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.

It may be hypothesized that sexual steroids could exert beneficial effects through a modulation of the immune state with a lowering of the pro-inflammatory lymphocyte responses of the Th1 type and an enhancement of the anti-inflammatory responses of the Th2 type. They may also play a direct role in the remyelination of central nervous system lesions, as they do in the peripheral nervous system. The POPART'MUS study is a European, multicentre, randomized, placebo-controlled and double-blind clinical trial, which aims to prevent MS relapses related to the post-partum condition, by administering high doses of progestin (nomegestrol acetate), in combination with endometrial protective doses of estradiol. Treatment will be given immediately after delivery and continuously during the first three months post-partum. Assuming the results of the trial to be positive, this new treatment could be considered in the relapsing-remitting phase of the disease in women afar from pregnancy and post-partum.