Babies born prematurely have a much higher risk of developing severe mental disorders including psychosis, bipolar disorder and depression, according to a study to be published on Monday.
Scientists in Britain and Sweden found that people born very prematurely - at less than 32 weeks' gestation - were three times more likely than those born at term to be hospitalised with a psychiatric illness at aged 16 and older.
The researchers think the increased risk may be down to small but important differences in brain development in babies born before the a full 40 week gestation period.
The risk varied depending on the condition - psychosis was 2.5 times more likely for premature babies, severe depression 3 times more likely, and bipolar disorder 7.4 times more likely for those born before 32 weeks.
The study, to be published in the Archives of General Psychiatry journal, also found smaller but significant increased psychiatric risks for babies born only moderately early, at between 32 and 36 weeks.
Chiara Nosarti from the Institute of Psychiatry at King's College London, who led the research, said it showed "a very strong link" between premature birth and psychiatric disorders.
"Since we considered only the most severe cases that resulted in hospitalisation, it may be that in real terms this link is even stronger," Nosarti told reporters at a briefing.
She stressed, however, that: "The majority of individuals who are born prematurely have no psychiatric or cognitive problems are absolutely healthy and well functioning."
The disorders affect between 1 and 6 percent of the population as a whole, she said.
Nosarti worked with researchers at the Karolinska Institute in Sweden and analysed data from 1.3 million medical records in Sweden between 1973 and 1985 to find all those admitted to hospital with their first episode of a psychiatric disorder by 2002.
They then looked back to find which of these people had been born either very or moderately prematurely.
"The strongest association we found was to mental health disorders known to have a strong biological basis, such as bipolar disorder," Nosarti said.
She said that suggested that subtle alterations in the brain development of those born early may play an important role in mental health later in life.
Previous studies have found that premature babies have higher risks of various health and developmental problems but this was the first to look in detail at links between severe psychiatric disorders and pre-term births.
A United Nations-backed report in May said 15 million babies were born prematurely in 2010, and rates of the phenomenon are soaring around the world partly because of advances in medicine which allow even extremely premature babies to survive.
Asked whether this increase could account in part for similar rises in mental illnesses and neurodevelopmental disorders such as autism, Nosarti said a small fraction of that may be explained by pre-term births.
The study found around 6 percent people with severe depression and 6 percent of those with psychosis were born pre-term, as were around 11 percent of people with bipolar disorder.
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