Does one size fit all? The case for ethnic-specific standards of fetal growth

BACKGROUND: Birth weight for gestational age is a widely-used proxy for fetal growth. Although the need for different standards for males and females is generally acknowledged, the physiologic vs pathologic nature of ethnic differences in fetal growth is hotly debated and remains unresolved. METHODS: We used all stillbirth, live birth, and deterministically linked infant deaths in British Columbia from 1981 to 2000 to examine fetal growth and perinatal mortality in Chinese (n = 40,092), South Asian (n = 38,670), First Nations, i.e., North American Indian (n = 56,097), and other (n = 731,109) births. We used a new analytic approach based on total fetuses at risk to compare the four ethnic groups in perinatal mortality, mean birth weight, and "revealed" (< 10th percentile) small-for-gestational age (SGA) among live births based on both a single standard and four ethnic-specific standards. RESULTS: Despite their lower mean birth weights and higher SGA rates (when based on a single standard), Chinese and South Asian infants had lower perinatal mortality risks throughout gestation. The opposite pattern was observed for First Nations births: higher mean birth weights, lower revealed SGA rates, and higher perinatal mortality risks. When SGA was based on ethnic-specific standards, however, the pattern was concordant with that observed for perinatal mortality. CONCLUSION: The concordance of perinatal mortality and SGA rates when based on ethnic-specific standards, and their discordance when based on a single standard, strongly suggests that the observed ethnic differences in fetal growth are physiologic, rather than pathologic, and make a strong case for ethnic-specific standards.