Average weight boys may be at risk if they were small babies

He said the research means people will have to take a fresh approach to tackling obesity in the young as some would face more danger from being overweight than others.

At the moment, obesity and being overweight are defined by body mass index - the relation between height and weight. Children of a certain BMI are classed as obese.

But Dr Barker said his study suggested it was not the BMI alone that was important, but how the child got to that level.

"If you went into a school and you took out the most overweight 15% of boys and did something special to them, like give them more exercise, you'd be missing the point," he said.

"For most of those children the effects of being overweight are not very severe, it's just a tiny increase in risk of CHD.

"It's the ones who got there by accelerated weight gain rather than the ones who were great big babies and are now great big children."

Explanations

He said there were three possible explanations for the findings:

• Thin babies are less muscular so their weight gain is likely to consist of fat; or
• They have hormonal settings that drive the weight gain, and will continue to do so for the rest of their lives. This would be bad for the growth of their cardiovascular systems; or
• There is something intrinsically bad about accelerated post-natal growth. This could be characterised by a smaller number of cells expanding rapidly, and perhaps disrupting the communication between cells.

However, few babies are born fat, he said. High birth weight indicates muscle and height except in those born to diabetic mothers.

Around seven or eight is the right time to look for weight-related health warnings. In future anybody looking for such danger signs should also examine the journey from birth to seven years old, he suggested.