Large infants, or those who grow rapidly in the first two years of life are at increased risk of obesity as children and adults, research shows.
Weight gain in infancy is important for later obesity risk
The findings come from an analysis in the British Medical Journal of 24 studies involving over 400,000 infants.
Factors such as feeding and weaning are important, the authors believe.
The World Health Organization says babies should be exclusively breastfed for the first six months of life and some think this guards against obesity.
Feeding and weaning
Data suggests one in five UK children is overweight or obese.
On present trends, half of all children in England in 2020 could be obese.
The paper's authors say there is an urgent need to tackle rising obesity levels but it is not clear how early in life prevention could begin.
Dr Janis Baird, of the Medical Research Council's Epidemiology Resource Centre at Southampton University, and other UK colleagues say their findings suggest obesity risk begins from infancy, if not younger.
They found the heaviest infants (those who had the highest body mass index) and those who gained weight rapidly during the first and second year of life, were more likely to be obese at all stages of life - childhood, adolescence, and early adulthood - than other infants.
Ms Baird said: "What we need now is more research to look at what factors are associated with patterns of growth and also to look at whether there are benefits or harms to altering infant growth other than obesity."
She said it would be interesting to see whether the WHO recommendations on breastfeeding that came into force in 2003 would make a difference to obesity rates.
She said researchers were looking at whether maternal diet during pregnancy and birth weight might also be important.
"The factors that influence infant growth are many and varied," she said.
Genes and shared lifestyles both contribute to the trend of obesity running in families, experts believe.
Dr David Haslam of the National Obesity Forum said: "The most important thing in those first two years of life is breastfeeding and weaning.
"Breastfeeding protects both mother and baby against obesity."
He said topping up feeds too much with bottle milk could be a factor, as could weaning inappropriately.
"We hear the horror stories of how infants are fed burgers and fries mashed up in the food processor as part of their food.
"Some parents think that giving the kids what they eat is a good thing when it is not always."