To longitudinally investigate body mass index (BMI) in young children in Ireland and identify factors and critical time points associated with changes in BMI.

Data on 11 134 children were collected in the nationally representative Growing Up in Ireland infant cohort study. Height and weight were measured at 9 months, 3 years, and 5 years of age. Multilevel regression was used to identify risk factors associated with changes in BMI over time (n = 10 377), combining a unique set of covariates collected from the child and the 2 main caregivers (usually the mother and father).

The proportion of children ≥85th percentile of World Health Organization growth criteria was 39% at 9 months, 44% at 3 years, and 30% at 5 years. Children born large for gestational age (13%) and those with rapid infant weight gain (25%) consistently had higher BMI. Low average BMIs were consistently seen in children born small for gestational age (10%) or before 37 weeks (7%). Smaller variations in BMI existed for other factors including ethnicity, household structure, caregiver weight status, breastfeeding, sex, socioeconomic status, sleeping hours, childcare, and region.

In this study, differences at birth and in infancy appear to be most strongly associated with variation in BMI at all ages. Nevertheless, belonging to a number of other high-risk groups cumulatively could lead children to develop critical weight states. Policy-makers should target families with interventions before and during pregnancy when dominant risk factors are still modifiable. Longer-term follow-up of children may be needed to study associations later in childhood.