Birth weight and child behaviour

Being underweight at birth has been linked to depression among schoolchildren, The Guardian reports. Children who had “a birth weight of less than 2.5kg (5lbs 8oz) were more susceptible to anxiety, depression and being withdrawn at school, and likely to have a history of aggressive outbursts”, the newspaper says. Children who were born in inner cities and had a low birth weight also had “worse attention spans” it adds.

The study looked at a variety of behavioural problems in children in the US, based on the reports of their parents and teachers. It is not possible to say from this study whether low birth weight babies were at greater risk of conditions such as depression or ADHD, as the children were not assessed for diagnosis by a doctor. However, these findings highlight the need to ensure that both mother and baby receive the best care during pregnancy and the importance of reducing inequalities between urban and suburban communities.

Where did the story come from?

Drs Kipling Bohnert and Naomi Breslau from Michigan State University carried out this research. The study was funded by the National Institute of Mental health and the National Institute on Drug Abuse. It was published in the peer-reviewed medical journal: Archives of General Psychiatry .

What kind of scientific study was this?

This was a cohort study looking at the possibility of a link between low birth weight and psychiatric problems in children as they grow up, and whether this was affected by socioeconomic status.

The researchers obtained the newborn discharge lists from two hospitals in Michigan, one serving middle-class suburbs and one serving an inner city area, for the period 1983 to 1985. They randomly selected children from this list who had either a low birth weight (2.5kg or less) or normal birth weight. They did not include children with severe neurological impairment. They contacted the parents of the children to invite them to participate in the study. Children whose parents agreed to participate were assessed for psychiatric problems at age six, 11 and 17 years.

At each assessment, the researchers used standard questionnaires to investigate particular psychiatric problems (internalising, externalising and attention problems) over the previous two to six months. The questionnaires were completed by either the children’s mothers or their teachers. Internalising problems include withdrawal, physical complaints such as aches and pains, and symptoms of anxiety or depression. Externalising problems include delinquent and aggressive behaviour. Children who scored over the accepted normal range on these tests were classified as having problems in these areas.

Of the 1,095 eligible children, 823 (75%) were assessed at age six, 717 (65%) at age 11 and 713 (65%) at age 17. The researchers then used statistical methods to look at whether birth weight affected risk of having psychiatric problems at each age. They also looked at whether this was affected by where the children lived (urban or suburban areas), which assessment questionnaire was used (teacher or mother completed) and the child’s gender. Each analysis was controlled for these other factors.

What were the results of the study?

There were quite a lot of differences in demographic characteristics between the urban and suburban groups, for example, the urban group were more likely to be black, have single mothers and to have mothers with a lower level of education. However, there was less difference in these characteristics between the low birth weight and normal birth weight groups within each of these areas.

The proportion of children with psychiatric problems varied between the different areas, birth weights and ages, with attention problems in between 4% and 22%, internalising problems in between 11% and 25%, and externalising problems in between 9% and 26%. In general, psychiatric problems were more common among children and adolescents living in the urban community than among those in the suburban community.

Children in the low birth weight group were about 53% more likely to have externalising problems and 28% more likely to have internalising problems than those in the normal birth weight group in both areas. The odds of having attention problems was about 2.8 times higher among low birth weight children than normal birth weight children in the urban community, but this difference was not seen in the suburban community. The effect of low birth weight on psychiatric problems did not vary across the different ages.

What interpretations did the researchers draw from these results?

The researchers concluded the effect of low birth weight on psychiatric problems seems to remain the same throughout a child’s school life. They suggest that the different effect of low birth weight on attention problems in the urban and suburban communities implies that social environment may interact with prenatal conditions.

What does the NHS Knowledge Service make of this study?

This study provides evidence that children with low birth weight may be at greater risk of psychiatric problems. The study does have some limitations which should be taken into account when interpreting these results:

As with all studies of this type, the problem is that the groups compared (low birth weight and normal birth weight) differ on a range of characteristics, not just the factor being investigated, and these other characteristics may be responsible for the results seen. For example, socioeconomic factors are known to be strongly correlated with the effects of low birth weight. The researchers tried to take these into account by separating children living in urban areas, which are usually more socially disadvantaged, from those living in suburban areas, which tend to be more socially advantaged. Although this broad grouping by area of residence will remove some of the effects of socioeconomic factors on low birth weight, this is a relatively crude way of determining socioeconomic status, and may not fully remove its effect. In addition, many other factors may be confounding the results, such as maternal drug use.
It is not possible to say for certain whether low birth weight itself increases the risk of behavioural or mental problems, or whether it is confounding factors that are associated with being underweight at birth that could underlie the associations. For example, maternal smoking, substance abuse, younger age and being of single marital status are associated with having a child of low birth weight; these social factors may also increase the risk of mental health problems in both the parent and child.
This article did not look at whether the children had particular psychiatric diagnoses (such as ADHD, depression or anxiety) but looked at more general psychiatric problems, such as internalising, externalising and attention problems. It is not possible to draw conclusions about the effect of birth weight on specific psychiatric diagnoses.
This study was carried out in the US, and results may not apply to other countries.
The study lost a relatively large number of participants by age 17 (35%) and this may have affected the results.

These results support the importance of measures to reduce inequalities between different communities, and of programmes aimed at reducing risk of low birth weight. Further studies will be needed to determine whether programmes that target low birth weight children from poorer socioeconomic backgrounds might improve their behaviour and mental health.

Sir Muir Gray adds...

Prevention of low birth weight remains a public health priority, but it is difficult to achieve.