ADHD 'more common in overdue children'

“Babies who spend too long in the womb are twice as likely to suffer behavioural problems in early childhood,” the Daily Mail has warned today.

The story comes from a large study exploring whether or not babies who are born “late” (defined as at or after 42 weeks of pregnancy) are more likely to have behavioural or emotional problems in early childhood. The study found that the parents of children who had been born late were twice as likely to report behavioural problems as the parents of those born within the normal range of between 37 and 42 weeks. Parents of late-born children were also more likely to report symptoms of attention deficit hyperactivity disorder (ADHD) in their children. The parents in the study were questioned twice, once when their children were 18 months old and again at three years old.

The findings of this large study are interesting but do not show that being born after 42 weeks leads to behavioural problems or ADHD. This is because the study had several limitations, including its reliance on parents reporting their child’s later behaviour. Parental reporting can be less reliable than a formal diagnosis from doctors. It is also possible that both gestational age and childhood behaviour might have been influenced by some other unknown factor.

At present, pregnant women who go beyond term are closely monitored and may be induced if there are signs of the baby being in distress. It is already known that babies born post-term may have an increased risk of some problems around the time of birth. Further research is required to assess whether there are any longer-term effects.

Where did the story come from?

The Dutch study was carried out by researchers from Erasmus University and Erasmus MC university medical centre. It was funded by the Sophia Children’s Hospital Fund and the WH Kroger Foundation.

The study was published in the peer-reviewed International Journal of Epidemiology. It was covered fairly although uncritically in the newspapers. The Daily Telegraph correctly pointed out that it was not clear whether behavioural problems were caused by babies being overdue or whether either or both outcomes were caused by an underlying medical or social factor.

What kind of research was this?

This was a cohort study of more than 5,000 pregnancies that aimed to explore whether babies born late (post-term) had a higher risk of behavioural and emotional problems (including ADHD) in early childhood. The authors said that research on post-term birth has shown increased risks to the baby’s health during the first year of life, but that the long-term consequences are unclear. They also point out the long-term problems associated with preterm birth (usually defined as before 37 weeks of pregnancy) are well established.

In a cohort study, researchers typically follow a group of people for a period of time to find out if there is any association between a particular event (in this case, post-term birth) and an outcome (behavioural problems). This type of study is useful but on its own can't prove that one factor causes another, and therefore in this instance can't prove that post-term birth leads to behavioural problems down the line. Both outcomes could be due to some other unknown factor driving the two.

What did the research involve?

Researchers recruited pregnant women resident in Rotterdam, the Netherlands, who gave birth between 2002 and 2006. Of the 7,484 children born to this group, a total of 5,145 children were followed in the study (a response rate of 78%).

Researchers assessed the “gestational age” of each baby at birth, which was based on a foetal ultrasound exam given during the pregnancy. Gestational age is often based on the number of weeks that have passed since the end of a woman’s last menstrual period, but an ultrasound scan which measures the size of the foetus is thought to be more accurate.

The babies were classified into three main groups:

those born between 37 weeks and 41 weeks, six days (ie within the normal range)
those born before 37 weeks (preterm)
those born at 42 weeks or after (post-term)

An additional sub-group was also included, of babies born before 35 weeks.

The parents of these babies were asked to complete a standard, validated checklist called the Child Behaviour Checklist, which was sent as a postal questionnaire. The checklist is designed for assessing toddlers and looks at a child’s behaviour when they are 18 months old and again when they are three years old. Mothers were asked to complete the questionnaire when their child was 18 months old and both parents were asked to complete it when their child was three years old.

The checklist had 99 questions about a child’s behaviour in the preceding two months, each scored on a three point scale (0 = not true, 1 = somewhat true, 2 = very true or often true). From this, each child was given a total score. The researchers said the score on the checklist was matched against other formal diagnoses of emotional disorders, including ADHD, but a clinical diagnosis of ADHD was not made for any child in the study.

The researchers then used several methods to analyse the relationship between gestational age at birth and the presence of emotional or behavioural problems, as indicated by the checklist. The results were adjusted for factors that might influence a child’s behaviour, such as:

the mother’s age and education
parental psychological problems
whether smoking or drinking occurred during pregnancy
the child’s gender
family income

What were the basic results?

Among the 5,145 children recruited, 88.2% were born at within the normal time range (to term), 7.4% were born late (post-term) and 4.4% were born prematurely (preterm).

The researchers found that babies born prematurely and those born late scored higher for behavioural and emotional problems at 18 months old and three years old than those born at term.

Compared with children born to term, children born post-term had a higher risk for overall problem behaviour [odds ratio 2.10, 95% confidence interval 1.32 to 3.36] and were almost two-and-a-half times as likely to have attention deficit or hyperactivity problem behaviour (OR 2.44, 95% CI 1.38 to 4.32), according to their parents.

How did the researchers interpret the results?

The researchers said that children born late were more likely than those born to term to have emotional and behavioural problems, including ADHD, in early childhood. They said that there are several possible explanations for this association, including the possibility that an “old” placenta at the end of longer pregnancies offers fewer nutrients and oxygen than a full-term foetus requires, which may predispose them to abnormal development.

Also, it is possible that a disturbance of the “placental clock”, which controls the length of pregnancy, could lead to abnormalities in the way that hormones interact with the brain. This could increase a child’s vulnerability to behavioural problems later in life. They also suggested that the association between late delivery and birth problems such as prolonged labour might have long-term effects, but said their results did not suggest increased foetal stress at the time of labour and childbirth for babies born late.

The results, they said, suggest that late-born babies may experience neurodevelopmental delays. Further research is needed to determine the cause of post-term birth and to reduce post-term birth rates, they argued.

Conclusion

The exact causes of attention deficit hyperactivity disorder (ADHD) are not fully understood, and this large study raises the possibility that late birth may be associated with a higher risk of the disorder in childhood. While this doesn’t mean it has found any cause-and-effect relationship between the length of time a baby spends in the womb and their behaviour as a young child, it certainly raises some interesting possibilities about what factors might contribute to the increasingly common condition. For example, there have also been suggestions that being born early (preterm) may also be linked to an increased risk of ADHD.

Although the study’s design means it can’t prove a cause-and-effect relationship, it does have some strengths. For example, the researchers used foetal ultrasound to get an accurate assessment of probable gestational age at birth and also used a validated checklist for childhood behaviour to assess children for behavioural and emotional problems.

However, the study also relied on parents assessing and reporting their children’s behaviour themselves. This introduces the possibility of bias and it's important to note that only symptoms of ADHD were assessed, as the diagnosis of ADHD was not clinically confirmed. This is not the ideal way of assessing behavioural disorders such as ADHD. Furthermore, assessments of behaviour have so far only been conducted up until the age of three, so it is unclear whether the children’s behavioural symptoms would persist into later childhood or if the children would naturally grow out of them.

As the researchers note, the trial was not “blinded” for gestational age, which means parents were made aware of whether or not their child had been born late. Although parents were not aware of the aim of the research, mothers who were conscious that their babies were born late (as well as early) might subjectively perceive more behavioural problems in those children later on.

Finally, although the researchers controlled for several factors which might affect the results of the study, it is possible that some confounding factors (such as family dynamics) influenced the results. It is also possible that both late birth and behavioural problems were influenced by an underlying, as yet unrecognised, social or medical factor.

Further research is required in this important area.