A case for not adjusting birthweight customized standards for ethnicity: observations from a unique Australian cohort

Abstract

Low birthweight is more common in infants of indigenous (Aboriginal and/or Torres Strait Islander) than of White Australian mothers. Controversy exists on whether fetal growth is normally different in different populations. We sought to determine the relationships of birthweight, birthweight percentiles, and smoking with perinatal outcomes in indigenous vs nonindigenous infants to determine whether the White infant growth charts could be applied to indigenous infants. Data were analyzed for indigenous status, maternal age and smoking, and perinatal outcomes in 45,754 singleton liveborn infants of at least 20 weeks gestation or 400 g birthweight delivered in New South Wales, Australia, between June 2010 and July2015. Indigenous infants (n=6372; 14%) had a mean birthweight 67 g lower than nonindigenous infants (P<.0001; with adjustment for infant sex and maternal body mass index). Indigenous mean birthweight percentile was 4.2 units lower (P<.0001). Adjustment for maternal age, smoking, body mass index, and infant sex reduced the difference in birthweight/percentiles to nonsignificance (12 g; P=.07). Disparities exist between indigenous and non-indigenous Australian infants for birthweight, birthweight percentile, and adverse outcome rates. Adjustment for smoking and maternal age removed any significant difference in birthweights and birthweight percentiles for indigenous infants. Our data indicate that birthweight percentiles should not be adjusted for indigenous ethnicity because this normalizes disadvantage; because White and indigenous Australians have diverged for approximately 50,000 years, it is likely that the same conclusions apply to other ethnic groups. The disparities in birthweight percentiles that are associated with smoking will likely perpetuate indigenous disadvantage into the future because low birthweight is linked to the development of chronic noncommunicable disease and poorer educational attainment; similar problems may affect other indigenous populations.