Seasonality of low birthweight in Indigenous Australians: an increase in pre-term birth or intrauterine growth retardation?

Analysis of birthweights of Aboriginal and Torres Strait Islander livebirths in the Kimberley region of Western Australia from 1981‐93 suggests that adverse environmental conditions may be associated with increased risk of very low birthweight (VLBW) and suggests that seasonality of birthweight may be due to an increase in pre‐term births rather than a increase in intrauterine growth retardation.

Abstract

We have analysed birthweights of 4,508 Aboriginal and Torres Strait Islander livebirths in the Kimberley region of Western Australia from 1981-93. Mean birthweight varied significantly according to month of birth (F(11) = 2.57, p = 0.003) and low birthweight babies were more common during the wet season. A significant increase in the proportion of very low birthweight (VLBW) babies was observed during the wet season compared with the dry season (OR 2.73; 95% CI 2.3-3.67; p < 0.001); whereas babies weighing 1,500-2,499 g were not significantly more common during the wet season (OR 1.06; 95% CI 0.96-1.17; p = ns). The results indicate that adverse environmental conditions may be associated with increased risk of VLBW. Since newborns weighing less than 1500 g are very likely to be pre-term (< 37 weeks' gestation), the findings also suggest that seasonality of birthweight may be due to an increase in pre-term births rather than an increase in intrauterine growth retardation. Further research is required to identify the underlying causes of an increase in VLBW babies during the wet season.