More than 17 percent of five-year-olds were heavier than they should be, with "12.9 percent overweight and 4.3 percent obese," the report said.
The study, conducted by doctoral student Karina Huus at Linköping University and released on Tuesday, tracked 17,000 babies born in southeastern Sweden between October 1997 and October 1999.
They were measured at birth, one year, two-and-a-half years, five years and eight to nine years of age. The parents also filled in questionnaires, including information about their children's eating habits.
Low physical activity was associated with higher weights while certain foods, such as sugar-sweetened fruit drinks "may contribute to overweight/obesity at five years" of age while porridge may protect children at two-and-a-half years of age.
The report also showed that children whose parents were better educated, especially the father, had a lower chance of becoming fat.
In 2006, Sweden announced it would begin screening four-year-olds as part of their routine check-ups following a slew of reports showing an increased incidence of Sweden's traditionally healthy children being overweight or obese.