Pre-pregnancy obesity linked to childhood ADHD and autism

Monday November 27 2017

"Women who are overweight or obese before they get pregnant are more likely to have a child who is autistic or with behavioural problems, a new review found," reports the Mail Online.

The news comes from a review that pooled the findings of 32 studies looking for a possible link between whether a woman was overweight or obese before she became pregnant, and neurodevelopmental disorders like attention deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASD) in their children.

The causes of these conditions, aside from possible genetic factors, aren't well understood.

Overall, the study found women who were overweight before they became pregnant had about a one-third increased risk of having a child with ADHD, and a 10% increased risk of having a child with autism spectrum disorders, compared with normal-weight women.

If the mothers were obese, the risks were slightly higher (two-thirds and one-third increased risk).

Although this is a useful review, it has its limitations. The findings are based on observational studies that varied widely in their studied populations, how they assessed weight status and neurodevelopmental outcomes, and the other factors taken into account.

It's possible that genetics, health, lifestyle and other family environmental factors could have played a role in the likelihood of having a child with one of these conditions.

As such, the studies aren't able to prove there's a direct link between these disorders and women who were overweight or obese before falling pregnant.

But the various risks of being overweight or obese are well established.

Obviously, not all pregnancies are planned. But if you're planning for a baby, it's recommended that the mother-to-be achieves or maintains a healthy weight before trying to conceive.

Read more about body weight and pregnancy.

Where did the story come from?

The study was carried out by a team of researchers from two institutions in the US: Duke University Medical Center and Virginia Commonwealth University.

It was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

The study was published in the peer-reviewed journal Pediatric Obesity.

The Mail Online article generally takes the findings at face value without recognising the limitations of this study – that is, we don't know that obesity is a causative factor of these disorders.

The story also mainly focuses on autism when in fact the study looked at several behavioural conditions, such as ADHD and cognitive and intellectual impairment.

What kind of research was this?

The researchers carried out a systematic review followed by a meta-analysis.

They looked at existing evidence to investigate the association between mothers being obese or overweight before they were pregnant, and neurodevelopmental conditions such as ADHD or autism spectrum disorders in their children.

The prevalence of child behavioural and developmental conditions in western countries is rising, but the causes are poorly understood.

Antenatal exposure to environmental toxins and maternal stress and nutrition have been suggested as possible causes in previous research. This study looked at the possible link with a mother's weight.

Systematic reviews are the best way of gathering the published literature on a topic to look for a potential association between an exposure and an outcome.

The difficulty is that the findings of a systematic review are only as good as the studies the researchers include as part of their analysis.

As the studies included in this research are observational, it's difficult to account for many other factors that could have had an influence on the findings.

What did the research involve?

The researchers searched for observational studies looking at the link between a mother being obese or overweight before pregnancy and neurodevelopmental disorders in her child, including autism spectrum, ADHD, and cognitive and intellectual impairment.

They assessed the quality of these studies, looking at factors such as:

  • type of study
  • sample size
  • loss to follow-up
  • how participants were recruited
  • characteristics of the cohorts (the group being studied)
  • criteria for determining and categorising pre-pregnancy weight
  • comparability of obese and non-obese groups (ideally, the two groups should be matched in other important characteristics, such as age or smoking history)
  • how neurodevelopmental outcomes were measured or diagnosed

Forty-one studies met the inclusion criteria, and the findings of 32 were pooled in a meta-analysis (6 case-control studies and 26 cohort studies).

Twenty studies came from the US, with a handful from the UK, the Netherlands, Denmark, Finland, Sweden, Norway and Australia.

What were the basic results?

Pooling data from 22 cohort studies looking at being overweight, mothers who were overweight before becoming pregnant were more likely to have a child with one of the conditions being investigated (odds ratio [OR] 1.17, 95% confidence interval [CI]: 1.11 to 1.24).

From 25 cohorts that included obese women, obesity before pregnancy was associated with an even greater increase in risk of having a child with one of the conditions (OR 1.51, 95% CI: 1.35 to 1.69).

More specifically, mothers who were overweight before pregnancy were more likely than normal-weight mothers to have a child with:

  • ADHD (OR 1.30, 95% CI: 1.10 to 1.54)
  • autism spectrum disorders (OR 1.10, 95% CI: 1.01 to 1.21)
  • cognitive or intellectual developmental delays (OR 1.19, 95% CI: 1.09 to 1.29)

There was no link between a mother being overweight and having a child with emotional or other behavioural problems.

Mothers who were obese before pregnancy were even more likely to have a child with these conditions:

  • ADHD (OR 1.62, 95% CI: 1.23 to 2.14)
  • autism spectrum disorders (OR 1.36, 95% CI: 1.08 to 1.70)
  • cognitive or intellectual delay (OR 1.58, 95% CI: 1.39 to 1.79)
  • emotional or behavioural problems (OR 1.42, 95% CI: 1.26 to 1.59)

How did the researchers interpret the results?

The researchers concluded: "Results show that children born to mothers who are overweight or obese are at a higher risk of neurodevelopmental problems, including ADHD, ASD, greater emotional and behavioural problems, and cognitive delay."

They say that a critical next step could be to start looking at biological causes for the links, such as maternal obesity possibly influencing inflammation levels during a child's development in the womb.

Conclusion

This review gathered a large body of existing observational studies that investigated the association between mothers being overweight or obese before pregnancy and neurodevelopmental disorders in their children, such as ADHD.

The limitations are:

  • The findings are based on pooled data from observational studies. This means that many other genetic, health, lifestyle and environmental factors could have had an influence both on the mothers' risk of being overweight or obese and the risk of child developmental disorders. The authors acknowledged this limitation, noting that the studies differed in the confounding factors that were taken into account.
  • The study reports on the relative increase in risk compared with normal-weight mothers, but it isn't clear what their baseline risk level was. For example, the risk for any mother of having a child with an autism spectrum disorder is low to start with, so the 10% increase on that risk for overweight mothers may not give such a high overall risk.
  • Related to this, the number of children in these studies who developed these conditions may have been quite small, and analyses including small numbers may give imprecise risk figures.
  • The studies varied in terms of what measurement scales they used to look at child outcomes.
  • Pre-pregnancy body mass index (BMI) and maternal weight was in some cases self-reported, which may have been inaccurate.
  • Though the pooled analyses found positive links, there was a high degree of difference (heterogeneity) in the results of the individual studies. This most likely results from the differences in study methods, included populations, sample sizes, and how researchers assessed weight and outcomes. It suggests some of these studies may not have been appropriate to pool together and decreases confidence in the findings.

The causes of conditions such as ADHD and autism spectrum disorders remain largely unknown. But we do know that being overweight or obese has adverse health effects.

If you're planning to have a baby, taking steps to lose weight (if you're overweight) before conceiving could benefit both you and your baby, as well as reduce the risk of complications during pregnancy.

The NHS Weight Loss Plan provides information on dieting and exercise techniques that can lead to weight loss over time.

Analysis by Bazian
Edited by NHS Choices