Scientists have discovered for the first time that a mother's nutrition during pregnancy can alter her child's DNA and cause obesity in later life.
The international study, led by the University of Southampton and including teams from New Zealand and Singapore, found a mother's diet during pregnancy can alter DNA in a process called epigenetic change.
This can lead to her child tending to lay down more fat, the research claimed.
The study showed that this effect acts independently of how fat or thin the mother is and of the child's weight at birth.
Keith Godfrey, professor of epidemiology and human development at the University of Southampton, who led the study, said: "We have shown for the first time that susceptibility to obesity cannot simply be attributed to the combination of our genes and our lifestyle, but can be triggered by influences on a baby's development in the womb, including what the mother ate.
"A mother's nutrition while pregnant can cause important epigenetic changes that contribute to her offspring's risk of obesity during childhood."
Researchers measured epigenetic changes in nearly 300 children at birth and showed that these strongly predicted the degree of obesity at six or nine years of age.
The scientists were surprised by the size of the effect - children vary in how fat they are - but measurement of the epigenetic change at birth allowed the researchers to predict 25% of this variation.
The epigenetic changes, which alter the function of our DNA without changing the actual DNA sequence inherited from the mother and father, can also influence how a person responds to lifestyle factors such as diet or exercise for many years to come.
Prof Godfrey said: "This study indicates that measures to prevent childhood obesity should be targeted on improving a mother's nutrition and her baby's development in the womb.
"These powerful new epigenetic measurements might prove useful in monitoring the health of the child."
Professor Mark Hanson, of the British Heart Foundation, said: "This study provides compelling evidence that epigenetic changes, at least in part, explain the link between a poor start to life and later disease risk.
"It strengthens the case for all women of reproductive age having greater access to nutritional, education and lifestyle support to improve the health of the next generation, and to reduce the risk of the conditions such as diabetes and heart disease, which often follow obesity."
The study was primarily funded by the UK Medical Research Council (MRC), the National Institute for Health Research, WellChild (previously Children Nationwide), Arthritis Research UK and the University of Southampton.
Their findings will be published on April 26 in the printed journal Diabetes.