No major birth order effect on the risk of multiple sclerosis.

In the present study, we found no association between multiple sclerosis (MS; definite and probable, n = 211) and birth order (p = 0.1411). The observed number of first-born patients did not differ significantly from the expected number (p = 0.0871). While there was a significantly high birth order (n = 258, p = 0.0381) and a marginally significant low number of first-borns (p = 0.0475) when possible MS cases were included, an artefact due to the population structure may have accentuated this result. In comparison with the control birth cohort, there was no significant association with birth order (p = 0.0742) or the proportion of first-borns (p = 0.220) in a subgroup from the MS incidence cohort born between 1915 and 1929 (n = 158). Birth order had no major impact on the risk of subsequent MS in this study.