Multiple sclerosis (MS) is the most common chronic disabling neurological disease affecting young women. Paradoxically, our knowledge of the relationship between pregnancy and MS is limited. However, several conclusions emerge from the literature: 1) The rate of relapse in MS decreases during pregnancy, and it rises significantly during the first three months post partum before coming back to its level prior to pregnancy. 2) Although pregnancy and delivery cause changes of the relapse rate, they have no influence on mid and long term residual disability. 3) Breast-feeding and epidural analgesia do not seem to have any deleterious effect on the disease. 4) Lastly, MS does not seem to influence pregnancy, delivery or the child's health. The studies available to date suffer from methodological limitations. They need to be confirmed by prospective studies. This is the purpose of the study entitled "Pregnancy in multiple sclerosis, PRIMS", which has been carried out since 1992 at the European level.