Older mothers who exercise while pregnant can halve the risk of their baby being born with congenital heart disease, research in the US suggests. The condition, which affects up to nine in every 1,000 babies born in the UK, is caused when development of the heart is disrupted.

Now experiments with young and old mice has suggested the condition is not due to ageing eggs, but the age of the mother - and that the risk can be alleviated with regular exercise.

Scientists hope the findings, published in the journal Nature, will lead to therapies to tackle the condition.

Critically, researchers found that voluntary exercise - whether begun early or later in life - was found to reduce this risk of the condition in the mice.

Congenital heart disease remains a leading cause of death among children. Previous studies have focused primarily on changes taking place in the embryo, but the results have led to no possible way of preventing it.

And until now, whether the basis of the risk is tied to the age of the mother or her eggs has remained uncertain.

But the new findings now suggest there may be actions an ageing woman can undertake to reduce the risk of her unborn baby developing congenital heart disease.

Professor Patrick Jay, of Washington University in the US, said: 'In my lab we are interested in understanding why certain individuals who are exposed to a known cause of congenital heart disease - whether genetic or environmental - escape the condition and others don't.

'We study mice with a mutation that increases the risk of heart defects. The mutation first was found in people. But not every mouse with the mutation gets a heart defect just as in humans.'

The experiment involved using mice genetically prone to relatively high rates of congenital heart defects. Researchers then took ovaries from older mothers and transplanted them into younger ones.

Likewise, they took the ovaries of younger mothers and transplanted them into older mothers.

They then examined the offspring to determine if higher rates of heart defects tracked with the age of the mothers or the age of the ovaries.

Young mice with old ovaries bore offspring with low rates of heart defects - similar to young mice with young ovaries. And older mice - even with young ovaries - bore offspring with higher rates of heart defects similar to older mice with older ovaries.

The scientists then looked at exercise, putting the mice on running wheels.

They found that the risk of heart defects in pups of older mothers dropped from about 20 per cent for sedentary mothers to 10 percent for exercising mothers.

The rates remained at 10 per cent in young mothers, regardless of physical activity.

Prof Jay added that exercise did not have to be life-long to produce a measurable benefit.

Older mouse mothers who exercised for at least three months prior to birth saw an effect similar to that seen in older mothers who had exercised since they were the equivalent of teenagers.

Prof Jay said: 'I hope this study will change the way investigators think about congenital heart disease. Right now the field is very focused on the embryo - finding genetic mutations and figuring out the biology to see how they affect cardiac development.

'That research is important and necessary - but this opens up a whole new conversation.'

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