Month of birth as a risk factor for multiple sclerosis: an update Torkildsen Ø, Grytten N, Aarseth J, Myhr K-M, Kampman MT. Month of birth as a risk factor for multiple sclerosis: an update. Acta Neurol Scand: 2012: 126 (Suppl. 195): 58–62. © 2012 John Wiley & Sons A/S. Background – Several studies have indicated month of birth as a risk factor for multiple sclerosis (MS) susceptibility and disease progression. Methods – We performed a systematic search on PubMed and Medline up to May 2012 using the search string ‘multiple sclerosis’ and ‘month of birth’ or ‘season of birth’. In addition, congress abstracts and the reference lists of the publications identified were examined for further citations of relevance. Results – A total of fifteen published studies and two congress abstracts were found on the effect of month or season of birth on MS risk (sixteen in the northern and one in the southern hemisphere). Most studies in the northern hemisphere detected an excess of MS births in spring and a decrease in autumn. In the southern hemisphere, a reverse pattern was detected, with an excess in November and a decrease in April. Only three studies did not report any month of birth effect, all in low-risk areas for MS. Five studies have analysed a possible effect on disease course by month of birth. Of these, two studies reported an association between month of birth and age at onset of relapsing- remitting MS, with a younger disease onset for those born in the winter months. No consistent findings have been detected on the association between month of birth and disease progression. Discussion – The month of birth effect is consistently found to influence the risk of MS, and the effect seems to be most prominent in high-risk areas of the disease, especially in areas with low sunlight exposure. There seems to be little or no month of birth effects in areas with high sunlight exposure. These findings indicate a possible role for vitamin D concentrations during pregnancy or early life of the newborn. A possible effect of vitamin D supplementation needs to be further investigated. Ø. Torkildsen 1,2 , N. Grytten 1,2 , J. Aarseth 1,2 , K.-M. Myhr 1,2 , M. T. Kampman 3,4 1 Norwegian Multiple Sclerosis Competence Centre, Department of Neurology, Haukeland University Hospital, Bergen, Norway; 2 Kristian Gerhard Jebsen MS Research Centre, Department of Clinical Medicine, University of Bergen, Bergen, Norway; 3 Institute of Clinical Medicine, University of Tromsø, Tromsø, Norway; 4 Centre for clinical research and education, UNN, Tromsø, Norway Key words: multiple sclerosis; epidemiology; month of birth; season of birth Ø. Torkildsen, Department of Neurology, Haukeland University Hospital, N-5021 Bergen, Norway Tel.: +47 55 97 70 39 Fax: +47 55 97 59 01 e-mail: oivind.torkildsen@gmail.com Accepted for publication June 4, 2012 Introduction Multiple sclerosis (MS) is a chronic, demyelinat- ing disease of the central nervous system (CNS), which is thought to be caused by a complex inter- play between genetic and environmental risk fac- tors. A number of studies have indicated that the risk of MS is strongly associated with place of residence in early life (1). Further, early life and gestational events have frequently been suggested to contribute to MS susceptibility. Findings of seasonal birth patterns that differ significantly from the general population suggest that environ- mental factors acting in early life or in the intra- uterine period are important for disease susceptibility (2). We have reviewed the current literature on month and season of birth in MS and whether they can indicate possible risk fac- tors for the disease. Methods We searched PubMed and Medline up to May 2012, using the search string ‘multiple sclerosis’ and ‘month of birth’ or ‘season of birth’. Addi- tionally, we searched abstract books and abstract CDs for relevant abstracts presented in the past 3 years at meetings or congresses held by the European Committee for Treatment and Research in Multiple Sclerosis (ECTRIMS), the 58 Acta Neurol Scand 2012: 126 (Suppl. 195): 58–62 DOI: 10.1111/ane.12040 Ó 2012 John Wiley & Sons A/S ACTA NEUROLOGICA SCANDINAVICA