New mothers who go out to work may increase the risk of their children becoming overweight, a study indicates.

For every ten hours a week a mother works, the odds of having a child who is overweight by the age of 3 are increased by about 10 per cent, according to results from a study of almost 13,000 mothers.

The team from the Institute of Child Health in London believes this is because long hours of work, rather than a lack of money, may make it harder to provide a child with healthy food and opportunities for activity.

The effect is stronger among households earning more than £33,000 a year, the research found. This does not imply that middle-class children as a whole are fatter than working-class children – national statistics tend to show the opposite – but that the link between working mothers and overweight children is more pronounced among those who earn more.

The team used data from the Millennium Cohort Study, a UK-wide study of children born since 2000. They looked at children born between 2000 and 2002 for whom they had height and weight data at the age of 3, and records of parental working patterns. Writing in the International Journal of Obesitythey report a link between maternal employment and a child’s body mass index. The longer hours a woman worked – and especially in the higher-income groups – the greater the effect.

For households with incomes over £33,000 a year, the chances of having an overweight three-year-old were increased by 19 per cent. For those in the £22,000 to £33,000 group, the extra risk was 13 per cent, and for those between £11,000 and £22,000 8 per cent. Only the result for the £33,000plus households was said to be statistically significant.

This suggests that, if the authors are right in suggesting that a lack of time to prepare nourishing meals or take infants for walks is the cause, more middle-class occupations place greater time constraints on mothers than do the same hours worked in less well-paid occupations. But speculation may well be pointless, as the effects are small and the results are on the margin of statistical significance.

The authors themselves are cautious. “Although we found that maternal employment is associated with overweight among British preschool children, our results and the larger evidence base suggests there are many risk factors for overweight” they write, adding that more research is needed.

Among the further work would be a study showing whether or not children’s diets or activity levels are actually different when their mothers work. If not, then the trends found in this study will need an alternative explanation. National statistics from the Health Survey for England do tend to suggest that social class is relevant to a child’s likelihood of being overweight.The new study did not look at absolute levels of overweight for each income group, but the effect of mothers working on the risks of overweight.