Commentary: Ethnic differences in gestational age exist, but are they ‘normal’?

David A Savitz

Department of Epidemiology, CB #7435, University of North Carolina School of Public Health, Chapel Hill, NC 27599-7435, USA. E-mail: david_savitz{at}unc.edu

Research on preterm birth and low birthweight has been successful at two levels—the broad, social level, where international, socioeconomic, and racial/ethnic differences are clear, and at the mechanistic, clinical level, where pregnancy complications, prior adverse outcomes, and certain pre-existing chronic diseases are strongly related to adverse pregnancy outcome. Research on everything in between, i.e. the more readily modifiable behaviours or health services, has been less successful with the exception of cigarette smoking and reduced birthweight. Despite intensive effort, research on diet, stress, physical activity, environmental pollutants, and prenatal care has not taught us a great deal about what causes or how to prevent these adverse pregnancy outcomes.

Patel and colleagues1 have developed and analysed a rich resource for describing duration of gestation, finding clear and consistent reductions in duration of gestation among Blacks originally from Africa or the Caribbean, and Asians, specifically south Asians, compared with white Europeans. Median gestational age at delivery was lower in these groups by around one week and they had a greater risk of delivering preterm (<37 weeks' completed gestation), suggesting a shift in the entire distribution, not just an increase in the lower tail of the distribution of gestational ages. However, a more explicit look at those underlying distributions would help in understanding and proposing explanations.

Assuming that ethnic differences result from a shift of around one week in the entire gestational age distribution for Blacks and Asians compared with Europeans, two questions logically follow. First, is the relationship between gestational age and infant survival (or other infant health outcomes) a function of absolute gestational age or relative position within the distribution? In the case of birthweight, as Wilcox and colleagues have demonstrated quite clearly,2,3 shifts in the distribution across groups (gender, race, smoking) are associated with corresponding shifts in the mortality curve, such that within the dominant distribution, z-scores are more informative than absolute values and the size of the lower tail of the distribution has great significance independent of the position of the dominant distribution. Clearly, at the extreme, births of 30 or 32 weeks' gestation are detrimental regardless of the overall distribution of gestational ages. What is less clear is whether the implications of being born at 35 versus 37 weeks, or 38 versus 40 weeks confers a comparable disadvantage across ethnicity, given that their overall gestational age distributions differ. The data resource might be examined to determine whether more advanced gestation confers comparable survival advantages across ethnicity as it does within populations.4,5

Limited evidence on the survival of European and Asian preterm infants suggests that survival across the gestational age spectrum does not appear to differ by ethnicity, i.e. absolute gestational age is what matters.6 The information presented by Patel et al.1 on meconium staining of amniotic fluid suggests that Black and Asian fetuses ‘mature faster,’ but this is not nearly as persuasive as evidence of a shift in the mortality curves would be. The meconium data and other evidence from the literature would predict more favourable survival at early gestational ages for Blacks and Asians.

The use of the word ‘normal’ to describe the patterns suggests that there is some immutable underlying reason why infants born to Blacks and Asians mature and are thus appropriately born earlier than infants of Europeans. The suggestion of the term ‘normal’ is that there are genetic aspects of the geographical origins of these populations that result in a different set point to define the normal duration of pregnancy. Political correctness aside, this is a reasonable hypothesis given that fetal and infant survival is so intimately tied to human evolution and selective pressures in various regions of the world may well have had such a modest but discernible effect on the normal duration of pregnancy. Where the testing of this hypothesized causal pathway falls far short, however, is when biological underpinnings are inferred based solely on the failure of simplistic indicators of cultural, socioeconomic, or behavioural factors to account for the phenomenon.7 There are many reasons why the observed differences in gestational age might exist, with myriad possible causes given the differences in life experiences among these ethnic groups, and only with careful measurement and control of such attributes would the evidence for a genetic basis be enhanced. Few of those determinants could be considered in this analysis, and none could be examined in fine detail.

Thus, the possibility of a modifiable social or behavioural explanation for these patterns remains highly plausible, but largely untested. This is where the concept ‘normal’ or ‘natural’ can take on unintended negative scientific and social implications. The potential for scientific advances is diminished in suggesting that a hypothesized biological basis for these differences does not demand affirmative support, or the inference that social and behavioural explanations have been disproved; neither is true. The danger for policy in defining situations as ‘normal’ is the implication that change is unattainable. For an intermediate outcome like gestational age, of concern only because it is so strongly predictive of infant health and survival, this may not be so worrisome if the important consequences of gestational age truly do differ across ethnic subpopulations. However, precisely the same type of evidence and logic could be applied to such clinically important outcomes as infant mortality, where the failure of social and behavioural factors to explain differences across ethnic groups could be misused to infer the presence of immutable biological differences. The goal of such research is to elucidate the pathways by which ethnic differences operate, a step towards understanding why they occur. Describing the differences in duration of gestation across these subgroups of the British population is a very useful step in this direction. However, we still are far from understanding why such ethnic differences occur, whether these differences are amenable to modification or the appropriate response to such information on the part of clinical and public health service providers.


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1 Patel RR, Steer P, Doyle P, Little M, Elliott P. Does gestation vary by ethnic group? A London based study of over 122 000 pregnancies with spontaneous onset of labour. Int J Epidemiol 2004;33:107–113.[Abstract/Free Full Text]

2 Wilcox AJ, Russell IT. Birthweight and perinatal mortality: II. On weight-specific mortality. Int J Epidemiol 1983;12:319–25.[Abstract]

3 Wilcox AJ. On the importance—and the unimportance-of birthweight. Int J Epidemiol 2001;30:1233–41.[Abstract/Free Full Text]

4 Copper RL, Goldenberg RL, Creasy RK et al. A multicenter study of preterm birthweight and gestational age-specific neonatal mortality. Am J Obstet Gynecol 1993;168:78–84.[ISI][Medline]

5 Kramer MS, Demissie K, Yang H et al. The contribution of mild and moderate preterm birth to infant mortality. JAMA 2000;284:843–49.[Abstract/Free Full Text]

6 Draper ES, Manktelow B, Field DJ, James D. Prediction of survival for preterm births by weight and gestational age: retrospective population based study. BMJ 1999;319:1093–97.[Abstract/Free Full Text]

7 Kaufman JS, Cooper RS, McGee DL. Socioeconomic status and health in blacks and whites: the problem of residual confounding and the resiliency of race. Epidemiology 1997;8:621–28.[ISI][Medline]





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