Pregnancy is associated with a lower risk of onset and a better prognosis in multiple sclerosis.

The effects of pregnancy were studied in a multiple sclerosis incidence cohort. In order to eliminate interaction bias between the disease and pregnancy, analysis of the risk of relapse during pregnancy and the puerperium was limited to the onset bout, using fecundity figures for Sweden. The risk of onset bout was significantly reduced during pregnancy while the risk of onset bout in the post-partum period did not differ significantly from the risk during non-pregnancy periods. We also found a decreased risk of multiple sclerosis onset in parous compared with nulliparous women. The association between nulliparity and multiple sclerosis tended to increase with age. Furthermore, the effect of pregnancy on the long-term prognosis in established multiple sclerosis was analysed by comparing the risk of change from a relapsing-remitting to a chronic progressive course and the risk of reaching level 6 of the Disability Status Scale in women with pregnancy after multiple sclerosis onset with that in non-pregnant control patients, matched for neurological deficit, disease duration and age. There was a significantly decreased risk of a progressive course in women who were pregnant after multiple sclerosis onset.