The researchers base their findings on 571 New Zealand children of European heritage, who were regularly monitored from birth to the age of 7 years.
Their percentage body fat, a reliable indicator of overweight and obesity, was calculated at the age of 3˝ and 7 years. TV viewing time and the amount of physical activity they took part in were also measured at the age of 7.
Children who had acquired a high percentage body fat by the age of 3˝ were significantly more likely to be obese at the age of 7 than those with low percentage body fat.
Children who had rapid growth spurts were more at risk.
But having a mother who was overweight or obese, being a girl, and spending a lot of time in front of the TV were all independently associated with the chances of becoming obese.
Children with overweight or obese mothers had around 4% more body fat than those whose mothers were of normal weight.
It is not clear if genes, exposure to specific factors during pregnancy, or shared lifestyle factors are to blame, say the authors.
Children who spent more than 3 hours a day in front of the TV had 5% more body fat than those who watched TV for less than an hour.
Unsurprisingly, doing very little exercise also significantly boosted the chances of obesity. Every additional hour of inactivity added almost 1% of body fat.
Children "start on the trajectory to overweight and obesity early in life," warn the authors. Consequently, counter measures also need to be deployed early too, they say.