The 50,000 babies born prematurely each year in England and Wales may be up to three times more likely to develop a severe psychiatric disorder later in life, researchers have determined.
The finding, from one of the largest studies to investigate the link between prematurity and later admission to a psychiatric hospital, shows the explosion in premature births around the world could be fuelling a rise in mental illnesses.
Premature births have risen by 30 per cent in the past 30 years, thought to be the result of older mothers and increasing obesity. The effects have been seen in all developed countries.
Today, about one in 13 babies is born prematurely – before 37 weeks – in England and Wales, with 8,000 born very prematurely at less than 33 weeks' gestation.
Previous research has shown that premature babies are more likely to have emotional and behavioural problems after the age of four, affecting their intellectual and social development.
Now researchers from the Institute of Psychiatry in London and the Karolinska Institute in Stockholm have found the very premature babies born before 32 weeks have a three-fold increased risk of hospital-based care with mental health problems after the age of 16.
The risk varied with the disorder – it was 2.5 times higher for psychosis, three times for severe depression and 7.5 times for bipolar disorder. Moderately premature babies born between 32 and 36 weeks had a doubled risk of being taken to hospital with the disorders.
But even with the increased risk, few babies were affected – six in 1,000 for the very premature and four in 1,000 for the moderately premature compared with two in 1,000 for those born at term.
"Most premature babies were fine," Dr Chiara Nosarti, of the Institute of Psychiatry and lead author of the paper, said. But she said they might underestimate the true impact of prematurity on mental health because the researchers only considered the most severe conditions.
Around one in four of those born prematurely has some psychiatric problem by the age of 18, compared with just over one in 10 of those born at term.
The findings, published in the Archives of General Psychiatry, were based on 1.5 million birth records in Sweden between 1973 and 1985.
Dr Nosarti said: "We believe that the increased risk of mental disorders in those born very prematurely can be explained by subtle alterations of brain development."
Neuroprotective measures such as head cooling at birth, giving special nutrients in the diet and cognitive training at school could help ameliorate the effects of prematurity on the brain, the researchers said.