Background Rates of overweight and obesity have been shown to vary across socio-economic groups (SEG) from at least the age of three years but little is known about whether SEG differentials vary after adipose rebound and into adolescence and whether these trajectories differ by national context. This study examines socio-economic differentials in children’s body mass trajectories in infancy and extending across childhood into early adolescence in three contemporary child cohort studies.

Methods Data on body mass index (BMI) measured on at least three occasions between birth and adolescence was obtained from four prospective cohort studies – Generation 21 (G21–Portugal), Growing Up in Ireland (GUI–Ireland) (infant and child cohorts), and the Millennium Cohort Study (MCS–UK) – involving a total sample of 44 136 children. SEG differentials in children’s BMI trajectories was modelled by maternal educational level (primary, secondary, tertiary) using hierarchical models with fixed and random components for each cohort study.

Results Child BMI growth trajectory was greater for children of lower educated mothers but only from three years of age. In G21, the educational differential emerged by 4 years of age and increased from 0.25_boys [CI₉₅=0.14, 0.38] and 0.44_girls [CI₉₅=0.30, 0.58] to 0.45_boys [CI₉₅=0.25, 0.64] and 0.70_girls [CI₉₅=0.48, 0.92] by 7 years of age. In GUI, the mean difference in BMI between polarised educational groups increased from 0.21_boys [CI₉₅=0.08, 0.35] and 0.35_girls at 3 years of age [CI₉₅=0.21, 0.49] to 0.92_boys [CI₉₅=0.63, 1.21] and 1.40_girls [CI₉₅=1.09, 1.71] by 13 years of age. In MCS, the educational differential was first observed at 5 years of age and increased from 0.14_boys [CI₉₅=0.06, 0.23] and 0.19_girls [CI₉₅=0.10, 0.28] to 0.66_boys [CI₉₅=0.49, 0.83] and 0.61_girls [CI₉₅=0.42, 0.79] by 11 years of age.

Conclusion Socio-economic factors are strongly implicated in the aetiology of childhood obesity. This study shows that the socio-economic differentials emerge in early childhood and widen over time providing important policy evidence about the timing of potential policy interventions designed to eliminate the adverse life course health effects associated with early emerging adiposity.