Aim Our aim was to examine whether breast feeding is associated with behavioural development in children aged 5 years.

Methods We used data from a large, prospective, nationally representative UK cohort, the Millennium Cohort Study. Breast feeding was ascertained from parent-interviews at baseline and child behaviour from parent-rated outcome, the Strengths and Difficulties Questionnaire (SDQ). Our analyses included 10 037 mother-child pairs with data on breast feeding, SDQ and potential confounders. 9525 of the children were born at term and 512 were preterm. We used logistic regression models to investigate the associations of breast feeding duration with abnormal parent-rated SDQ total and sub-scores at age five in term and preterm children separately.

Results Overall, abnormal SDQ scores were less common in breast fed than formula-fed children. Term children breast fed for four months or longer had lower odds of an abnormal total SDQ score (multivariable-adjusted OR compared to never breast fed children: 0.65, 95% CI 0.52 to 0.82). This effect was similar for all the SDQ sub-scores. In preterm children prolonged breast feeding was generally associated with lower odds of abnormal SDQ total and sub-scores but the effect estimates were imprecise. The associations between exclusive breast feeding and abnormal SDQ scores were similar to those of any breast feeding and abnormal SDQ scores.

Conclusions Our findings suggest that, at least in term children, prolonged breast feeding is associated with having fewer parent-rated behavioural problems at the age of 5 years.