Nationwide shock after the 9/11 terrorist attacks on New York may have led to an unexpected surge in miscarriages across the United States, say scientists.
Researchers found that numbers of lost male babies spiked around the time of the September 11 2001 suicide attacks which destroyed the World Trade Centre.
Throughout the US, significantly fewer boys were born two months later than should have been. A review of all foetal deaths occurring at or after the 20th week of pregnancy showed that male losses rose 3% above expected levels in September 2001.
Dr Tim Bruckner, assistant professor of public health at the University of California, Irvine, said: "The theory of 'communal bereavement' holds that societies may react adversely to unsettling national events, despite having no direct connection to persons involved in these events.
"Our results appear to demonstrate this, as the shocks of 9/11 may have threatened the lives of male foetuses across the US."
Almost 3,000 people lost their lives after al Qaida terrorists flew two hijacked passenger jets into the 110-storey World Trade Centre twin towers, causing both to collapse. Most of those who died were trapped in the floors above the impacts.
Dr Bruckner's team analysed foetal death rates from all 50 US states between January 1996 and December 2002 to calculate how many male losses would be expected in a normal September. The researchers focused on boys because, across many species, stress is known to reduce male birth rates.
"This is commonly thought to reflect some mechanism conserved by natural selection to improve the mother's overall reproductive success," said Dr Bruckner.
A previous study had found that male foetal death rates in California increased after the 9/11 attacks. The new findings appear in the online journal BMC Public Health.
The scientists wrote: "The terrorist attacks of September 11 2001 induced widespread social and economic disruption, leading to high levels of stress and anxiety in the United States population."