Pregnant women who witnessed the World Trade Center attacks on 9/11 passed on biological signs of stress to their babies, researchers suggest.
The women were either in or near the World Trade Center on 9/11
Scientists from Edinburgh and New York say tests on infants when they were a year old showed they had low levels of the stress hormone cortisol.
Their mothers also showed low cortisol levels, a sign someone is affected by PTSD the researchers say.
The study is in the Journal of Clinical Endocrinology and Metabolism.
The researchers will follow the babies as the grow up to see if those with lower cortisol levels go on to develop psychological disorders when they are older.
A separate study by Columbia University Medical Centre researchers found one in three New York schoolchildren suffered mental disorders in the wake of the September 11 attacks.
And a quarter experienced one or more of six anxiety disorders in the six months directly after the terrorist outrage.
Cortisol helps the body to raise blood sugar levels and blood pressure in response to stress, but also causes changes in mood and memory.
Previous research, which had largely focussed on children of Holocaust survivors, also found low cortisol levels in the offspring.
However, scientists then concluded the finding was due to the stress of hearing their parent describe their experiences, or living with a parent who was distressed or anxious.
In this study, carried out in 2002, scientists from the UK's University of Edinburgh and Mount Sinai School of Medicine in New York, the US, examined 38 women who were pregnant while at or near the World Trade Center when it was attacked.
Women were asked what stage of their pregnancy they were at on 9/11 and were assessed for PTSD.
Saliva tests were then carried on the women and their babies when the infants were a year old.
Women who developed PTSD had lower than average levels of cortisol that those who did not.
In addition, babies born to mothers with PTSD also had lower cortisol levels than those born to women who did not develop the disorder.
The changes were most apparent in babies born to mothers who were in the last trimester of their pregnancies on 9-11.
Professor Jonathan Seckl of the University of Edinburgh said: "The mother-to-offspring transmission of trauma has often been linked to the way the mother communicates her experience of trauma to the child, or to other consequences of parental trauma such as neglect or inconsistent behaviour towards the baby.
"However, because the babies were about a year old at the time of testing, this suggests the trauma effect transfer may have to do with very early parent-child attachments, cortisol 'programming' in the womb or shared genetic susceptibility."
He added: "It may be that stress has an effect on the developing brain of a foetus."
He said the study showed the effect on the baby depended on the stage of pregnancy the woman was at when she experienced the stressful event.
Professor Stafford Lightman, Director of the Henry Wellcome Laboratories for Integrative Neuroscience and Endocrinology at the University of Bristol, said the findings could be interpreted in a different way.
"Since only very small proportion of people who had the trauma of 9/11 got PTSD, you could say that only those people who
had a genetic predisposition to getting PTSD actually developed it.
"Part of this predisposition might have been the fact that they had relatively lower levels of cortisol before 9/11.
"If this is the case then their infants would also be likely or more likely to inherit the same genetic predisposition and thus to have genetically inherited a relatively lower level of cortisol."
He added: "This paper, therefore, isn't definitive one way or the other. It is however very interesting and it will be very interesting to follow up these children."