The present study was performed to investigate whether breast-feeding is associated with early pubertal development among children 7–9 years old in Korea.

Children were divided into those who did and did not receive breast-feeding for 6 months or longer in accordance with the recommendations of the WHO. Pubertal status was determined by clinical examination using Tanner staging.

Prospective observational study.

We conducted a follow-up study of children aged 7–9 years in 2011 who had taken part in the Ewha Birth & Growth Cohort study.

Fifty (22·8 %) of the total of 219 children were in early puberty, with the proportion being slightly higher for girls (24·1 %) than boys (21·4 %). Children who had entered early puberty were taller, weighed more and had a higher concentration of insulin-like growth factor 1. Moreover, the change in weight Z-score from birth to follow-up was significantly lower in children who were breast-fed than in those who were not (weight Z-score change: 0·32 (sd 1·59) v. 0·77 (sd 1·61), respectively, P=0·04). Comparison of breast-feeding by puberty status indicated a preventive association with early puberty in children who were breast-fed for 6 months or longer (OR=0·37; 95 % CI 0·18, 0·74). This association remained significant after adjustment for relevant covariates.

These results demonstrate a beneficial association between breast-feeding and early pubertal development, especially in those breast-fed for 6 months or longer. The study suggests that interventions would need to start early in life to prevent early pubertal development.