1 MRC Social and Public Health Sciences Unit, University of Glasgow, 4 Lilybank Gardens, Glasgow G12 8RZ, UK
2 Department of Epidemiology and Public Health, Queens University, Belfast BT12 6BJ, UK
3 Clinical Epidemiology Group, Unit of Chronic Disease Epidemiology, University of Manchester Medical School, Manchester M13 9PT, UK
Correspondence: MRC Social and Public Health Sciences Unit, University of Glasgow, 4 Lilybank Gardens, Glasgow G12 8RZ, UK. E-mail: seeromanie{at}msoc.mrc.gla.ac.uk
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Abstract |
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Method Live singleton births to mothers present at the 1991 Census in a national longitudinal study were classified by mother's country of birth and ethnic origin as reported in the census. During 19832000, 52 554 White, 1788 Indian, 1538 Pakistani, 995 Bangladeshi, 300 Black Caribbean, and 299 Black African live singleton births were identified. Mean birthweights were adjusted for maternal age, socio-economic circumstances, gender, year of birth, and birth order.
Results Adjusted mean birthweights were: 3400 g (95% CI: 3395, 3405) for infants of UK-born White mothers; 3033 g (95% CI: 2980, 3087) of UK-born Indian mothers and 3066 g (95% CI: 3034, 3097) of migrant Indian mothers; 3110 g (95% CI: 3049, 3172) of UK-born Pakistani mothers and 3123 g (95% CI: 3087, 3159) of migrant Pakistani mothers; 3026 g (95% CI: 2922, 3130) of UK-born Bangladeshi mothers and 3110 g (95% CI: 3076, 3145) of migrant Bangladeshi mothers; 3268 g (95% CI: 3177, 3359) of UK-born Black Caribbean mothers and 3238 g (95% CI: 3089, 3388) of migrant Black Caribbean mothers; and 3167 g (95% CI: 3004, 3330) of UK-born Black African mothers and 3302 g (95% CI: 3208, 3395) of migrant Black African mothers. The proportions of low birthweight infants (<2500 g), generally greater among migrant mothers than White UK-born mothers, were similar by generational status within the ethnic groups.
Conclusion There are no significant differences in mean birthweights of infants by generational status among mothers from these main ethnic minority groups in the UK.
Accepted 2 March 2004
Birthweight is strongly correlated with maternal health and nutrition, factors which seem to contribute substantially to the consistent difference in birthweights between developing and developed countries. Birthweight provides a measure of growth and fetal nutrition in utero, is the single most important determinant of neonatal and infant survival and general health, and is linked to an infant and child's position on postnatal growth centiles, as well as probably to later development of chronic disease.1 Babies born in the UK to women born in the Caribbean, sub-Saharan Africa, and the Indian subcontinent have been and continue to be lighter than the UK average.2 Stillbirth and infant mortality rates are higher for babies whose mothers were born in Pakistan, the Caribbean, and West Africa.3 Similar findings occur for African Americans compared with other ethnic groups in the US.4
The high prevalence of diabetes and coronary heart disease in South Asians (defined as those of Indian subcontinent origin), and of diabetes and hypertension in African origin people is well known,5,6 and the relationship between constrained intrauterine growth and these outcomes is under intense debate.79 This issue is of considerable importance for the excess of hypertension-related disease in African Americans whose mean birthweights are lower, and hence proportions of defined low birthweight are greater, than other US ethnic minority groups. One method of examining how and whether rapid environmental changes and improvements affect birthweights is through migrant studies. For example, if the more affluent environment of Britain compared with home countries leads to better pre-conceptional health, we would expect UK-born babies of UK-born mothers to be heavier than those of migrant mothers. Three local studies have investigated intergenerational differences in birthweights of babies born to South Asian mothers in Britain.1012 Two10,12 found no change and one,11 the smallest, that there had been a significant increase in birthweight in babies of second generation South Asians.
Ethnic origin is not recorded in the UK at-birth registration and the previous studies were conducted on local area samples in which South Asian ethnicity was identified by name or other information on maternity records. However, ethnic origin was collected in the 1991 Census and because of the record linkage in the Office for National Statistics Longitudinal Study (LS), we were able to classify births registered in the UK by reported ethnicity in the 1991 Census. This provided the opportunity to use nationally representative data on all major ethnic minority groups (Indians, Pakistanis, Bangladeshis, Black Caribbeans, and Black Africans) to test the hypothesis that birthweights of babies born to UK-born mothers would be higher than that of those born to migrant mothers in the same ethnic group.
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Method |
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Birth order was constructed using data from UK-registered births. Mean birthweights were adjusted for maternal age at birth registration, gender, year of birth of infant, birth order, and socio-economic circumstances using linear predictions from regression models, derived separately for each ethnic group. More than half of the women could not be classified by occupational social class at birth registration and this was supplemented by social class at the nearest census. Socio-economic position was also measured from other census indices nearest to the birthaccess to cars, housing tenure, and overcrowding. Low birthweight (LBW) was defined as <2500 g. Gestational age is recorded in obstetric records and not in civil registrations. Obstetric records are not linked to the LS so it was not possible to control for gestational age in these analyses.
It is possible, in spite of the restrictions imposed to ensure that women were nulliparious at the start of 1983, that reporting of previous births at the 1971 Census could be inaccurate. Our analyses showed that birth order was positively related to birthweight. An underestimation of birth order would therefore contribute to an upward shift in mean birthweight. A sensitivity test was conducted by using births from women who were <13 years at the 1971 Census and were, therefore, very unlikely to have ever had a birth before arriving in the UK. This additional restriction resulted in much smaller samples (a total of 43 325 births), and provided a measure of replicability and reliability of the results for the Indian and Pakistani groups. This was not possible for the other groups as the number of Bangladeshis was too small (births to UK-born mothers 45, migrant 79), as were the number of births to migrant Black Caribbeans (10) and Africans (10). This reflects differences in migration histories and in the absolute sizes of the groups (Indians arrived mainly in the 1950s and 1960s and Bangladeshis, the most recent migrants, in the 1980s. Indian migration was, in contrast to Black Caribbeans, more family centred, which resulted in sizeable numbers of those <13 years at the 1971 Census. Indians are also the biggest non-white ethnic minority group in the UK.)
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Some similarities can be found in US studies, where migrant Black women have better pregnancy outcomes than their US-born Black counterparts.4,1417 It has been suggested that migrant women were less likely to engage in behaviours (such as smoking) that negatively affect pregnancy outcomes, and that they were less economically disadvantaged. A lack of improvement in birthweights across generations in the UK is surprising given the evidence of intergenerational upward social mobility for most groups.18 Health-related data on second-generation ethnic minority groups is sparse but there is some evidence of an increase in the prevalence of adverse health behaviours across the generations. UK-born South Asians are more likely to smoke and consume alcohol and Black Caribbeans are more likely to smoke than their migrant counterparts.19,20 A change in health behaviours is also assumed to have contributed to the higher reported rates of limiting long-term illness in second generation ethnic minority groups compared with the first generations18 and also to the increasing cardiovascular mortality rates with increasing duration of residence among South Asians.21 This discordance between improvement in social conditions and reduction in health advantage requires research about the environmental stresses underlying the generational shift in behavioural norms.
Maternal birthweight itself may be the more important predictor22,23 if birthweights of UK-born and migrant minority mothers themselves were not different. This would be explicable on the basis of stronger selection effects (better health and favourable socio-economic circumstances) from later migration of the migrant mothers compared with the earlier migration of the mothers of UK-born ethnic minority mothers. It is possible that the end of right of entry from British colonies to the UK after the 1962 Commonwealth Act of the British Parliament resulted in more selective migration as most migrants would have had to come as foreign students or of independent means rather than as workers. Furthermore, the environments of the home countries would have changed over time with different consequences for the health capital of migrant mothers; for example if birthweights have increased in developing countries over the last 5060 years, then the birthweight of migrant mothers would be greater than the birthweight of the migrant mothers of the UK-born ethnic minority mothers. Arguably, birthweights could have increased across the vertical generations such that birthweight of the UK-born mother is greater than that of her own mother but less than that of her infant. Testing such relationships require multigenerational data.
Maternal birthweights of ethnic minority mothers are probably lower than that of White mothers in the UK, which could contribute to the lower mean birthweights among ethnic minority babies compared with White babies. This has been suggested as an important determinant of the BlackWhite differences in birthweights in the US.23,24 Maternal birthweight and adult height, measures of health capital, are likely to be influenced by the health and social and economic circumstances of generations. So although there has been considerable upward inter/intra generational social mobility among ethnic minorities in the UK, it is likely that because of a historical lag in health and socio-economic circumstances the health advantage an ethnic minority mother transfers to her infant has not yet resulted in parity in risk between White and ethnic minority babies.
A potentially confounding factor in this and other studies using birth registration data is that birth order could be inaccurately estimated if babies born before migration of the mother were not reported at the registration of UK-born births. The longitudinal design of the LS allowed us to construct true birth order for mothers who were unlikely to have ever had a birth before the start of follow-up in 1971. There were no significant shifts in mean birthweights between the generations in the ethnic groups when we imposed these restrictions on the sample. It was not possible to determine how these findings were related to gestational age, as it is not recorded at birth registration. The use of ethnic origin classifications in the census is also problematic. There is undoubtedly much heterogeneity among these prescribed categories. For example Black Caribbeans born in the Commonwealth Caribbean refer to those who were born in islands with diverse economic, political, and socio-cultural environments. Similarly the category 'Indians' obscures linguistic and religious heterogeneity, factors which could have influenced these results differently.
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KEY MESSAGES
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References |
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