The worldwide explosion in premature births is fuelling a rise in emotional and behavioural problems, researchers say.

Premature births have risen 30 per cent in the past 30 years, thought to be the result of older mothers and rising obesity. About 50,000 babies a year are born prematurely – before 37 weeks – in England and Wales. Of these, 23,561 were born between 32 and 35 weeks in 2009.

The risks to babies born extremely premature – at less than 26 weeks – are well known, with high rates of physical disabilities and learning difficulties. Now researchers have examined those born moderately premature – at 32-35 weeks – whose numbers are increasing fastest. Babies born after 32 weeks comprise 85 per cent of all premature births.

The results showed that they had higher rates of emotional and behavioural problems at age four on all the measures used. The problems are thought to be the subtle effects of damage caused to the developing brain when the foetus is separated too early from its protective womb and subjected to the trauma of premature birth.

Boys were more likely to have problems such as aggression while girls suffered difficulties such as anxiety. Overall, babies born moderately prematurely were almost twice as likely to have emotional and/or behavioural problems as those born at full term (40 weeks).

The study was conducted among 1,500 children born in the Netherlands. The researchers, from the University of Groningen, say: "Behavioural and emotional problems in early childhood tend to persist in later childhood and adolescence. Moreover mental health problems have a greater impact on the developmental and social competencies at school and in the community. Interventions might include extra support at school or even specialised school services and psychological assistance."