Pregnant women who were traumatised by witnessing the Sept. 11 attacks on the World Trade Center may have passed on a biological sign of stress to their unborn babies, scientists said on Tuesday.

Researchers found the women and their babies had reduced levels of the stress hormone cortisol, which is a sign someone has been affected by Post-traumatic Stress Disorder (PTSD).

Low levels of cortisone have been observed in the children of Holocaust survivors, but researchers had put them down to living with a depressed parent or hearing stories about what had happened to them.

The latest study suggests however that a mother can pass on low cortisol levels to her unborn child, researchers at the Mount Sinai School of Medicine in New York and the University of Edinburgh in Scotland said.

“This shows that exposure to severe stress in pregnancy is associated not only with PTSD in the mothers but also with the biologic marker of it - low hormone levels in the saliva - in the offspring long before they could have been listening to tales,” Professor Jonathan Seckl, of the University of Edinburgh, said in an interview.

Seckl said it was too early to tell if the children would suffer any ill effects. The researchers plan to follow up the children during their development.

In a study of 38 pregnant women Seckl and Dr Rachel Yehuda, of Mount Sinai School of Medicine, found lower than normal levels of cortisol in saliva samples from the women who suffered PTSD linked to the 9/11 attacks, and in their infants.

The mothers and their babies had lower levels of the hormone than women who did not develop PTSD following the tragedy. Even a year after the children’s birth, babies of stressed mothers had lower levels than other children.

“Something happens in PTSD that makes the brain more sensitive to cortisol,” said Seckl. “It marks some change that has happened in the brain.”

The low levels were most apparent in babies born to mothers who were in the final three months of their pregnancy on 9/11, according to the study published in The Journal of Clinical Endocrinology and Metabolism.

“The findings suggest that mechanisms for transgenerational transmission of biologic effects of trauma may have to do with very early parent-child attachments,” Yehuda said in a statement.

She added the effects of cortisol programing could even begin before birth.

The findings suggest a mother suffering from PTSD may provide a less effective, or different, form of care for her child or there may have been some sort of biological signal sent through the placenta to the fetus while it was developing.