The association between birth weight, season of birth and latitude

Background: Birth weight is known to fluctuate with month of birth and with latitude-, however the mechanisms underlying these gradients remain unclear. Aim: The study examined within-year fluctuations in birth weight across four regions in Australia. Subjects and methods: We obtained the birth weight of singleton births of at least 37 weeks gestation from four regions: (a) southeast Queensland, (b) Sydney and surrounding districts, (c) Victoria, and (d) Tasmania. Seasonal fluctuations were examined after long-term trends had been removed. The amplitude, timing and variability of seasonal fluctuations were examined and compared across regions. Results: Each of the four regions had prominent annual periodicity with the peak birth weight occurring in spring. The amplitude of this signal ranged from 1.4g (Victoria) to 7.7g (southeast Queensland). There was no significant correlation between the amplitude of these fluctuations and the latitude of four regions. In addition, a second smaller autumn peak was identified. Conclusion: Environmental factors that have regular within-year variation influence birth weight in Australia. The evidence suggests that at least two distinguishable seasonal processes contribute to the findings. Identifying the factors underlying these seasonal fluctuations could have implications from a public health perspective.