Timing of birth and risk of multiple sclerosis in the Scottish population

Development of multiple sclerosis (MS) is believed to result from interplay of genes and environmental factors, which may be anticipated to act in a seasonal manner. Scotland has the highest prevalence of MS in the world. We aimed to determine if risk of multiple sclerosis is associated with season of birth in the Scottish population. Data for MS patients within the West of Scotland (n = 1,309) was obtained. Birth rates of MS patients were compared with national (n = 6,198,352) and regional (n = 664,447) controls. Excess MS births occur in the spring months, with 22% more than expected (401 observed vs. 328 expected, p < 0.0001). Peak month in both sexes was April. Fewer MS births occurred in autumn, with 16% fewer births compared with regional controls (275 observed vs. 328 expected, p = 0.01). Our study, the largest in the Scottish population, demonstrates season of birth and risk of MS are associated. Elucidating potential environmental factor(s) accounting for such an association remains a challenge.