The 50,000 babies born prematurely each year in England and Wales may have an up to three-fold higher risk of developing a severe psychiatric disorder later in life, researchers have found. 

The finding, from one of the largest studies to investigate the link between prematurity and later admission to psychiatric hospital, shows the explosion  in premature births seen around the world could be fuelling a rise in mental ill health.

Premature births have risen by 30 per cent in the last 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 at age 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 hospitalisation with mental health problems after the age of 16.

The risk varied with the disorder - it was 2.5 times higher for psychosis, 3 times for severe depression and 7.5 times for bi-polar disorder.

Moderately premature   babies born between 32 and 36 weeks had an overall doubled risk of being hospitalised with the disorders.

However, even with the increased risk, few babies were affected - 6 in 1,000 for the very premature and 4 in 1,000 for the moderately premature compared with 2 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.

However, she said they might underestimate the true impact of prematurity on mental health because the researchers  only considered the most severe cases.

Around one in four of those born prematurely has some psychiatric problem by the age of 18 compared with just over one in ten of those born at term.

The findings, published in 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. The immature nervous system in those born prematurely is particularly vulnerable to neonatal brain injury resulting from birth complications.”

“The strongest association we found in this study was to mental health disorders known to have a strong biological basis, such as bipolar disorder, further adding to the theory that neurodevelopmental differences in those born prematurely may play an important role for later mental health.”

Dr Abraham Reichenberg, a co-author of the study said that 20 years ago babies born before 32 weeks had a low chance of survival.

“Today most will survive. We were anxious to look at the consequences. But these children were born in the 1970s. Does this [increase in mental disorders] still exist for babies born in the 1990s, when medical care has advanced? We don’t know. In ten years time we could be sitting here and saying there is no excess risk for babies born from 32 to 36 weeks - only for those born very prematurely at less than 32 weeks.”

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.

7.5 Babies born before 32 weeks have a 7.5 times higher risk of developing bipolar disorder