Children whose mothers were overly stressed during pregnancy may themselves be more vulnerable to anxiety as a result, research suggests.
The researchers looked at the stress hormone cortisol
High levels of stress hormone may cross the placenta and affect the baby in the womb in a way that carries long-term implications, UK scientists believe.
A Bristol University team found anxiety in late pregnancy was linked to higher cortisol levels in children aged 10.
The work in Biological Psychiatry tallies with earlier animal findings.
Past studies have shown stress in animals during pregnancy affects the body's stress response system - the hypothalamic-pituitary-adrenal (HPA) axis which controls stress hormone levels, including cortisol.
But scientists have not been able to show that it also affects humans in the same way.
US psychologist Dr Thomas O'Connor, from the University of Rochester in New York, working with UK colleagues from Bristol University and Imperial College London, studied 74 children aged 10.
They analysed saliva samples first thing in the morning and three times a day on three consecutive school days to monitor levels of stress hormones.
The children's mothers had completed questionnaires 10 years previously, when they were expecting, about any stress or anxiety they were experiencing during their pregnancy.
The researchers looked back at this data to compare the results with those of the saliva tests.
The children with high levels of cortisol in their saliva tended to be born to the mothers who reported the most stress during their pregnancy.
Dr O'Connor said: "These results provide the strongest evidence to date that prenatal stress is associated with longer term impact on the HPA axis in children.
"Several human studies of children and adults suggest that elevated basal levels of cortisol are associated with psychological risk...notably depression and anxiety.
"Our findings point to a possible mechanism by which prenatal stress or anxiety may predict these disturbances in early adolescence, and possibly into adulthood."
However, he said much more work was needed to check that this was the case.
He also pointed out that it was not clear whether high cortisol itself could cause psychological disturbance. Some psychiatric disorders have been linked with low rather than high cortisol levels.
Other factors, such as the personality of the child and the environment they are living in, may play a part in childhood stress too.
Gillian Fletcher of the National Childbirth Trust said: "It's certainly something we need to look at in more detail.
"We don't want to make women who are pregnant more anxious than they already are by saying stress could have long term implications for the growing child."
She said there were many things pregnant women could do to alleviate stress and anxiety.
"Antenatal classes can help allay fears women might have about pregnancy, the birth and the health of the baby.
"They can also teach a women about stress and relaxation and taking life more slowly can help. It's trying to find a balance."