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.