Month of birth as a latitude-dependent risk factor for multiple sclerosis in Norway

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.