Background Overweight and obesity in childhood are socially patterned, with higher prevalence in more disadvantaged populations, but it is unclear to what extent early life factors attenuate the social inequalities found in childhood overweight/obesity.

Methods We estimated relative risks (RRs) for being overweight (combining with obesity) at age 11 in 11 764 children from the UK Millennium Cohort Study (MCS) according to socio-economic circumstances (SEC). Early life risk factors were explored to assess if they attenuated associations between SECs and overweight.

Results 28.84% of children were overweight at 11 years. Children of mothers with no academic qualifications were more likely to be overweight (RR 1.72, 95% CI 1.48 to 2.01) compared to children of mothers with degrees and higher degrees. Controlling for prenatal, perinatal, and early life characteristics (particularly maternal pre-pregnancy overweight and maternal smoking during pregnancy) reduced the RR for overweight to 1.44, 95% CI 1.23 to 1.69 in the group with the lowest academic qualifications compared to the highest.

Conclusions We observed a clear social gradient in overweight 11-year-old children using a representative UK sample. Moreover, we identified specific early life risk factors, including maternal smoking during pregnancy and maternal pre-pregnancy overweight, that partially account for the social inequalities found in childhood overweight. Policies to support mothers to maintain a healthy weight, breastfeed and abstain from smoking during pregnancy are important to improve maternal and child health outcomes, and our study provides some evidence that they may also help to address the continuing rise in inequalities in childhood overweight.