Pregnant women who eat a diet high in soya beans may increase the risk of long-term developmental damage in their children, researchers have found.

Soya beans contain compounds called phytoestrogens or isoflavones, that have been found to mimic the effects of the female sex hormone oestrogen.

These effects may help to prevent a range of conditions including the unpleasant symptoms of the menopause.

But scientists at the Third International Symposium on the Role of Soy in Preventing and Treating Chronic Disease in Washington DC were told the impact on foetuses and children could be negative.

A team from Cedars-Sinai Medical Center presented data from two studies - one on animals and one on humans - to the conference.

Organising tissue development

Dr Claude Hughes, director of the Center for Women's Health at Cedars-Sinai Medical Center, said sex hormones appeared to play an important role in organising the development of tissues early in life.

He said: "These hormones influence the way the brain is organised, the way the reproductive organs and cells develop, even the way immune function develops.

"Therefore, if mom is eating something or has in her body fat something that can act like sex hormones, it is logical to wonder if that could change the baby's development."

The researchers analysed amniotic fluid samples of 54 pregnant women from the Los Angeles area.

They found that about 80% of the foetuses were exposed to phytoestrogens at levels up to 180 times the norm.

The amniotic fluid samples were taken during routine amniocentesis between 16 and 20 weeks of gestation - after a baby's organs have formed but during a critical stage of development.

The researchers tested the impact of exposure to phytoestrogen by feeding them to pregnant rats at a similar stage of their pregnancy.

'Masculinising' effect

The phytoestrogen had a "masculinising" effect on both the male and female rat foetuses.

Male rat pups also experienced early onset of puberty.

Dr Hughes said the rat experiment was likely to give some indication of the effect of phytoestrogens on human foetuses.

He said: "There is no reason to assume that there will be gross malformations of foetuses but there may be subtle changes, such as neurobehavioral attributes, immune function, and sex hormone levels."

"There are many long-term health questions that come about when the little clocks in our heads are changed. There may be subtle things occurring and we don't know it.

"Or it could be that humans are much more resistant to these effects than are other animals, and this is not an issue. We will not know until we get the data."