Month of birth and risk of multiple sclerosis in a Portuguese population

Background: Several studies analyzing the month of birth (MOB) of multiple sclerosis (MS) patients and the risk of the disease have been published; as a whole, MS patients were found to be predominantly born in spring months, leading to the current assumption that MOB is somewhat related to the risk of MS. Objective: Estimate the risk of MS by MOB in a Portuguese population. Methods: MS patients sample was obtained from the database of patients attended at our MS clinic and born in the districts of Porto, Braga and Viana do Castelo. The control sample was composed of the live births records in the same time period and geographical area. We applied the Hewitt test for seasonality. Results: We found 421 patients that satisfied the conditions to enter the study. The rank-sums for successive 6-month segments indicate the July December period as of higher incidence; however, the corresponding rank-sum (48) was not statistically significant according to the Hewitt test (p > 0.05). Conclusion: Our data does not support the seasonality hypothesis of MOB as risk factor for MS in Portugal. However we are aware that the analysis of a larger MS sample could shed more light in this issue. (C) 2013 Elsevier B.V. All rights reserved.