title: The impact of nonlinear exposure-risk relationships on seasonal time-series data: modelling Danish neonatal birth anthropometric data.
abstract: BACKGROUND
  Birth weight and length have seasonal fluctuations. Previous analyses of birth weight by latitude effects identified seemingly contradictory results, showing both 6 and 12 monthly periodicities in weight. The aims of this paper are twofold: (a) to explore seasonal patterns in a large, Danish Medical Birth Register, and (b) to explore models based on seasonal exposures and a non-linear exposure-risk relationship.
  
  
  METHODS
  Birth weight and birth lengths on over 1.5 million Danish singleton, live births were examined for seasonality. We modelled seasonal patterns based on linear, U- and J-shaped exposure-risk relationships. We then added an extra layer of complexity by modelling weighted population-based exposure patterns.
  
  
  RESULTS
  The Danish data showed clear seasonal fluctuations for both birth weight and birth length. A bimodal model best fits the data, however the amplitude of the 6 and 12 month peaks changed over time. In the modelling exercises, U- and J-shaped exposure-risk relationships generate time series with both 6 and 12 month periodicities. Changing the weightings of the population exposure risks result in unexpected properties. A J-shaped exposure-risk relationship with a diminishing population exposure over time fitted the observed seasonal pattern in the Danish birth weight data.
  
  
  CONCLUSIONS
  In keeping with many other studies, Danish birth anthropometric data show complex and shifting seasonal patterns. We speculate that annual periodicities with non-linear exposure-risk models may underlie these findings. Understanding the nature of seasonal fluctuations can help generate candidate exposures.
authors.affiliation: Queensland Centre for Mental Health Research, The Park Centre for Mental Health, Wacol, Q4076, Australia. john_mcgrath@qcmhr.uq.edu.au
authors.forename: John
authors.initials: J
authors.lastname: McGrath
authors.forename: Adrian
authors.initials: A
authors.lastname: Barnett
authors.forename: Darryl
authors.initials: D
authors.lastname: Eyles
authors.forename: Thomas
authors.initials: T
authors.lastname: Burne
authors.forename: Carsten B
authors.initials: CB
authors.lastname: Pedersen
authors.forename: Preben Bo
authors.initials: PB
authors.lastname: Mortensen
mesh_terms: D000886:Anthropometry; D001724:Birth Weight; D001827:Body Height; D003718:Denmark; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D018811:Maternal Exposure / Q000009:adverse effects / Q000706:statistics & numerical data*; D017711:Nonlinear Dynamics; D010507:Periodicity; D016012:Poisson Distribution; D011247:Pregnancy; D012042:Registries; D018570:Risk Assessment / Q000379:methods / Q000706:statistics & numerical data*; D012621:Seasons; D013696:Temperature; D014808:Vitamin D Deficiency