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Comparisons of growth of children exposed to different environments are sometimes complicated by differences among the children in their ethnic backgrounds, or in the degree of ethnic heterogeneity in their backgrounds. The influence of these factors is difficult to estimate even when the range of ethnic variation is largely limited to within one geographic area (e.g., among Europeans). To determine the effect of ethnic heterogeneity upon child growth, a study was conducted of children predominantly of European descent living near Philadelphia, Pennsylvania. Ethnic heterogeneity was measured by the number of different nations named by each child’s family to represent their ethnic identification, and by the number of different nations of grandparents’ birthplaces. Children were grouped according to the degree of ethnic heterogeneity by each scale, and their postnatal (n = 288) and prenatal growth (n = 524) measures were compared. Of five postnatal and three prenatal growth measures, adjusted groups’ means differed significantly for three (triceps skinfold, height z-score, and birth weight), and only among groups distinguished on the first of the two scales. In addition, the differences were in different directions. Overall, there was no consistent effect of heterogeneity across all the prenatal and postnatal growth measures. Comparisons of the postnatal growth of U.S.-born children of European descent are not likely to be biased by the uncontrolled factor of ethnic heterogeneity.