Harvard researchers have found another explanation for the national child obesity epidemic: mothers’ weight gain during pregnancy.

The seven-year study, led by Harvard Medical School instructor Emily Oken, discovered that children whose mothers gained at least the recommended amount of weight during pregnancy were four times more likely to be overweight at age three, compared to those whose mothers gained less.

The recommended weight gain for women with a normal body mass index (BMI) is 25 to 35 pounds. This number varies depending on the BMI of the expecting mother.

The study is published in the April issue of the American Journal of Obstetrics and Gynecology.

Oken and her research team monitored 1,044 pregnant women in the Boston area and compiled data on the BMI of their children until they reached three years of age.

Oken said that she was interested in studying why more young children are overweight now than in previous years.

“They aren’t eating too much fast food, watching too much television, or not exercising,” Oken said.

“It was important to look at other factors in early life, including in the uterus,” she added.

Kenneth P. Kleinman, an associate professor at the Medical School who also worked on the study, said that its findings could very well change federal recommendations about gestational weight gain.

“I don’t think that this one paper has enough weight to get the Institute of Medicine to change anything, but it could be part of the weight that causes them to revisit the question,” he said.

Leslie J. Sim, who directs the Committe on the Impact of Pregnancy Weight on Maternal and Child Health at the Institute of Medicine, said that any change would require another investigation by the National Academy of Science and would take over 18 months.

Oken said that her team will continue to monitor the children and will expand the scope of the research by evaluating the weight of the children who are now seven years old.

“We’re planning to look at the children and see if this association continues beyond the preschool age,” Oken said. “We would like to find more findings in different groups of people.”