Overdue babies are just as likely to suffer problems in later life as premature babies, according to a report.
In the first study of its kind, researchers in the Netherlands said children who spend too long in the womb are twice as likely to suffer from the behavioural problem ADHD when they get older.
The risk affects babies born after a pregnancy of 42 weeks, who were found to be twice as likely to suffer in the long-term compared with those born after the normal 40-week length of pregnancy.
Writing in the International Journal of Epidemiology, the study's lead author Dr Hanan El Marroun said: "Children born too late are more than twice as likely as term-born children to have clinical ADHD."
But she added that the risk for those born too early is "similar".
"We were aware of potential complications caused by babies born prematurely, but these findings suggest there is also a greater risk of long-term problems among babies born too late," she warned.
She explained that those who spend longer than 40 weeks in the womb tend to be bigger.
The "old" placenta cannot provide the level of nutrients and oxygen they require, she said, which may lead to abnormal physical, emotional and behavioural development.
Dr El Marroun has now called for women to be induced or to have a Caesarean at between 40 and 41 weeks, "because it might reduce emotional problems and has the advantage of preempting some of the other complications involved when women give birth to bigger babies".
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