Multiple sclerosis (MS) affects 1 in 1000 people in western countries, mainly women in their childbearing years. It is an autoimmune disease of the central nervous system (CNS), which results in a chronic focal inflammatory response with subsequent demyelination and axonal loss. Recent prospective studies reported a significant decline by two-thirds in the rate of relapses during the third trimester of pregnancy and a significant increase by two-thirds during the first three months post-partum by comparison to the relapse rate observed during the year prior to the pregnancy (Confavreux et al., 1998). These dramatic changes in the relapse rate occur at a time when the impregnation of many substances (among which, sexual steroids) is at its highest, before a dramatic decline to the pre-pregnancy levels, immediately following delivery.