Birthweight and behavioural problems in children: a modifiable effect?

Yvonne J Kellya, James Y Nazrooa,b, Anne McMunna, Richard Borehamb and Michael Marmota

a International Centre for Health and Society, Department of Epidemiology and Public Health, University College London, 1–19 Torrington Place, London WCIE 6BT, UK.
b National Centre for Social Research, 35 Northampton Square, London EC1V 0AX, UK.

Dr Yvonne J Kelly, International Centre for Health and Society, Royal Free and University College London Medical School, 1–19 Torrington Place, London WCIE 6BT, UK. E-mail: y.kelly{at}public-health.ucl.ac.uk

Abstract

Background Low birthweight has been shown to predict behavioural problems in children. Less is known about the effect of birthweight, and how this may interact with the social environment in determining behaviour in a general population sample. We have examined the relationship between birthweight and social factors on childhood psychological well-being.

Methods Cross-sectional analysis of data on 5181 children aged 4–15 years from a randomly selected household population, the 1997 Health Survey for England. The main outcome measures were behavioural problems as defined by the Strengths and Difficulties Questionnaire (SDQ) in relation to birthweight and social environmental factors.

Results Birthweight was a significant predictor of total difficulties score (odds ratio [OR] = 1.27, 95% CI : 1.07, 1.49), hyperactivity in boys (OR = 1.25, 95% CI : 1.05, 1.51), and peer problems in girls (OR = 1.24, 95% CI : 0.99, 1.53). There was a strong social class gradient in the prevalence of behavioural problems for all birthweight tertiles. Bivariate analysis showed that high total difficulties score was significantly more common in lower birthweight tertiles for social classes III non-manual and III manual (P-value for trend 0.05 and 0.03, respectively). There were smaller, non-significant effects of birthweight on the prevalence of behavioural problems in social class I and II, and IV and V. Statistical tests for an interaction between birthweight and social class were not significant.

Conclusions Early life factors, such as birthweight and social class have important influences on psychological well-being in children. The birthweight effect is influenced by social factors, with the possibility that an advantaged social environment protects against the development of behavioural problems, and a disadvantaged environment increases the risk of behavioural problems, regardless of birthweight.

KEY MESSAGES

Keywords Health inequalities, birthweight, behavioural problems, social environment, children, SDQ

Accepted 17 October 2000

Early life events (ante- and post-natal) are important both in terms of development during childhood and disease throughout the life course.1 Such early life factors can be both biological, for example birth outcome and cigarette exposure, and environmental, for example social class and poverty. These biological and social environmental influences are not necessarily discrete entities.

Children born with reduced birthweights, both pre- and full-term, have more health problems than those born at term with normal birthweights. These morbidities include respiratory problems, cognitive, neurological and psychological deficits. Several previous studies have considered psychological aspects of health in children. These have focused on cognitive functioning and academic success in highly selected groups with different birth outcomes,24 and behavioural problems in children of low birthweight.510 Studies have not considered whether a ‘birthweight effect’ exists across the full spectrum of birthweight, in the general population. Identifying the specific nature of the effect of birth outcome on childhood behaviour has been hampered by inconsistencies in previous findings. These inconsistencies appear to be largely a consequence of methodological problems. For example, not considering the impact of the social environment; use of small sample sizes; studying highly selected groups; using inappropriate controls (if any), and varying methods of psychological assessment.

This paper examines the importance of birthweight as a predictor of behavioural problems in children. This analysis adds to existing research findings by examining the relationships between birthweight and social environment (early life influences) and behavioural problems in children, using data collected during a national household survey of health in 1997. We used birthweight as a continuous variable across the entire range of birthweights, rather than using arbitrary cut-offs to define groups of children. This means that the results of this analysis can be placed in the context of the general population rather than apply only to highly selected groups of low birthweight children.

We aim to test the hypotheses that birthweight and social factors exert independent effects on childhood behavioural outcomes, and that the ‘birthweight effect’ is potentially modifiable.

Subjects and Methods

Data were collected during the 1997 Health Survey for England, an annual survey that is carried out across England. Details of the sampling method and survey response are published elsewhere.11 Briefly, a stratified probability sampling design was used. The sampling frame was the small user Postcode Address File from which 720 postcode units were chosen as the Primary Sampling Units. Once an address had been selected up to two of the children living there were sampled by the interviewer using a KISH grid (a means of randomly allocating participants), and included in the sample. Social class was assigned using the occupation of the head of household, the same procedure was used for lone parents. If the head of household was not working their last occupation was used to assign social class.

In addition to health, interviews covered demographic and social factors. Reported birthweight was collected for children who were living with their natural mother.

Weighting was applied to the data in order to ensure that the survey sample was representative of the population of England. This accounted for the probability of a child being selected into the sample. The weighted sample size was slightly smaller than the unweighted sample. The estimated effect sizes in the multivariate analysis are therefore slightly conservative.

In all, 85% of households agreed to participate in the survey. The total sample size for bivariate analysis was 5705 children aged 4–15 years. For multivariate analysis complete data were available for 5181 children.

Strengths and difficulties questionnaire
For children aged 4–15 years parents were asked to complete the Strengths and Difficulties Questionnaire (SDQ) as a measure of psychological health.12 The SDQ is a 25-item questionnaire which enquires about five dimensions of behaviour, namely conduct problems, hyperactivity, emotional symptoms, peer relationships, and prosocial behaviour. Other commonly used questionnaires tend to focus on negative aspects of behaviour.13,14 In contrast to this, SDQ questions are phrased in such a way that both strengths and difficulties are represented. The SDQ is a validated tool and compares favourably for identifying hyperactivity and attention problems with the Rutter questionnaire.12 Attributes for each of the sub-scales are shown in the box. The parent marked each of these attributes as ‘Not true’, ‘Somewhat true’ or ‘Certainly true’, responses were coded as 0, 1 and 2 (those in italics were reverse scored). In each sub-scale scores for each item were summed, giving a range of 0 to 10. Scores from the conduct problems, emotional symptoms, hyperactivity and peer relationship sub-scales were summed to give the total difficulties score, with a range of 0 to 40. According to the author of the instrument the cut-offs for ‘high’ scores for each sub-scale were as follows: conduct problems >=4, hyperactivity >=7, peer relationships >=4, emotional symptoms >=5 and prosocial behaviour <=4. A ‘high’ total difficulties score was defined as >=17.12


Strengths and difficulties questions

Hyperactivity: ‘Restless, overactive, cannot stay still for long’, ‘Constantly fidgeting or squirming’, ‘Easily distracted, concentration wanders’, ‘Thinks things out before acting’, ‘Sees tasks through to the end, good attention span’.

Conduct problems: ‘Often has temper tantrums or hot tempers’, ‘Generally obedient, does what adults request’, ‘Often fights with other children or bullies them’, ‘Often lies or cheats’, ‘Steals from home, school or elsewhere’.

Emotional symptoms: ‘Often complains of headache, stomach-ache or sickness’, ‘Many worries, often seems worried’, ‘Often unhappy, down-hearted or tearful’, ‘Nervous or clingy in new situations, easily loses confidence’, ‘Many fears, easily scared’.

Peer relationships: ‘Rather solitary, tends to play alone’, ‘Has at least one good friend’, ‘Generally liked by other children’, ‘Picked on or bullied by other children’, ‘Gets on better with adults than with other children’.

Prosocial behaviour: ‘Considerate of other people's feelings’, ‘Shares readily with other children (treats, toys, pencils, etc.)’, ‘Helpful if someone is hurt, upset or feeling ill’, ‘Kind to younger children’, ‘Often volunteers to help others (parents, teachers, other children)’.

 

Data analysis
All analyses were performed using SPSS version 7.5 for Windows. The proportions of children with ‘high’ behavioural problem scores were compared for mean birthweight and a number of social and demographic factors. Chi-squared and P-values (for trend where appropriate) were generated for categorical variables, analysis of variance (ANOVA) was used for continuous data. For multivariate analyses logistic regression was used. Deviation from linearity in the relationship between birthweight and behavioural problems was tested for by looking at the difference in the log likelihood statistic between two models. The first model included birthweight categories as a categorical variable, and the second included birthweight categories as a continuous variable. Tests for statistical interactions between birthweight and social class were performed. The main outcome variables, total difficulties, hyperactivity, conduct problems, peer relationships, emotional symptoms and prosocial behaviour were dichotomized as ‘high’ score versus ‘low/borderline’ score, as recommended by the author of the instrument.12 For the first set of models the covariates were birthweight as a continuous variable, social class of head of household (four categories: I + II, IIINM, IIIM, IV + V), lone parenthood (yes or no), smokers in the household (yes or no), age band (4–9 and 10–15) and gender. A second set of models were generated for a sub-sample (n = 1621) of children for whom the following additional data (because the mother had been interviewed about her own health) were available: maternal smoking (>=20/day, 10–20/day, <10/day, ex and never smokers); maternal age at the time of birth (<20, 20–24, 25–29, 30–34, >=35); and maternal psychological status (GHQ12 score of 0, 1–3, >=4). The sub-sample of children for whom maternal variables were available were not different to the total sample in terms of social and demographic factors.

Results

The prevalence of high total difficulties score varied by demographic, social and maternal psychological factors. A high total difficulties score was significantly more common in boys compared with girls, the younger versus the older age group, in children from lower social classes and from lone parent families, those living with smokers (mother and/or other), and in children whose mothers had more psychological problems (Table 1Go).15


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Table 1 Prevalence of high total difficulties score by social, psychological and demographic factors
 
Considering more specific outcomes, boys were more likely than girls to have high scores for the conduct (16.7% versus 11.5%, P < 0.001), hyperactivity (17.5% versus 8.5%, P < 0.001), peer relationships (15.5% versus 11.4%, P < 0.001), and prosocial problems (5.6% versus 2.4%, P < 0.001) sub-scales. High scores on the emotional symptoms component were more common in girls than boys (12.7% versus 10.7%, P < 0.05).15

We explored the relationship between mean birthweight (3.33 kg, SD 0.56, range 0.91–6.01) and total difficulties score, and found it to be linear, there was no deviation from linearity (Table 2Go).


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Table 2 Relationship between total difficulties score and birthweight in categories, odds ratios (OR) and 95% CI adjusted for age and gender
 
Table 3Go shows differences in mean birthweight by gender and social factors. There were significant differences in mean birthweight by social class, family structure, maternal smoking and by gender.


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Table 3 Differences in birthweight by family and maternal factors
 
Overall, children with high hyperactivity, peer problem, conduct problem and total difficulties scores were lighter at birth than those with low/borderline scores. There was a gender difference here; boys with high hyperactivity, peer and conduct problem scores were significantly lighter than boys with low/ borderline scores on these scales. Girls with peer problems were significantly lighter than girls without such problems (Table 4Go).


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Table 4 Difference in mean birthweight (g) (95% CI) between high and low/borderline Strengths and Difficulties Questionnaire (SDQ) scores
 
Table 5Go shows adjusted OR and 95% CI based on the logistic regression analysis of the relationship between birthweight, age, gender and social factors, and SDQ sub-scale and total difficulties scores. Birthweight was a significant predictor of high scores on the hyperactivity (OR = 1.17, 95% CI : 1.01, 1.35), peer relationship problems (OR = 1.19, 95% CI : 1.03, 1.37) and total difficulties (OR = 1.27, 95% CI : 1.07, 1.49) scales but it was not significantly related to scores on the conduct problems sub-scale and entirely unrelated to the emotional problems and prosocial components of the scale. Younger age was associated with high scores on the hyperactivity, conduct problems, and total difficulties scales. Boys were at increased risk of high scores on the hyperactivity, conduct, peer, prosocial problems and total difficulties scales. Girls were more likely to have high scores on the emotional symptoms sub-scale. Social class, lone parenthood and the presence of smokers in the home all had significant effects on behaviour. Lone parenthood was the strongest predictor of a high score on the emotional problems sub-scale (OR = 1.78, 95% CI : 1.46, 2.14) and was also associated with a high score on the hyperactivity, conduct problems and total difficulties scales.


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Table 5 Adjusted odds ratios (OR) and 95% CI for the effect of birthweight and social factors on behavioural problems in boys and girls (n = 5181)
 
The same series of logistic regression models were constructed for boys and girls separately. In girls birthweight predicted high scores on the peer relationships sub-scale (OR = 1.24, 95% CI : 0.99, 1.53), and in boys birthweight predicted high scores on the hyperactivity (OR = 1.25, 95% CI : 1.05, 1.51) and total difficulties (OR = 1.31, 95% CI : 1.06, 1.63) scales. The effect of age on hyperactivity, conduct and total difficulties scores was constant for boys and girls, with an increased risk in younger children. Age did not predict scores on the peer relationships scale. For both girls and boys prosocial behaviour was poorly predicted by any of the variables included in these separate-gender models.

Data on current maternal smoking status, age at time of birth and psychological status were available for a representative sub-sample of children (n = 1621). When these variables were entered into the model the predictive value of birthweight was significant for total difficulties score (OR = 1.42, 95% CI : 1.06, 1.92) and was attenuated for the peer relationships score (OR = 1.25, 95% CI : 0.97, 1.64). Heavy maternal smoking predicted a high score on the hyperactivity (OR = 1.76, 95% CI : 1.11, 2.79), peer relationships (OR = 1.60, 95% CI : 1.04, 2.48), conduct problems (OR = 3.14, 95% CI : 2.07, 4.74), emotional symptoms (OR = 1.82, 95% CI : 1.14, 2.88) and total difficulties (OR = 3.24, 95% CI : 1.99, 5.26) scales. Maternal psychological status was strongly predictive of all behavioural problems. Maternal age was not predictive for any of the SDQ scales.

Gradients in the prevalence of problem behaviour scores across the social classes and birthweight tertiles are shown in Figure 1Go. High total difficulties score was significantly more common in lower birthweight tertiles for social classes III non-manual and III manual, (P-values for trend 0.05 and 0.028, respectively), but the birthweight effect was smaller and not significant for social classes categories I + II and IV + V (Figure 1aGo). There was a significant social class gradient in behaviour problems for all birthweight tertiles. The prevalence of high scores on the hyperactivity and peer problems sub-scales followed a similar pattern (Figures 1b and 1cGo). Statistical interactions between birthweight and social class were not significant.





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Figure 1a, 1b, 1c Prevalence of high total difficulties, hyperactivity and peer relationship problems scores by social class and birthweight tertile *P <= 0.05.

 
Discussion

These data were collected from a randomly selected population sample, giving us confidence in the generalizability of the results. We have demonstrated that for boys birthweight is a specific independent risk factor for hyperactivity and in girls birthweight is a predictor of the quality of peer relationships. The magnitude of the birthweight effect is considerable even when adjustments are made for social factors, with an approximately 30% increased risk of overall behavioural problems per kilogram drop in birthweight.

Previous studies have reported a variety of behavioural problems in low birthweight children, most commonly defined using cut-offs of 1000 g, 1500 g and 2500 g. These problems have included hyperactivity and attention problems,5,6,8,1620conduct disorders,10,18,21 problems with peer acceptance,7 social adjustment,7,2123 and emotional symptoms.7,8,10,18,19,22,24 We have demonstrated a continuous ‘birthweight effect’ on behavioural outcomes across the birthweight spectrum.

We have shown that the presence of smokers in the home, whether it is the child's mother or another person(s), is a strong predictor of behavioural problems in childhood. Parental smoking and maternal smoking during pregnancy have been reported as a risk factor for behavioural problems.2527 These observations suggest that exposure to cigarette smoke, (pre- and post-natal) has a direct biological effect leading to behavioural problems, but smoking is also a marker of social disadvantage and so may exert an effect via indirect pathways. We have demonstrated strong influences of social and parental factors on childhood behaviour. Indicators of social disadvantage such as lone parenthood and poverty have previously been shown to have profound effects on behavioural sequellae in childhood.28,29 In the present study there were strong inter-relationships between lone parenthood, social class and mother's smoking status, and all of these factors are related to poor behaviour outcomes in children.

Boys were more likely to have behavioural problems than girls, with the exception of emotional symptoms which were more common in girls. Our results are consistent with a number of previous studies that have reported boys with lower birthweights having more behavioural problems compared with girls.9,10,21,30,31 Additionally a recent report showed that feeding practices in infancy dramatically affected IQ in low birthweight pre-term boys.32 These differences might reflect the increased prevalence of psychiatric problems among male children.33 Alternatively, as there are gender differences in cognitive development in children of low birthweights,34 boys may be more susceptible to perinatal complications.

Our data show that birthweight has differential associations with different behavioural outcomes. The clearest associations being between the hyperactivity, peer relationships, and total difficulties scales, with no association for the other SDQ sub-scales (conduct problems, emotional symptoms and prosocial behaviour). These differences could prove important in suggesting mechanisms by which a ‘birthweight effect’ could act. The effect of birthweight on childhood behaviour can be interpreted in different ways. Firstly, as a direct biological insult mediated via some neurological pathway. Secondly, reduced birthweight is associated with social disadvantage and so birthweight is an indicator of social factors. Thirdly, a socially disadvantaged environment compounds the direct biological effect of birthweight. The social class gradient in morbidity and mortality is well established.35 In keeping with this our data show a clear social class gradient in the prevalence of behavioural problems and in the bivariate analysis the ‘birthweight effect’ appears to be minimal in social classes I and II, suggesting that an advantaged social environment protects against the development of behavioural problems. Birthweight had stronger effects for children in social classes III non-manual and III manual, but in classes IV and V there appeared to be a ceiling to the ‘birthweight effect’, suggesting that in disadvantaged environments the biological impact of birthweight is outweighed by social factors. However, this assertion remains speculative as tests for statistical interactions between birthweight and social class were not significant.

Methodological issues and sources of bias
There are two key limitations of these data that should be borne in mind. Clearly, analysis and subsequent interpretation of cross-sectional data are limited and should be approached with caution. We have reported associations between early life factors and psychological health in childhood. We have made assumptions about smoking and social environment during pregnancy and in the first few years of life. These assumptions are based on evidence from the literature, for example, most women start to smoke as teenagers, many continue during pregnancy, and the failure of pregnancy-related smoking cessation trials is well documented.36 However, some women may start to smoke after having had children. So, though it is likely that mothers who smoked at the time of interview also smoked pre- and post-natally, we cannot be certain that current smoking status reflects lifelong smoking patterns. Data on changes in social class were not available. There is some evidence from the 1958 birth cohort that social class during childhood reflects that at the time of birth.37 However, these data are from some 30 years ago and may not apply to data collected in the 1990s, especially in the case of lone parent families who are likely to have experienced substantial changes in social and economic status. An additional limitation of these data is that the potential contribution of gestational age to the ‘birthweight effect’ on childhood behavioural problems cannot be assessed, and so it is difficult to fully estimate the potential contribution of birthweight on behavioural outcomes. However, it would be interesting to pursue our findings in appropriate, contemporary longitudinal data.

There are important strengths of this paper that should be considered. Using an arbitrary cut-off, e.g. <2501 g, as the birthweight predictor variable would only be useful if there were substantial increases in behavioural problems below this specific weight. As there was a linear relationship between birthweight and SDQ score we used the continuous variable for our analyses.

Problems with inter-rater agreement, for example between teachers and parents in reporting behaviour in children is well established. This was not relevant in our data as questions were answered by the parent only (most often the mother). Previous work suggests that bias relating to mother's psychological status does not exist in these data (personal communication).

Inconsistencies in previous reports that have used psychological assessment methods, such as the CBCL or Rutter questionnaire have largely been due to methodological problems. These problems include poor follow-up of the original sample,21,29,30 small sample sizes,6,8,17,19,21,29,30 and highly selected groups being compared with large normative samples.17,19,29 Additional problems arise when studies using different age groups are compared, as pre-school children may be less likely to exhibit certain behaviours compared with school age children who are likely to have encountered more challenging environments. For these reasons it is difficult to apply the results of many such studies to the general population. A strength of the present analysis is that we have examined the effect of birthweight on behaviour in children across the birthweight spectrum.

Conclusions

Early life (ante- and post-natal) events are important determinants of psychological health in childhood. Early life factors are of several types with biological and social origins. However, these influences are not necessarily discrete entities, for example birthweight may be classified as a biological parameter which is in part socially determined. Social disadvantage (low social class, lone parenthood and in utero cigarette smoke exposure) contribute via different pathways to reduced birthweight. In turn, birth outcome, cigarette smoke and social disadvantage exert independent effects on behaviour in children.

We have speculated that the effect of birthweight on childhood behaviour may be influenced by the social environment. An advantaged social environment appears to protect against the development of behaviour problems in lower birthweight children, and a disadvantaged social environment appears to increase the risk of behavioural problems regardless of birthweight. Potential interactions between social class, birthweight and children's behaviour have clear policy implications, although such interactions are speculative and require exploration elsewhere. The Independent Inquiry into Inequalities in Health emphasized the importance of policies aimed at improving the circumstances of families with children because of the long-term effects of inequalities in health.38

Clearly these conclusions have serious implications for psychological health, and it is not clear whether behavioural problems such as those described in this paper will persist into later life. It may be that factors shown to have more subtle effects on behaviour in childhood such as that reported for birthweight may exert a greater effect as age progresses.

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