WASHINGTON — The more mothers work during their children’s lifetimes, the more likely their kids are to be overweight or obese, according to a US study published on Friday.

Researchers from American University in Washington, Cornell University in New York state and the University of Chicago studied data on more than 900 elementary- and middle-school-aged children in 10 US cities.

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They found that the total number of years the children’s mothers worked had a cumulative influence on their children’s body mass index (BMI) — the weight to height ratio used to measure if a person is overweight or obese.

“Every period of time (averaging 5.3 months) a mother was employed was associated with an increase in her child?s BMI of 10 percent of a standard deviation,” says the study which was published in the journal Child Development.

“For a child of average height, this is equivalent to a gain in weight of nearly one pound (half a kilogram) every five months above and beyond what would typically be gained as a child ages.”

The findings were strongest among sixth graders, the oldest children for whom data was studied. Sixth graders are typically 11 years old.

Changes in the children’s physical activity, time spent unsupervised or watching television did not explain the link between maternal employment and children’s BMI, the study says.

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Moreover, a mother working odd hours or overnight was not significantly associated with their children’s BMI.

The researchers were unable to clearly explain the findings but theorized that because working mothers have little time to shop for healthy food and prepare meals, they and their children eat more fast- and packaged foods, which tend to be high in fat and calories.

Childhood obesity in the United States has tripled in 30 years.

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Today, one in three US kids is overweight or obese, meaning they are more likely than their normal-weight counterparts to grow up to be obese adults and suffer from obesity-related conditions including diabetes, cardiovascular disease and fatty liver disease.

Childhood obesity has also been linked to “behavior and academic problems in adolescence and adulthood,” said the lead author of the study Taryn Morrissey, assistant professor in public administration and policy at American University, calling for healthy foods to be made more accessible to working families.

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“Given that more than 70 percent of US mothers with young children work, the importance of providing support to these families is clear,” the study says.