More than a quarter of children in Britain are overweight
The "tipping point" that sets children on the way to a lifetime of obesity often occurs before the age of two, say US researchers.
A study of more than 100 obese children and teenagers found more than half were overweight by 24 months and 90% were overweight by the age of five.
A quarter were overweight before they were five months old, the researchers reported in Clinical Pediatrics.
In the UK, around 27% of children are now overweight.
The children in the study - who had an average age of 12 - were all overweight or obese by the age of 10.
Although the reason for rapid weight gain in early life is not well understood, contributing factors are likely to be poor diet, early introduction of solid food, and not getting enough exercise, the researchers said.
They added that food preferences may be set by the age of two, so changing a child's eating behaviour at a later stage may be difficult.
Study leader Dr John Harrington, an assistant professor at Eastern Virginia Medical School, said the results should be a "wake-up call for doctors".
He went on: "Too often, doctors wait until medical complications arise before they begin treatment.
"Getting parents and children to change habits that have already taken hold is a monumental challenge fraught with road-blocks and disappointments.
"This study indicates that we may need to discuss inappropriate weight gain early in infancy to effect meaningful changes in the current trend of obesity."
A Department of Health spokesman said: "What happens in the first years of a baby's life has a big effect on how healthy they are in the future.
"Despite recent encouraging statistics which show that childhood obesity may be levelling off, obesity levels are still too high and it is important we keep the momentum going."