Introduction In western populations, informal child care is associated with childhood obesity. However, in western populations, informal child care and childhood obesity are associated with lower socio-economic position (SEP), making these observations vulnerable to residual confounding. In this situation, evidence from non-western developed settings can be valuable.

Methods We used multivariable linear and logistic regression to estimate the association of child care at 6 months, 3 years, 5 years and 11 years with body mass index (BMI) z-score and overweight (including obesity) at 11 years in a large population-representative Hong Kong Chinese birth cohort, “Children of 1997”, comprising 88% of births in April and May 1997. We also assessed if the associations varied with sex or SEP.

Results Of the original 8327 cohort members, 7933 are alive, participating and living in Hong Kong. At approximately 11 years, 6796 had clinically assessed BMI. Higher SEP was associated with informal care. Informal care at each of 3, 5 or 11 years was separately associated with higher BMI z-score (3 years: 0.11, 95% CI 0.03 to 0.19; 5 years: 0.13, 0.05 to 0.20; 11 years: 0.16, 0.05 to 0.27) and the presence of overweight at 11 years. Current informal care had the strongest association, however, informal child care at 5 years also made a contribution. There was no evidence of differences by sex or SEP.

Conclusions In a developed non-western setting, informal child care was associated with childhood obesity. Modifiable attributes of informal child care warrant investigation for obesity prevention.