The need for a Pacific-specific low birth weight threshold: a comparison between Samoan babies born in Samoa with Samoan babies born in Aotearoa New Zealand
There are ethnic differences in mean birth weight, proportions of low birth weight, and infant mortality in the multiethnic society of Aotearoa New Zealand. Low birth weight in particular is strongly associated with infant mortality. Pacific babies born in Aotearoa New Zealand have a 40 percent greater risk of death than the average Aotearoa New Zealand baby. This is despite Pacific babies having a lower prevalence of pre-term and low birth weight deliveries compared to other Aotearoa New Zealanders. Therefore, it is important to consider the definition of low birth weight and whether an ethnic specific low birth weight threshold would better identify at risk Pacific infants. Study aims The aim of this project is to investigate whether the current standard measure used to determine low birth weight LBW < 2500 grams, as determined by the 10th percentile of full term births is appropriate for Pacific infants, or whether the suggested ethnic-specific low birth weight measure of < 3000 grams is more appropriate to identifying at risk Pacific infants. Design and Methods An international comparative cross-sectional study design was employed to examine birth weight such as LBW < 2500 grams and preterm births, relevant demographics such as age, parity and any other information available from birth records of the Samoan born population sample. The Samoan born sample consisted of 1,054 mothers and babies from birth registrations with the Samoan National Hospital – Tupua Tamasese Meaole TTM between the years January 2006–2007. The comparative sample of Aotearoa New Zealand born Samoans were sourced from the Pacific Island Families PIF birth cohort study and numbered 647 Samoan mothers and their babies. Results Samoan TTM babies were on average 3,319 grams whereas Samoan PIF babies were on average 3,553 grams, some 234 grams heavier p<0.001. Similarly, the 10th percentile for full term births measured at 2,840 grams for Samoan TTM babies and 3,065 grams for Samoan PIF babies, a difference of 225 grams. Significant predictor variables of mean birth weight for the two sample populations were country p<0.001; maternal age categories p<0.001; parity p<0.001 and infant sex p=0.02. Mothers’ smoking habits prior to conceiving were non-significant in predicting the mean birth weights of the two birth samples, although recall biases may have been responsible for this null finding. When all births were considered, Samoan PIF babies were more likely than those born in Samoa to be either preterm or low birth weight. Marital status as a main effect was the only significant predictor of either preterm or low birth weight outcomes, with PIF mothers in de facto relationships having higher odds of birthing either a preterm or low birth weight infant than TTM mothers Discussion There were significant differences in the birth weight distributions and profiles of Samoan TTM babies and Samoan PIF babies even after adjusting for the limited number of predictor variables available. However, both Samoan samples had a 10th percentile birth weight threshold that was significantly higher than the 2,500 grams threshold currently used to identify at risk births. The presence of other individual, cultural, societal and environmental factors, which were unavailable to the present study, may offer valid explanations for the results obtained. The significant differences observed between the proportions of either preterm or low birth weight within the two samples suggest other important factors excluding those which were available may be at play. Such factors may include differences in delivery procedures. For example, Aotearoa New Zealand may have a greater tendency to induce earlier labour once at risk infants are identified, and to conduct higher risk deliveries. Conclusions The mean birth weights and 10th percentile birth weights for the two Samoan sample populations were different; but both populations are significantly higher than the average Aotearoa New Zealand birth weights and 10th percentile birth weights. The results suggest an ethnic specific low birth weight threshold of < 3000 grams may be more appropriate to identify at risk Samoan infants.