhood (2-4 years) and pubertal onset. Methods: A total of 610 healthy, term-born children (260 boys) aged 6 to16 years, from the Bergen Growth Study 2 (2016) underwent ultrasound-based assessments of breast development and testicular volume. Longitudinal length/height and weight data were retrospectively collected from child health centres. Growth trajectories were modeled using piecewise linear mixed-effects models. Logistic regression was used to analyze sex-specific associations between postnatal growth and ultrasound-measured breast stage ≥ 2 and menarche in girls, ultrasound-measured testicular volume ≥ 2.7 mL, and testosterone level ≥ 0.5 nmol/L in boys, and pubarche in both sexes. Results: In girls, increased linear growth and weight gain during infancy and early childhood were associated with earlier breast development. Increased weight and body mass index gain in early childhood were linked to earlier menarche. In boys, increased length gain in early infancy was associated with earlier testicular growth, and increased length and weight gain during later periods were associated with higher testosterone levels (all P < .05). No statistically significant associations were found for pubarche in either sex. Conclusion: Increased linear growth during early infancy was associated with earlier pubertal onset in both sexes, with more consistent associations for weight gain from late infancy, particularly in girls.