title: The role of birth weight in myopia--the genes in myopia twin study.
abstract: OBJECTIVE
  It was the aim of this study to assess the role of birth weight in the development of myopia using a large cohort of Caucasian monozygotic (MZ) and dizygotic (DZ) twins that took part in the Genes in Myopia (GEM) twin study.
  
  
  METHODS
  The recruitment of all twins in the GEM twin study was facilitated by the Australian Twin Registry. Each twin underwent a standard questionnaire and a comprehensive ocular examination, which included a dilated objective refraction through autorefraction. Myopia was defined as spherical equivalent equal to or worse than -0.50 diopters. Birth weight was determined through self-report as part of the standard questionnaire.
  
  
  RESULTS
  A total of 1,224 twins (690 MZ and 534 DZ twins) aged between 18 and 86 years (mean age 52.36 years) were recruited into the GEM study. The mean birth weight was similar between MZ (2.34 kg) and DZ twins (2.46 kg; p > 0.05). Logistic regression showed no significant association with birth weight and myopia for all twins (p = 0.26), as well as for MZ (p = 0.18) and DZ twins (p = 0.70) separately, with no gender effect (p = 0.23). Moreover, there was no significant difference in mean birth weight between discordant (presence/absence) MZ (myopes = 2.33 kg, non-myopes = 2.39 kg) and DZ twin pairs (myopes = 2.39 kg, non-myopes = 2.43 kg; p = 0.91 and 0.95, respectively).
  
  
  CONCLUSIONS
  Birth weight appears to have little to no role in the development of myopia. In addition, birth weight was not a predictor of the discordance of myopia in MZ and DZ twin pairs.
authors.affiliation: Centre for Eye Research Australia, University of Melbourne, Melbourne, Vic, Australia. m.dirani@pgrad.unimelb.edu.au
authors.forename: M
authors.initials: M
authors.lastname: Dirani
authors.forename: F M A
authors.initials: FM
authors.lastname: Islam
authors.forename: P N
authors.initials: PN
authors.lastname: Baird