A timing-of-birth effect on multiple sclerosis clinical phenotype.

BACKGROUND A month-of-birth (MOB) effect has been shown in multiple sclerosis (MS). METHODS Our chi(2) analyses looked at whether this MOB effect differed by MS phenotype ("bout onset," "primary progressive"). RESULTS The MOB effect was derived from "bout onset" MS patients (May/November ratio = 1.43; chi(2) = 17.32, df = 1, p = 0.000032). CONCLUSIONS An unspecified environmental effect in early development can influence both multiple sclerosis susceptibility and phenotype.