Analysis of the Relationship between the Month of Birth and Risk of Multiple Sclerosis in a Spanish Population

Some studies have suggested an association between the month of birth and risk of multiple sclerosis (MS), related to environmental factors, mainly sun exposure and maternal vitamin D levels. Few studies have been conducted in Southern Europe countries. Madrid has a continental climate with considerable variation of sun hours between winter and summer, so it may be relevant to study this relationship.

MS patients, born between 1932 and 2003, 1,335 of them from our database between 2004 and 2015, were analysed. The weighted average number of births per month in Madrid from 1996 and 2012 (n = 1,098,774) was considered the control population. The month and season of birth were analysed using chi-square, Hewitt's and Roberson's seasonality tests.

Birth rate increased in June, July, and September, and decreased in November, January, and February. Births were 29% more frequent in summer (July-September) than in winter (January-March), with a ratio of 0.79. Hewitt's test for seasonality gave a rank sum of 53 between May and October (p = 0.12). Rogerson's variation was applied to 3-, 4-, and 5-month periods. Substantial differences were noted in the 5-month periods (k = 5), although the largest rank sum (June-October) was not significant (p = 0.09).

Our analysis seemed to suggest that pre-natal sun exposure may have an influence on the incidence of MS, most likely in combination with other environmental or genetic factors.