Birthweight varies according to ethnic group but height at the age of five does not. Why might that be? Does it tell us anything about the lives of second and third generation immigrants? And does it offer any useful guidance to health professionals hoping to target disadvantaged groups? Professor Yvonne Kelly outlines recent research with colleagues at the ESRC International Centre for Lifecourse Studies looking at differences in birthweight and early growth between ethnic groups.

Birthweight is important. There is a large body of work that suggests links between low birthweight and the development of chronic disease. Height at the age of five is a less straightforward indicator but still an important measure. The relationship between the two is important as well. Rapid post-natal growth may also have a role in later disease risk, and any correlation may tell us something about the lives of people born in the UK to parents born elsewhere.

Earlier research shows that babies born to South Asian and Black mothers weigh up to 300g less than those with White mothers. They are also up to two and a half times more likely than their White counterparts to have low birthweight.

Our study made use of the rich information available in the Millennium Cohort Study and enabled us to drill down further into ethnic differences. We were able to look at White, Indian, Pakistani, Bangladeshi, Black Caribbean and Black African groups.

These, of course, are groups that have very different migration histories. The Black Caribbeans and Indians mainly migrated to the UK in the 1950s and 1960s. The Pakistanis arrived in the 1960s and 1970s, the Bangladeshis in the 1980s and the Black Africans in the 1990s.

Social v biological

If you accept that ethnicity is a social not a biological construct, these variations must be the result of factors that are not intrinsic to the group but tend to go with membership. Relevant factors are likely to be either socioeconomic or maternal.

So, if one group tends to have higher incomes and higher levels of educational attainment, it is likely to have fewer babies with low birthweight. Similarly, if mothers within one group are less likely to smoke they too are likely to have heavier babies.

Because our research compared data on birthweight to those on ethnicity, socioeconomic status and maternal characteristics, it was possible to identify which were most closely associated.

The results suggest that socioeconomic factors are important in explaining birthweight differences in Black Caribbean, Black African, Bangladeshi and Pakistani infants. Maternal characteristics are important in explaining birthweight differences in Indian and Bangladeshi groups. Clearly, both must operate to some extent in all cases.

Our study identifies the dominant factor for each ethnic group and recommends policy-makers pay attention to the different socioeconomic and culturally related profiles of ethnic minority groups when devising policies aimed at reducing inequalities in birthweight.

A question of height

One key maternal characteristic identified was height. Mothers from the Indian, Pakistani and Bangladeshi groups were on average 8cm shorter than White mothers. We speculate that it might take several generations for individuals within ethnic groups to reach their height potential.

And it could be that increases in maternal height do not happen so much for the first couple of migrant generations due to the ‘accumulated effects of disadvantage, including racism, discrimination and poverty that are disproportionately experienced by migrants’. That idea was put to the test in a second study also using MCS data.

The primary aim of this research was to investigate ethnic differences in height at 5 years of age. The same ethnic groups were used. Again, the sample was large and broadly representative of the whole UK.

Playing catch up

In contrast to the findings on birthweight, Indian, Pakistani, Black Caribbean and Black African children were taller than White children at age 5. Bangladeshi children were the same as White children. Birthweight was not entirely irrelevant. It was a weak to moderate predictor of height in White, Pakistani, Bangladeshi and African children.

All the measured variables favoured the White group over all ethnic minorities. This is consistent with the suggestion floated towards the end of the first study that what is happening is that a generation is ‘catching up’, earlier generations having been previously thwarted by such factors as poor nutrition in underdeveloped home countries.

Saying that, catch-up growth is likely to explain only a part of the ethnic height differences identified and further research is important here. It is also important to note that taller children are more inclined to obesity and so the height advantage of ethnic minority children might not translate into a health advantage in adulthood.

The links between ethnicity, birthweight and height in childhood are not, then, straightforward or by any means fully understood. It is clear that outcomes associated with different groups are the result of social and not biological characteristics.

The length of time a group has been established in the UK also appears to play a part with at least some suggestion of a generational ‘catch-up’ effect. Though there may be plenty of inequality left to address, that does at least suggest things are moving in the right direction.

Further information

Why does birthweight vary among ethnic groups in the UK? Findings from the Millennium Cohort Study is research by Yvonne Kelly, Lidia Panico, Mel Bartley, Michael Marmot, James Nazroo and Amanda Sacker and is published in the Journal of Public Health.

Ethnic differences in growth in early childhood: an investigation of two potential mechanisms is research by Amanda Sacker and Yvonne Kelly and is published in the European Journal of Public Health.

Photo credit: moinuddin forhad

[1] Ethnic differences in growth in early childhood: an investigation of two potential mechanisms. A. Sacker, Y. Kelly