We aimed to establish associations of duration of breast-feeding with mean BMI and waist circumference, as well as the likelihood of being overweight/obese, during early childhood.

Cross-sectional, population-based study. Height, weight and waist circumference were measured and BMI calculated. Interviewer-administered questionnaire determined whether the child was ever breast-fed and the duration of breast-feeding.

Sydney, Australia.

Infants and pre-school children (n 2092) aged 1–6 years were examined in the Sydney Paediatric Eye Disease Study during 2007–2009.

Of the children aged 1–6 years, 1270 had been breast-fed compared with 822 who were never breast-fed. After multivariable adjustment, 1–6-year-old children who were ever breast-fed compared with those who were not had significantly lower BMI, 16·7 (se 0·1) kg/m2v. 17·1 (se 0·2) kg/m2 (P = 0·01). Decreasing BMI was associated with increasing duration of breast-feeding (Ptrend = 0·002). After multivariable adjustment, each month increase in breast-feeding was associated with an average BMI decrease of 0·04 kg/m2 (P = 0·002) and 0·03 kg/m2 (P = 0·03) among children aged 1–2 years and 3–4 years, respectively. In 1–2-year-old children, each month increase in breast-feeding duration was associated with a 0·06 cm decrease in waist circumference (P = 0·04). Significant associations were not observed among 5–6-year-old children. Children who were ever breast-fed v. those never breast-fed were less likely to be overweight/obese (multivariable-adjusted OR = 0·54; 95 % CI 0·36, 0·83).

We demonstrated a modest influence of breast-feeding on children's BMI during early childhood, particularly among those aged less than 5 years.