This study in a representative community sample of births found striking ethnic differences in all birth outcomes studied and in most socio-demographic characteristics. The covariates available in the study, particularly maternal education, explained a considerable part of these differences.
Several limitations of the study should be considered when interpreting the results. First, more detailed data would provide a more complete picture of the family environment. It is likely that if information on economic and material conditions (e.g. income, housing, household amenities) would contribute further to the explanation of the ethnic differences. Data on husbands would also help but, as explained above, the proportion of missing data on fathers was too high to be used in the analyses. Data on household assets would also be valuable to improve the assessment of families' socioeconomic status. Similarly, data on factors that could mediate the link between ethnicity, socioeconomic characteristics and birth outcomes, such as nutrition, would be also useful. Finally, ethnic differentials in birth outcomes may be further exacerbated by unmeasured area-level characteristics [11
Second, some of the measurements might have been inaccurate. In particular, the definition of ethnicity is a difficult issue. Some misclassification is likely; some Roma mothers may have identified themselves as non-Roma. However, self-reported ethnicity, used in this study, has been suggested as the preferred method [3
], and alternative methods are not ideal either. In addition, mothers may have misreported some of the information (e.g. smoking or alcohol consumption). Birth outcome variables were taken from medical records; while in a few cases these variables may have been recorded inaccurately by the hospital personnel, the validity of such information in this study is high [12
]. In addition, assuming that such misclassification was random, it would tend to underestimate the associations studied, rather than lead to spurious findings.
Finally, the number of Roma births was much smaller than that of non-Roma births, and the unbalanced structure of the sample might have reduced the statistical power. However, given the overall size of the study, the statistical power was sufficient to demonstrate, in Roma vs. non-Roma comparisons, an odds ratio of 1.20 or higher at 95% confidence level. The probability of the beta error is therefore low.
In the research on ethnic differences in health in general, an important questions has been debated for some time, namely what are the reasons for such differences [3
]. For different health outcomes, the proposed explanations range from genetic to socioeconomic factors [13
] but addressing this questions has been hampered by limitations of available data [3
]. Our results shed new light at one specific type of ethnic differences in health – the poor birth outcomes of Roma mothers. It has been generally perceived that the poor health of Roma people is largely due to their unhealthy behaviours, such as smoking and drinking. Our results suggest that the explanation is more complex. Smoking before and during pregnancy was considerably more common in Roma women but it statistically explained a relatively modest part of the excess of poor birth outcomes of Roma mothers. Alcohol consumption, on the other hand, was not more common in Roma mothers, and cannot therefore be implicated in their poor pregnancy outcomes.
Maternal education made by far the largest single contribution to explaining the poor birth outcomes in Roma mothers. This is not surprising, because there were huge differences in educational attainment between Roma and non-Roma mothers in this study (table ), and because maternal education has been previously shown to be the key determinant of low birth weight, preterm birth, intrauterine growth and infant mortality in the Czech population [18
]. In this study, the crude differences in mean birth weight between infants born to mothers with primary and university education was 322 g. It is therefore entirely plausible that education plays an essential part in the differences between the two ethnicities, not least because maternal education is a good proxy for a variety of measures of deprivation. Marital status was also strongly associated with birth outcomes (e.g. the crude differences in birth weight between married and single mothers was 232 g) but its contribution to the ethnic differences was smaller than that of education.
Maternal size, particularly height, partly reflects socioeconomic conditions in earlier life [20
]. Reproductive history, indicated by the number of pregnancies, is also associated with social status. It is therefore likely that the ethnic differences in birth outcome in the Czech Republic are to a considerable extent determined by socioeconomic factors. This is consistent with the conclusions of a recent review of the literature on the ethnic differences in health in United States and Britain [3
]. It would be interesting to explore whether factors such as nutrition or use, access to and quality of antenatal care can help further clarify the pathways linking ethnicity, socioeconomic circumstances and health.