Premature babies are much more likely to become hyperactive children than full-term infants, research suggests.
Children born prematurely have an increased risk of ADHD
Danish researchers found babies born at between 34 and 36 weeks were 70% more likely to develop attention deficit hyperactivity disorder.
Those born at less than 34 weeks were three times more likely to have ADHD.
The study of 30,000 children in the Archives of Diseases in Childhood also found an increased incidence of ADHD in children with a low birth weight.
Previous small studies have shown a link between prematurity and increased risk of ADHD but have only looked at babies born very prematurely - at less than 28 weeks gestation.
Researchers used data from Danish national registers to compare the risk of ADHD or the related hyperkinetic disorder and preterm birth in 834 children with the conditions and 20,100 controls.
Babies born at term but weighing between 3lb 3oz and 5lb 5oz (1.5kg to 2.5kg) at birth were 90% more likely to develop hyperactivity disorder and ADHD than those of normal weight.
A birth weight of 5lb 6oz to 6lb 6oz was associated with a 50% increased risk.
Around 90% of the children with hyperkinetic disorder or ADHD were boys.
Writing in the journal, Dr Karen Linnet said: "Previous research shows that children born below 28 weeks have an increased risk of ADHD.
"However, most preterm children are born with higher gestational ages of 28 to 36 weeks.
"This large population-based study showed that preterm delivery near term and proxy measures of intrauterine growth in children born at or above term increase the risk of hyperkinetic disorder."
Charlotte Davies, spokesperson for baby charity Tommy's, said: "Although the correlation between babies who are born prematurely and the increased risk of developing learning difficulties and behavioural problems is well-documented, this study is of particular significance because it has massive implications for the UK.
"Currently, the rate of premature birth in the UK stands at the highest rate in Europe, with over 45,000 premature births every year, with a large proportion of these children born between 34 and 36 weeks.
She said there was a "desperate need" for more research into the causes of premature birth.
Premature baby charity Bliss said called for further research into why premature babies were more susceptible to developing hyperkinetic disorder.
A spokesman said: "In particular we need to investigate what can be done to offer them and their parents' additional support to ensure that they have the best quality of life."
Professor James Walker, consultant obstetrician at St James Hospital in Leeds said the study showed the risk of ADHD was not just confined to very preterm babies but that the earlier babies were born the more likely they were to have various developmental problems.
"It's probably to do with nutrition and I suspect some of the problems may have been overcome as nutrition for neonates has got a lot better," he said.