Parents who send their children off to day care may worry about missing time with the kids. Now scientists say they may have another reason to feel guilty: Day care seems to raise a child's risk for being overweight.

A team of researchers from the University of Montreal, University College London and other institutions followed a cohort of more than 1,600 children born in Quebec for several years. Mothers answered questionnaires about their child care arrangements, and the child's height and weight were used to determine body-mass index.

Children who primarily were left at a day care center or with an extended family member between 18 months and four years of age were 50 percent more likely to be overweight or obese, the scientists reported in the Journal of Pediatrics.

"This difference cannot be explained by known risk factors such as socioeconomic status of the parents, breastfeeding, body mass index of the mother, or employment status of the mother," University College London researcher Marie-Claude Geoffroy said in a statement Sunday.

It's unclear at the moment why children in day care should weigh more than those raised by their parents or a nanny.

“[Obesity-causing] features of these child care arrangements should be investigated in future studies,” the authors wrote.

Other studies have suggested that preschool years are a critical period to instill healthy habits in children.

The U.S.-based National Association for Sport and Physical Education recommends that preschoolers be physically active for at least two hours a day, and should not be sedentary for more than an hour at a time, except when sleeping.

But many surveys show that few children meet these standards: One meta-analysis of 29 studies representing more than 6,000 subjects found that on average, preschoolers were active for about 43 minutes a day.

Day care could actually be an ideal stage to implement programs to encourage children to be active, another group of scientists wrote in a paper published in the International Journal of Environmental Research and Public Health this past April.

“Many young children can be positively impacted by health promotion efforts associated with intervening in day care settings, similar to the rationale for school-based intervention,” the authors wrote.

But of course, day care programs can only do so much.

“I suggest to parents they ensure their children eat well and get enough physical activity, whether at home or at day care,” co-author and University of Montreal researcher Sylvana Cote said in a statement Sunday.

SOURCE: Geoffroy et al. “Childcare and Overweight or Obesity over 10 Years of Follow-Up.” J Pediatrics published online 8 November 2012.