The BMI represents the relationship between a child's height and weight.
These measures have been rising among American children.
Study authors used data from an ongoing project, the National Institute of Child Health and Human Development. They analyzed the BMI for 990 children from 10 U.S. cities, and their mothers' employment status. The children in the study were in the 3rd, 5th, and 6th grades.
The researchers found that for every 5.3 months a mother was employed, the kids' BMI was nearly a pound above what was considered typical for that age.
The biggest effect was seen in 6th grade, where the children of working mothers were 6 times more likely to be overweight.
To the researchers' surprise, the time spent watching TV, physical activity levels, and unsupervised time didn't account for the increase in the BMI.
It also didn't matter what schedule a mother worked - standard 9-to-5, or non-standard hours such as nights or weekends.
Critics say the study only makes an association, but doesn't establish that a mother's work CAUSES the higher BMI.
The study is published in the journal Child Development.
