A mother's stress can spread to her baby in the womb and may cause a lasting effect, German researchers propose.They have seen that a receptor for stress hormones appears to undergo a biological change in the unborn child if the mother is highly stressed, for example, because of a violent partner.
And this change may leave the child less able to handle stress themselves.
It has already been linked to mental illness and behavioural problems.
The findings, published in the journal Translational Psychiatry, are based on a small study of 25 women and their children, now aged between 10 and 19.
And the researchers point out that the women involved in the study had exceptional home circumstances and that most pregnant women would not be exposed to such levels of stress day in and day out.
Furthermore, the researchers say the findings are not conclusive - many other factors, including the child's social environment while growing up, might be involved.
But they suspect it is the child's earliest environment, the womb, that is key.For their study, they looked at the genes of the mums and the adolescents to find any unusual patterns.
Some of the teens had changes to one particular gene - the glucocorticoid receptor (GR) - that helps regulate the body's hormonal response to stress.
Such genetic alterations typically happen while the baby is still developing in the womb.
And the scientists believe they are triggered by the mum-to-be's poor state of emotional wellbeing at the time of the pregnancy.
Sensitive window In the study, these mums had been living with the constant threat of violence from their husband or partner. And it would appear this continued stress took its toll on the pregnancy.
When the babies were followed up one to two decades later as adolescents, they had changes in the genetics of their GR that other teenagers did not.
This "methylation" of GR appears to make the individual more tuned in or sensitised to stress, meaning that they will react to it quicker both mentally and hormonally.
As people, they tend to be more impulsive and may struggle with their emotions, explain the researchers, who carried out detailed interviews with the adolescents.
Professor Thomas Elbert, one of the lead researchers at the University of Konstanz, said: "It would appear that babies who get signals from their mum that they are being born into a dangerous world are faster responders. They have a lower threshold for stress and seem to be more sensitive to it."
The investigators now plan to carry out more detailed investigations following larger numbers of mothers and children to see if they can confirm their suspicions.
Dr Carmine Pariante, an expert in the psychology of stress at the Institute of Psychiatry, King's College London, said: "This paper confirms that the early foundation years start at minus nine months."
He added: "Pregnancy is uniquely sensitive to a challenging maternal psychosocial environment - much more than, for example, after the baby is born. As we and others have been advocating, addressing maternal stress and depression in pregnancy is a clinically and socially, important strategy."