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 Islande r livebirths in the Kimberley region of Western Australia from 1981-93. Mea n 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 seaso n. 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 th an 1500 g are very likely to be pre-term (<37 weeks' gestation). the findin gs also suggest that seasonality of birthweight may be due to an increase i n pre-term births rather than an increase in intrauterine growth retardatio n. Further research is required to identify the underlying causes of an inc rease in VLBW babies during the wet season.