The study is the first to show that unborn babies are exposed to their mother's stress at such an early stage in pregnancy

The study's authors said they did not wish to "unduly worry pregnant women", but highlighted the need to lead a "healthy balanced lifestyle" to avoid general stress.

The findings come days after the government changed its advice to pregnant women and those trying to conceive, warning them to abstain from drinking alcohol.

Previous guidelines had said they could drink up to two small glasses of a week.

The change in advice, which government health advisers said was made to avoid confusion, rather than in response to new medical evidence, prompt- ed claims from some critics that pregnant women are increasingly becoming targets in an obsessively anti-risk culture.

Researchers in the latest study led by Professor Vivette Glover at Imperial College London and the consultant obstetrician Pampa Sarkar, from Wexham Park hospital, Berkshire, UK, measured levels of the stress hormone cortisol in 267 pregnant women.

Cortiso1, which is pumped into the blood when we become , is good in the short term, as it helps the body to deal with a stressful situation, but long-term stress can cause tiredness, depression and make an individual more prone to illness.

Dr Sarkar said further research was needed into how high levels of stress in a mother affect the unborn baby "We are all a product of our developmental history," she added.

"Our research shows that the is exposed to cortiso1 in the maternal blood, and we also demonstrated that at and above 17 weeks, the in amniotic fluid had a strong positive relationship with cortiso1 in maternal

Dr Sarkar continued, "We found that the strength of this correlation became stronger with increasing gestational age. We now need to carry out further research to unravel the mechanisms by which maternal stress affects the foetus, both during foetal life and through into childhood."

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