Stress suffered by a pregnant woman can damage the unborn child from as early as 17 weeks, according to a new study.

While scientists have long suspected that stress can have a negative impact upon a foetus' health, the new research published in the Clinical Endocrinology journal is the first to point to clear evidence of a link.

Conducted by experts at Imperial College London and Wexham Park Hospital Berkshire, the study has calculated that, after 17 weeks, the quantity of a particular stress hormone in the amniotic fluid is strongly associated with the amount in the mother's bloodstream.

Scientists examined the levels of cortisol in blood samples from 267 women and the amniotic fluid surrounding their babies. The results point very definitely to a correlation, which suggests that many mothers could be unwittingly harming their unborn children.

Dr Pampa Sarkar said: "We are all a product of our developmental history. One of the times when we are most susceptible to the influences of our surrounding environment is when we are developing as a foetus in our mother's womb.

"We now need to carry out further work to unravel the mechanisms by which maternal stress affects the foetus, both during foetal life and through into childhood. We do not wish to unduly worry pregnant women. It should be remembered that one of the best ways for people to avoid general stress is to lead a healthy, balanced lifestyle."

To an extent, stress hormones are actually extremely useful, as they allow the body to deal with particularly stressful scenarios. Ultimately, however, prolonged stress can be dangerous, leading to depression, tiredness and other related conditions.

Separate research on animals has found that stress during pregnancy can have an enormous effect on brain function and behaviour.

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