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.