Forty years on: the effect of deprivation on growth in two Newcastle birth cohorts

Charlotte M Wright1,2 and Louise Parker3

1 Current affiliation: PEACH unit, Department of Child Health, Glasgow University, Glasgow, UK
2 Previous affiliation: Community Child Health Unit, Department of Child Health, Newcastle University, Newcastle, UK
3 Paediatric and Life Course Epidemiology Research Group, James Spence Institute, School of Clinical Sciences, Newcastle University, Newcastle, UK

Correspondence: Charlotte M Wright, Glasgow University, PEACH unit, QMH Tower, Yorkhill Hospitals, Glasgow G3 8SJ, UK. E-mail charlotte.wright{at}clinmed.gla.ac.uk


    Abstract
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 Abstract
 Method
 Results
 Discussion
 References
 
Background This study used two prospective birth cohort studies from the same city to examine the extent to which deprivation gradients for height and weight have changed over 40 years.

Methods Participants were 954 (86%) full term members of the Thousand Families (TF) 1947 and 3145 (99.9%) members of the Growth and Development Study (GDS) 1987–1988 Newcastle birth cohorts with both deprivation and growth data. Weights were available at birth, 6 months, and 1 year; weight and height at 4 (GDS only), 9, and 13 years (TF only), and in adulthood (TF only). These were stratified by level of deprivation, measured by Registrar's General social class in 1947 and area-based Townsend scores in 1987.

Results Both cohorts had similar birth and infancy weights, but there was no gradient by deprivation level at birth or in infancy in the 1947 cohort, while the 1987 cohort showed a consistent gradient from birth onwards. Height had increased from the 1947 to the 1987 cohort, but both showed very similar deprivation gradients, equivalent to a 4-cm difference between the most and least affluent strata at age 9 years. Body mass index was similar for both cohorts and only showed a deprivation gradient in adulthood.

Conclusions We found no evidence of a changing influence of socioeconomic deprivation on growth in childhood, despite increases in mean height over a 40-year interval.


Keywords Secular change, growth, birthweight, infancy, childhood, height socioeconomic deprivation

Accepted 28 March 2003

Variations in height between different groups are often used as markers of childhood nutritional status and general health.1 The inverse relationship between deprivation and attained height is well recognized, as is the continuing secular trend to increasing height.2 What is not known is what the interaction between these two phenomena is: as height increases, have deprivation gradients narrowed or indeed widened?

The Department of Child Health at Newcastle University is the custodian of data from a birth cohort of Newcastle children born in 1947 ‘The 1000 Families Study’.3 Growth data are also available from a second birth cohort, The Growth and Development Study,4,5 born in the city between 1987 and 1988. These two birth cohorts provided the opportunity to examine trends in growth over a 40-year period in the same city.


    Method
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 Abstract
 Method
 Results
 Discussion
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The Thousand Families Study
This study recruited parents of all children born in Newcastle in 2 months in 1947. Of 1142 identified births, 932 were studied for at least 1 year.6 Birthweight and gestation were passed to the study team by the midwife; children were weighed in infancy at Child Welfare Clinics by their own Health Visitor at varying times and these weights were kept with their study record. Subsequent school nurse measurements were also passed to the study. Adult height was requested in two postal questionnaires at ages 22 and 50 years, and measured at age 52. In the 1990s, as part of the latter follow-up, all the childhood growth data held were retrieved and re-entered.6

The Growth and Development Study
A complete birth cohort of 3655 infants, aged 18–30 months, was identified from the Child Health Computer System in 1989 along with their birthweight which was entered by the midwife shortly after birth. All their well-baby clinic records were then reviewed and their routinely collected weights retrieved, as well as gestation.4 Six years later when the children were aged 8–9 years, as part of a continuing study of weight gain in infancy, a 20% systematic sample was taken of the 2812 (82%) term infants for whom at least three weights had been retrieved. Consent and parental height was obtained from parents by post and the children were measured in school by a research nurse.5 The school health records of these children were also traced and their school entry measurements between the age of 4 and 5 years retrieved.

For both cohorts all available weights were grouped into age bands and the ages around which the largest number of measurements were available identified. The weight nearest to each target age, within defined age ranges, was then identified for each child.

All measurements were transformed into standard deviation scores (SDS) for their exact age and gestation compared with the UK 1990 reference standard.7 The UK 1990 reference was used for both groups, although in principle the earlier Tanner and Whitehouse standard was more appropriate for the 1947 cohort, to allow direct comparison of age- and sex-adjusted values between the two cohorts. For the 1987 cohort the height SD scores of both parents were averaged to obtain the mid-parental SDS.

Deprivation was classified for the 1947 cohort using Registrar General's occupational social class for the head of household at birth. For the main analyses social classes 1 & 2 were combined, as were social class 5 with unclassified. The 1987 cohort children were classified using the Townsend scores for their enumeration district8 identified from their postcode aged 18 months. These were then ranked and divided into four percentile groupings to correspond to the percentages in each of the four 1947 strata. For both cohorts these were termed: Affluent, intermediate, deprived, and very deprived. Subjects born before 37 weeks gestation were excluded from the analysis.


    Results
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 Method
 Results
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In the 1947 cohort 30 (2.6%) infants and in the 1987 cohort 237 (6.5%) were born before 37 completed weeks gestation and were excluded from further analyses.

In the 1947 cohort 12% of births were in social class 1 & 2, 57% in 3, 16% in 4, and 15% in 5 and unclassified. In the 1987 cohort, the subjects were therefore stratified into those percentile groupings using their ranked Townsend scores.

The number of weights available for each cohort at different ages is shown in Table 1. For both cohorts birthweight was available for the great majority and most childhood measurements were available for more than two-thirds. In the 1947 cohort data were relatively sparse in infancy with weights for only a half at ages 6 months and one-third at 12 months. Attrition was fairly consistent between deprivation strata apart from a trend to greater attrition in the affluent stratum which represented only 7–8% weights in all ages after birth.


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Table 1 Number (%) of children born at term with measurements and deprivation/social class information at different ages

 
In the 1987 cohort weights were available for nearly all at birth and for two-thirds or more (of those studied) at ages 6 and 12 months, 4 and 8.5 years. In this cohort attrition occurred evenly from all deprivation strata.

Overall mean weight SD scores at birth and in infancy were very similar for both cohorts but, while there was a significant and consistent gradient by deprivation strata in the 1987 cohort, no trend of any kind was seen in the 1947 cohort (Table 2, Figure 1). In the total 1947 cohort rates of low birthweight (< 2500 g) did vary by social class (2% social classes 1 & 2, 2.8% 5 & unclassified; P = 0.048) but this gradient also disappeared when pre-term births were excluded.


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Table 2 Mean weight standard deviation scores for both cohorts, by deprivation strata

 


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Figure 1 Socioeconomic gradients in weight in infancy for 1947 and 1987 cohorts

 
Height SDS in childhood was much lower than the 1990 reference values in the 1947 cohort, particularly at age 13 years, while the 1987 cohort matched the reference well (Table 3, Figure 2). The difference between the cohorts at age 9, 0.7 SDS, was equivalent to a 4-cm height difference. Both cohorts showed significant and consistent gradients by deprivation strata in childhood, which appeared to widen up to age 9 and narrow towards adulthood (Figure 3). The difference between the most and least deprived strata at age 9 was 0.69 SDS for the 1947 and 0.71 SDS for the 1987 cohort (Figure 2) again equivalent to a 4-cm height difference. In the 1947 cohort the adult height difference between the most and least deprived strata was 3.5 cm.


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Table 3 Height and body mass index (BMI) standard deviation scores for both cohorts, by deprivation strata

 


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Figure 2 Socioeconomic gradients in height and body mass index at age 9 in 1947 and 1987 cohorts

 


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Figure 3 Change in height gradients with age

 
In the 1987 cohort the influence of mid-parental height was also explored. The mid-parental height SDS showed the expected strong correlation with the child's height SDS (regression coefficient r = −0.51, P < 0.001) and a weaker association with deprivation stratum (r = −0.09, P = 0.055). To avoid the distorting effect of this strong inter-correlation, it was not appropriate to add both to the same regression model to test their independent effects. Instead we examined the effect of the height residual after allowing for parental height:

Where B = coefficient (0.51) for regression of height SDS on MPHT SDS. In a linear regression model this residual also varied significantly by deprivation stratum (r = −0.13, P = 0.007) and more so than mid-parental height (r = −0.08, P = 0.1), with a mean difference between the highest and the lowest stratum of 0.47 SD.


    Discussion
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 Abstract
 Method
 Results
 Discussion
 References
 
These data offer an opportunity to evaluate secular changes in growth over an exceptionally long interval. The strength of this study is the large numbers of subjects, both constituting total birth populations from the same geographical area, with 70% or more of each cohort followed up into childhood. Neither study was designed for this analysis but, serendipitously, growth measures were collected at sufficiently comparable times to allow a valid comparison. The main difference between the studies is the method used to classify deprivation. Registrar General's occupational social class was the only socioeconomic classification widely used at the time of the 1947 cohort. The census-data-based Townsend score approach8 was the only feasible method for the later cohort, since it requires only a postcode. Other methods of socioeconomic classification require information collected directly from parents, the majority of whom were not contacted directly for this study. Every measure of socioeconomic status produces some misclassification and the concern would be that the two might not be directly comparable. However, when both were used in another study they produced very similar results9 and the fact that both methods produced consistent and comparable height gradients in this study is reassuring. Townsend scores calculated per enumeration district have a much more fine-grained classification, which can be stratified into any size or number of categories.10 This had the advantage that categories could be defined so that exactly the same percentage of children in both cohorts fell into the most and least affluent categories. This avoids the tendency for secular changes to be masked by changing proportions in different social class over time.11

The city of Newcastle has changed substantially in 40 years. It is larger in land terms, having assimilated an adjoining suburb, yet has a much lower birth rate: only just over half the annual number in 1987 compared with 1947. The sort of bias this might introduce is not entirely clear. The assimilated suburb is more affluent than the inner city, but progressive depopulation of this part of the UK might lead to an increase in the average deprivation levels in those left behind. Demographic differences over time are inevitable in any long-term studies and the changes that have occurred in Newcastle are fairly typical of North Britain at least, apart from the lack of a major influx of ethnic minority groups.12 The mean height and weight SD scores in the 1987 cohort were similar to, but generally slightly below the contemporary UK growth reference, which would be in keeping with rather higher rates of deprivation than the UK. This would suggest that these observations are generalizable at least to UK children living in urban areas.

The observed secular change in height is compatible with those observed elsewhere in the UK over shorter time periods. The National Study of Health and Growth (NSHG)13 found a mean gain at age 9 years of 2.5 cm between 1972 and 1994. This would suggest a gain of 4.5 cm over a 40-year period, which is very close to our observed figure. By contrast a study in The Netherlands over a 42-year period found an average difference at age 9 of over 6 cm.14 The effects of deprivation on height found here appear greater than in the NSHG survey which found a difference of only 0.31 SDS between its highest and lowest strata of white inner city children. However, that analysis merged classes 1, 2, and 3 non-manual, reducing the potential variance between the most and least deprived categories and had no information at all on social class for 45% subjects.15 Both the 1946 and 1958 British longitudinal surveys have described significant social class gradients for height in childhood and adulthood,16 with the latter describing a difference between its highest and lowest strata at age 7 years of 2 cm in boys and 3 cm in girls, (equivalent to 0.39 and 0.58 SDS respectively) also with a smaller gradient in adulthood. Here, however, social classes 4 and 5 were combined, again reducing the potential variance between the most and least deprived categories.17

These findings raise interesting questions about the biological mechanisms of secular changes in growth. The expected increase in height over time was of similar magnitude in all deprivation strata, with no diminution of gradients over time. This is puzzling as, if the mechanism for the secular trend to increasing height is nutritional in origin,1 one would expect it to operate via a narrowing of that gradient rather than an equivalent increase for all social groups. Alternatively if the trend were inter-generational in origin, or reflecting intra-uterine growth, it should be present from birth. In the 1987 cohort, although a birthweight gradient is present, its range is half that seen in height at the age of 9 years, which is in keeping with the finding that mid-parental height explained around half the difference. In the 1947 cohort the gradient is not seen at birth or in the relatively sparse infancy weight data, but it is seen most prominently in childhood with narrowing again in adulthood. This suggests that while the gradient is partly of genetic or intra-uterine origin it also has a substantial childhood component. This would be in keeping with a nutritional origin and may reflect the protection provided by the British Welfare Food safety net in infancy, but not in the childhood years.

There was no deprivation gradient in childhood BMI in either cohort, as also seen in the NSHG,18 although higher levels of adult obesity were seen in those poorest at birth.19 While the later cohort were considerably taller, their mean BMI SD scores were very similar to the earlier cohort which suggests that the rapid increases in BMI described in children recently were not being seen in earlier decades.20

The lack of gradient in birthweight in the 1947 cohort is itself of interest. Social class gradients in birthweight in modern cohorts are regarded as the norm and are thought to be multifactorial in origin,21 but with smoking representing a powerful influence.22 The absence of this gradient in earlier cohorts has not been reported before. In the British 1946 cohort a social class gradient in low birthweight was reported, but this included pre-term infants.16 In our study this gradient was only evident when pre-term births were included. The British 1958 cohort, roughly midway in time between our two cohorts, did show a statistically significant gradient in birthweight equivalent to 0.15–0.2 SDS, but this also appeared to include pre-term births.17 Again however, the size of the gradient in birthweight is considerably smaller than that seen in the same cohort for height in childhood.

Birthweight data were slightly less complete in our 1947 cohort than our 1987 cohort and missing data did vary by social class, with 14% of classes 1 & 2 having no birthweight compared with 6–8% of the other four strata. However, this is unlikely to explain the absence of a socioeconomic effect as there was no trend to a gradient in those strata where rates of missing data were the same. For example, the very deprived stratum had a higher mean birthweight SDS than the intermediate stratum. It is also of note that the British 1946 cohort only obtained data for 88% of eligible births.

We would argue that the difference between the cohorts is likely to relate to changes in the social class distribution of maternal smoking rates. Although we do not have data on maternal smoking in these cohorts, we do know how rates of smoking have varied in women over time. In the 1940s smoking was most common in more affluent women.23 The prevalence of smoking, particularly in more deprived women rose to a peak in the 1970s, with subsequent falls predominantly in more affluent women.24 Thus the lack of birthweight gradient in the earlier cohort may reflect a reversal of the social class gradient in smoking rates, with higher rates of smoking in the more affluent classes in the 1940s cancelling out other adverse influences of social deprivation on birthweight. Another possible factor was the post-war food rationing that ensured much greater uniformity of diet across differing levels of deprivation.

In conclusion, changes over time in these two British inner city cohorts, spanning 40 years after World War II, showed little evidence of a lessening influence of deprivation on growth. While 9-year-old children in the 1990s were 4 cm or one centile space taller than children in the 1950s, for both cohorts the poorest children remained 4 cm shorter than the most affluent.


    Acknowledgments
 
Funding sources: Northern and Yorkshire NHS R & D, Wellcome Trust. Our thanks go to the study team for the Thousand Families Study: Melanie Cohen, Julian Smith, Douglas Lamont, and to Alan Craft and Sir George Alberti for their help in designing and directing the study as a whole. Similarly, thanks to the Growth and Development Study team: Jane Callum, Jenny Argyll, Sally Corbett, and Phillip Lowe.


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 Discussion
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21 Goldstein H. Factors related to birth weight and perinatal mortality. Br Med Bull 1981;37:259–64.[ISI][Medline]

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