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Keywords:

  • epidemiology;
  • etiology;
  • month of birth;
  • multiple sclerosis;
  • season of birth;
  • seasonality;
  • vitamin D

Objective –  To estimate the risk of multiple sclerosis (MS) by month of birth in Sweden.

Materials and 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.