title: Month of birth as a latitude-dependent risk factor for multiple sclerosis in Norway.
abstract: OBJECTIVE
  We aimed to determine if the risk of Multiple Sclerosis (MS) is associated with month of birth in Norway and to explore a possible latitudinal gradient.
  
  
  METHODS
  All patients with MS born between 1930 and 1979 registered in the Norwegian MS Registry or ascertained in Norwegian prevalence studies were included (n = 6649). The latitude gradient was divided in Southern, Middle and Northern Norway, according to the estimated regional yearly mean vitamin D effective UV dose.
  
  
  RESULTS
  Risk of MS was 11% higher for those born in April (p = 0.045), and 5% higher for those born in May (p = 0.229), 5% lower for those born in November (p = 0.302) and 12% lower for those born in February (p = 0.053) compared with the corresponding population, unaffected mothers and siblings. In Southern Norway the odds ratio of MS births in April and May was 1.05 (0.98-1.24), in Middle Norway 1.11 (0.97-1.27) and in Northern Norway 1.28 (1.0-1.63) compared with the other months.
  
  
  CONCLUSIONS
  This study confirms previous reports of increased MS births in spring and decreased MS births in the winter months. This could support the role of decreased sunlight exposure during pregnancy and vitamin D deficiency in prenatal life in MS.
authors.affiliation: Norwegian Multiple Sclerosis Competence Centre, Department of Neurology, Haukeland University Hospital, Bergen, Norway. nina.grytten@helse-bergen.no
authors.forename: Nina
authors.initials: N
authors.lastname: Grytten
authors.forename: Øivind
authors.initials: Ø
authors.lastname: Torkildsen
authors.forename: Jan Harald
authors.initials: JH
authors.lastname: Aarseth
authors.forename: Espen
authors.initials: E
authors.lastname: Benjaminsen
authors.forename: Elisabeth Gulowsen
authors.initials: EG
authors.lastname: Celius
authors.forename: Ole Petter
authors.initials: OP
authors.lastname: Dahl
authors.forename: Trygve
authors.initials: T
authors.lastname: Holmøy
authors.forename: Kristin
authors.initials: K
authors.lastname: Løken-Amsrud
authors.forename: Rune
authors.initials: R
authors.lastname: Midgard
authors.forename: Kjell-Morten
authors.initials: KM
authors.lastname: Myhr
authors.forename: Geir
authors.initials: G
authors.lastname: Risberg
authors.forename: Anita
authors.initials: A
authors.lastname: Vatne
authors.forename: Margitta T
authors.initials: MT
authors.lastname: Kampman
mesh_terms: D005260:Female; D006801:Humans; D008297:Male; D009103:Multiple Sclerosis / Q000453:epidemiology*; D009664:Norway / Q000453:epidemiology; D016017:Odds Ratio; D012042:Registries; D012307:Risk Factors; D012621:Seasons