Changes to a mother's genes may be passed on
The obesity crisis could deepen as the womb chemicals of increasingly overweight mothers set their baby's future risk, say US scientists.
A study, in the International Journal of Obesity, showed mice genetically prone to obesity getting fatter generation by generation.
"Epigenetic" tags, which affect the function of our genes, may be responsible, they say.
However, the researchers countered this effect in the mice through diet.
Our genes alone do not fully explain why we turn out the way we do - why some people will develop cancer or Alzheimer's disease, while others become obese.
Researchers increasingly believe that the effect of conditions in the womb on the developing foetus can play an important role in setting our future health.
While nothing can change our genes themselves, there is evidence that the womb environment can change the way our genes work.
Specifically, it may be able to produce chemical changes which control the level at which certain genes function. This process is called "epigenetics".
While there is strong evidence that a mother's weight has an impact on the future weight of her children, the reasons for this are not fully understood.
The researchers at the Baylor College of Medicine in Houston, Texas, used a type of mouse which, if offered a normal diet, will tend to put on weight.
While one population of mice was fed this normal diet, another was fed a diet fortified with vitamins and folic acid, designed to alter the epigenetic process by "silencing" genes.
The mice on the normal diet gained weight, as expected, and subsequent generations were progressively more obese.
But those on the supplemented diet did not gain weight through successive generations.
Dr Robert Waterland, who led the project, said he believed that the womb environment could alter the function of genes controlling the development of the part of the brain controlling appetite.
He said: "Why is everyone getting heavier and heavier? Maternal obesity could be promoting obesity in the next generation."
He said: "What we have shown is that in each generation, the maternal obesity of the previous generation influences the establishment of the body weight regulatory mechanism."
Dr Karen Lillycrop, from Southampton University, said that the paper was "interesting", but stopped short of proving that an epigenetic effect was responsible for the effects seen in the mice.
"There is now a lot of information that foetal environment - be it malnourished or over-nourished - has an permanent impact on the metabolism of that foetus as it tries to cope."
She said that while folic acid supplementation in the lab mice had stopped mice increasing in weight, there would be possible risks in similar fortification in humans.