title: The association between birth weight, season of birth and latitude.
abstract: BACKGROUND
  Birth weight is known to fluctuate with month of birth and with latitude; however the mechanisms underlying these gradients remain unclear.
  
  
  OBJECTIVE
  The study examined within-year fluctuations in birth weight across four regions in Australia.
  
  
  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 1.4 g (Victoria) to 7.7 g (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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Queensland Centre for Mental Health Research, The Park Centre for Mental Health, Wacol, QLD 4076, Australia. john_mcgrath@qcsr.uq.edu.au
authors.forename: John J
authors.initials: JJ
authors.lastname: McGrath
authors.forename: Adrian G
authors.initials: AG
authors.lastname: Barnett
authors.forename: Darryl W
authors.initials: DW
authors.lastname: Eyles
mesh_terms: D001315:Australia; D001724:Birth Weight; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D011247:Pregnancy; D012621:Seasons; D019371:Topography, Medical; D014807:Vitamin D / Q000378:metabolism
chemical_list: D014807:Vitamin D