By Madonna Behen

HealthDay Reporter

FRIDAY, Feb. 4 (HealthDay News) -- As if working mothers don't have enough to feel guilty about, a new study suggests that the more time they spend working, the heavier their children become.

Researchers at American University, Cornell University and the University of Chicago analyzed data on 900 school-aged children, and found that the cumulative time that a child's mother worked was associated with a small but measurable increase in the child's body mass index (BMI), a measurement that takes into account height and weight.

The research, which was sponsored by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), appears in the January/February issue of the journal Child Development.

"It's important to emphasize that it seems to be the environmental factors associated with the total time that moms work, and not maternal employment per se, that contributes to an increase in children's BMI," said study author Taryn W. Morrissey, an assistant professor of public administration and policy at American University.

Surprisingly, there was no evidence that the increase in BMI was linked to more TV viewing, a decrease in physical activity, or more time spent unsupervised.

The researchers concluded that it may be changes in children's eating and sleeping patterns (factors that were not included in the data) that account for the BMI changes. "While we weren't able to identify any specific environmental factors, it's clear from other research that nutrition and sleep are important," she said. "So, one possible policy implication is to do more to help working parents find quick and easy ways to prepare healthy foods."

Morrissey and her colleagues analyzed data from the NICHD's Study of Early Child Care and Youth Development, which began in 1991 and involved roughly 1,300 newborns in 10 cities across the country. For this study, they examined data on 900 children that was compiled from telephone and in-person interviews conducted in third, fifth and sixth grades. In addition, they looked at the mothers' employment status and schedule beginning from when their children were 3 months old.

"For a third grader of average height, the increase in BMI was equivalent to an extra one and a half to two pounds over what that child would normally gain in a year," Morrissey said.

The effect was even greater among children in fifth and sixth grades. "It is possible that because fifth and sixth graders generally have more independence and less adult supervision over their time use and food choices than third graders, maternal employment precipitates poorer food choices and more sedentary activity," the authors wrote.

Alison E. Field, an obesity researcher and associate professor of pediatrics at Harvard Medical School, said the study highlights the challenges that all working parents face as they juggle the demands of work and family responsibilities, and that one consequence may be relying too heavily on high-calorie, processed foods.

But, she added, the researchers' main findings "should be taken with a grain of salt."

"For one thing, we don't know why these mothers were going in and out of the workforce," Field said. "Some women choose to go back to work and others have to because they need the income. The reason the mothers are working can have a very different impact on how their families are eating, and that kind of gets glossed over in this study."

Another limitation of the paper, Field noted, is that because the children were studied around the time of puberty, it's possible that other developmental factors may have accounted for changes in BMI.

Nevertheless, Field agreed that a key take-home message for working parents is that they "need to think twice about feeding their families a lot of processed, pre-packaged foods."

The study didn't investigate what role the fathers' work played in their children's physical health, which Morrissey said wasn't particularly surprising.

"The data set we analyzed was from the early 1990s, and a study that starts today might have different employment patterns for fathers," she said. "On the other hand, even though dads today are participating a lot more in things like child care, cleaning and cooking, it seems that working moms still bear the brunt of family responsibilities."

More information

For more on childhood obesity, go to the U.S. Department of Health and Human Services.