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

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.