title: Season of birth and multiple sclerosis in Sweden.
abstract: OBJECTIVE
  To estimate the risk of multiple sclerosis (MS) by month of birth in Sweden.
  
  
  METHODS
  Cases (n = 9361) were obtained from the Swedish MS Registry. All births in Sweden 1900-2007 served as controls (n = 12,116,853). The risk of MS was analyzed for each month of birth separately compared with birth during the other 11 months.
  
  
  RESULTS
  More (11%) cases with MS than expected were born in June. Fewer (8% and 10%) cases with MS than expected were born in December and January (non-significant after correction for multiple analyses). More (5%) cases with MS than expected were born in February-July as compared with August-January.
  
  
  CONCLUSIONS
  This study supports previous results suggesting an association between the risk of MS and the season of birth. Decreased exposure to sun in the winter leading to low vitamin D levels during pregnancy is a possible explanation that needs further research.
authors.affiliation: Department of Neurology, Umeå University Hospital, Sweden. jonatan.salzer@neuro.umu.se <jonatan.salzer@neuro.umu.se>
authors.forename: J
authors.initials: J
authors.lastname: Salzer
authors.forename: A
authors.initials: A
authors.lastname: Svenningsson
authors.forename: P
authors.initials: P
authors.lastname: Sundström
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000453:epidemiology* / Q000209:etiology; D036801:Parturition; D012307:Risk Factors; D012621:Seasons; D013548:Sweden; D014807:Vitamin D; D055815:Young Adult
chemical_list: D014807:Vitamin D