Childhood obesity starts in the womb, with overweight mothers giving birth to fat babies.
New scanning techniques have allowed British scientists to examine fat levels in newborn babies.
They found some had built up fat around their abdomen in the same way as adults in their 50s.
It is the first direct link to be shown between the weight of a mother-to-be and that of her child.
Obese youngsters are more likely to develop a string of health problems, including heart disease, brittle bones, diabetes and asthma.
Almost half of women of child-bearing age are overweight or obese and more than 15 per cent of pregnant women are obese.
This also raises their odds of dying in pregnancy, of their baby being stillborn and a host of pregnancy complications, some of which can be fatal.
The study was led by Professor Neena Modi, the UK's top authority on high-risk health problems in newborns.
She said magnetic resonance imaging (MRI) scans had provided clear evidence that being overweight or obese in pregnancy could result in potentially harmful changes to a baby's fat levels while still in the womb.
Her team, which studied 54 boys and 51 girls at Chelsea and Westminster Hospital, found a total of 31 babies had more adipose, or fat, tissue around their abdomen than would have been expected.
This increased proportionately with the obesity of the mother.
Professor Modi, professor of neonatal medicine at Imperial College London, said: "The biological changes identified in the study were increased total fat, particularly around the tummy.
"I was very surprised to be able to detect such a clear continuum of effect of maternal BMI (body mass index) on the baby.
"This is a very important finding indeed, opening the door to a new understanding of how a mother's metabolism affects her baby.
"This shows how sensitive the baby is to the environment experienced within the womb and how lifelong effects may be initiated before birth."
In adults, adipose tissue is found mainly under the skin, but also in deposits between the muscles, around the intestines and around the heart - collecting fats which come from food eaten or produced in the body.
Newborn babies typically have about 700g of adipose tissue, but for each unit increase in maternal BMI, this increased by approximately 7g with a huge build-up in fat in the babies' livers.
"Normally, a newborn baby has minimal or no detectable liver fat," said Professor Modi.
"But for each unit increase in maternal BMI, the baby's liver fat content increased by eight per cent."
The findings come as experts predict obesity will cost the UK's National Health Service up to £6.3 billion (NZ$12.6bn) a year by 2015 unless more is done to tackle the spiralling problem.
Professor Modi said it was important that all women hoping to have families were aware that being obese could affect their unborn children's health prospects.
"Adipose tissue not only stores energy and provides insulation, but is also a source of variety of hormones. Being too fat is unhealthy and increases the risk of many diseases.
"In adults too much adipose tissue around the tummy is particularly bad and is associated with much greater risk of developing heart disease, high blood pressure and diabetes."
- DAILY MAILBy Jenny Hope