Premature birth linked to increased risk of ADHD

"Babies born just one month premature are more likely to develop attention deficit hyperactivity disorder (ADHD) in later life, new research suggests," reports the Mail Online.

Researchers in Norway compared a group of children born prematurely with a control group of children born at full term to see if either was more likely to develop symptoms of ADHD, such as hyperactivity and reduced attention span, at pre-school and school age.

Babies are considered to be full term if they are born at 37 weeks or later into the pregnancy. Before this, they're considered premature.

In this study, the researchers found that children who were born at least 4 weeks before full term (at 33 weeks or earlier) were more likely to show symptoms of ADHD than those born at full term. The association seemed to be stronger among girls than boys.

Although the media reported this as though it were a new finding, this link was already known from previous research. ADHD is a complex condition, and its causes are not fully understood. Environmental factors – such as whether a child was born prematurely – and genetics are thought to play a role.

While there is no guaranteed method of preventing premature birth, mums-to-be can reduce their risk by staying active, and avoiding drinking alcohol and smoking. Read more advice about staying healthy during pregnancy.

Where did the story come from?

The study was carried out by researchers from the University of Oslo, the University of Bristol and the Norwegian Institute of Public Health.

It was funded by the Norwegian Ministry of Health, and the Norwegian Ministry of Education and Research. It was published in the peer-reviewed Journal of the American Medical Association Pediatrics.

There were a few potentially misleading aspects in the reporting of this study. For example, the Mail Online's headline suggests that premature babies are more likely to develop ADHD in later life.

While the study did assess symptoms associated with ADHD – such as poor attention span, hyperactivity and impulsiveness – it did not follow up the children to see if they received a confirmed diagnosis of ADHD. It could be the case that some children would have "grown out" of some of the symptoms as they matured.

What kind of research was this?

This was a prospective observational study looking at whether premature birth was linked to increased risk of ADHD symptoms.

The advantage of this study design is that researchers can decide at the start what type of information they think they need to collect in order to answer their question, including information on possible confounders.

The main limitation is that, even with the best of methods, it's very difficult to single out the effect of one specific factor (prematurity) from others that could influence the result, such as family environment.

What did the research involve?

The study recruited pregnant women from across Norway between 1999 and 2008.

The researchers recorded whether the women's babies were born prematurely or not, and then measured the children's levels of ADHD symptoms at pre-school age (5 years old) and school age (8 years old). They then looked at whether those born prematurely were more likely to have increased levels of ADHD symptoms.

The researchers sent the women questionnaires about their pregnancies when they were at week 17 and week 30 of pregnancy, and 6 months after giving birth. They also obtained medical records that documented how long the mother had been pregnant when the baby was born (the "gestational age" of the baby) as well as other details of the birth.

For their analyses, they grouped the babies according to when they were born:

• early preterm (delivery at gestational weeks 22 to 33)
• late preterm (gestational weeks 34 to 36)
• early term (gestational weeks 37 to 38)
• term (at gestational week 40)
• late term (after gestational week 41)

During follow-up, the mothers also completed 2 standard questionnaires about their child's levels of inattention, and hyperactivity or impulsivity. These questionnaires asked, for example, how frequently the child experiences these symptoms and to what extent each had been a problem.

The researchers then tallied up scores on these questionnaires to indicate the child's level of ADHD symptoms.

A total of 113,227 children were included in this study, including 33,081 who were siblings.

The researchers used various different comparisons to look at the link between gestational age at birth and ADHD symptoms. They compared children born early or late with those who were born at term – first in all of the children in the sample and then only in the siblings.

They used siblings to try to rule out the possibility that genetic factors or unmeasured environmental factors shared by families might be causing the link.

The researchers also considered other factors that might affect the results, including:

• gender
• whether the mother had more than 1 child
• if the baby was born smaller than average
• if the baby had been born with any physical abnormalities
• how many times the mother had been pregnant
• if the mother had experienced bleeding before week 13 of the pregnancy

What were the basic results?

At age 5

Children born early preterm had more symptoms of ADHD than children born at term, based on the mothers' ratings. This was the case for ADHD symptoms overall, and for inattention and hyperactivity or impulsivity individually.

The results from siblings suggested this effect was not solely caused by shared genetics or other unmeasured environmental factors.

The link between early prematurity and ADHD symptoms was stronger for girls than boys at this age.

At age 8

Children born early preterm had higher levels of inattention symptoms, but not hyperactivity or impulsivity.

How did the researchers interpret the results?

The researchers stated that, after accounting for unmeasured genetic and environmental factors, early preterm birth was associated with a higher level of ADHD symptoms in pre-school children.

They said this shows the potential benefits of reducing preterm births, and the importance of providing support to children born preterm.

Conclusion

This study fits in with others that have found a link between premature birth and higher levels of ADHD symptoms in childhood. What it adds to these studies is that it used pairs of siblings to help account for any genetic or environmental factors that might be contributing to this finding.

However, there were limitations to the study.

Only 41% of pregnant women who were asked to participate did, which could mean the results are not representative of the overall population.

In particular, younger mothers, smokers and women with lower educational levels were under-represented in the study. These characteristics are also linked to the risk of ADHD, so this might have skewed the results.

ADHD symptoms were reported by the mothers and not verified by other observers. Mothers whose children were premature may have been more watchful for signs of ADHD, which could have influenced their ratings. It's also worth noting that this study had doctors assess the children to see whether any would qualify as having a diagnosis of ADHD.

We know ADHD is a complex condition and that many factors are likely to play a role in its development. While the researchers did what they could to account for the influence of potential confounders, the observational nature of the study means it's difficult to be certain that the increased risk was definitely caused by prematurity alone.

If parents are concerned their child is showing symptoms of ADHD, they should talk to their GP. Read more about seeking a diagnosis if you are concerned about your child's behaviour.