Children born a few weeks early are significantly more likely to suffer behavioural and/or emotional problems before they start school, researchers have said.
Those born between 32 and 35 weeks' gestation (moderately premature) are almost twice as likely to have behavioural and emotional problems as children born at term (38 to 41 weeks).
The study, published online in the journal Archives of Disease in Childhood, found moderately premature children had a range of issues. The boys were more likely to act out their problems through their behaviour than the girls, who tended to internalise them.
Researchers analysed data for 995 children who were moderately premature and 577 born at term.
Parents completed a questionnaire on when the children were aged four using an established behavioural checklist. This included questions on whether youngsters internalised or externalised their problems, whether they suffered anxiety or depression, were withdrawn, had sleep problems, unexplained medical complaints (somatic), issues in paying attention or aggressive behaviour.
Overall, those children born prematurely were almost twice as likely to suffer somatic complaints and externalise their problems through their behaviour. The tendency to act out was highest among the boys (one in 10 externalised problem behaviours), while a similar proportion of girls internalised their issues.
Premature children were almost twice as likely to suffer problems overall than those children born at term. This particularly affected premature girls, who were more likely than girls born at term to suffer a range of issues.
The authors, from the University of Groningen in the Netherlands, concluded: "Our results demonstrate that moderately premature children are more likely to already have behavioural and emotional problems before they enter school. Therefore, moderately premature children could be a potential target group for the prevention of mental health problems, as behavioural and emotional problems in early childhood tend to persist in later childhood and adolescence."
Professor Andrew Shennan, consultant obstetrician at the baby charity Tommy's, said: "It is increasingly appreciated that babies born even a few weeks early can have long-term behavioural issues, even into adulthood, so this data confirms previous findings. Therefore the moderately pre-term group remains very important and research needs to consider identifying and treating women even after 32 weeks gestation."
Data from the Office for National Statistics shows that 23,561 babies were born alive at 32 to 35 weeks' gestation in 2009. This is from a total of 705,810 live births in England and Wales that year.