Sex hormones in the womb could be a cause of the eating disorder anorexia, a study has found. The suspicion is that oestrogen may be overproduced by some mothers, affecting the baby’s brain and making it susceptible to the eating disorder.

Psychiatrists investigating the cause of the illness did so by studying records of thousands of Swedish twins, held in a database. They found, not unexpectedly, that the risk of developing anorexia was higher in girl twins than in boy twins. Anorexia is far commoner in girls than in boys.

But an exception to the pattern arose in the case of twins of different sexes. Boys who shared the womb with girl twins were found to be ten times more likely to develop the disorder in later life.

Many claims have been made that girls who become anorexic have been influenced by images of stick-thin models. The findings do not disprove this, but suggest that biology as well as culture plays an important part.

Marco Procopio, a psychiatrist from the University of Sussex and one of the authors of the study in Archives of General Psychiatry, said: “We know that women are ten times more likely to develop anorexia than men and this study goes a long way to explaining why. We know that oestrogen and other hormones can have a powerful effect on the body and it would seem that there is an ‘overexpression’ of oestrogen by the mother and the girl twin in some pregnancies.

“Oestrogen would be present in the amniotic fluid that bathes babies in the womb and would be swallowed by both the male and female twin. Oestrogen is needed in development of females but it is possible that too much affects the structure of the brain.”

The new study supports research from the United States that found that the brains of anorexics behaved in a different way to non-anorexics.

Dr Procopio said: “Research into twins is a way to examine the factors involved, as the single most important period for brain development is during the months of pregnancy.

“The one thing we are certain of is that there is a genetic disposition to anorexia. Some scientists have suggested that upbringing may be a factor in the gender difference in rates of occurrence of the disorder, but studies haven’t borne this out.”

Dr Procopio does not believe that thin models account for the condition.

“If that were the case, we would have many more anorexics,” he said. “There might be an effect on some girls but I doubt if these are truly anorexic but more likely a passing phase.” He believes it may be possible one day to monitor pregnant women for higher than normal oestrogen levels.

Dee Dawson, who runs the pioneering North London Rhodes Farm clinic for teenage anorexics said: “It’s an interesting study and there may well be some truth in the findings.

“I think that there is a genetic predisposition and the problem may well be to do with the formation of the brain in pregnancy.

“But often the triggers are problems at home so if you are susceptible in your brain make-up it could be triggered off by events in a teenager’s life.”

A new website has been set up for people suffering from eating disorders and their carers. The website www.eatingresearch.com is updated with the latest research into illnesses such as anorexia nervosa and bulimia nervosa, and has been started by the Institute of Psychiatry at King’s College London.

It is aimed at sufferers, carers and health professionals, with sections on getting help and treatment research. It also has areas on spotting a disorder, news, events and opportunities for people to participate in studies.

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