Like in other immune mediated diseases, pregnancy was shown to influence Multiple Sclerosis (MS) disease activity. It is marked by a decrease in relapse rate, especially during the third trimester. A temporary increase in relapses is observed within the first months postpartum, after which the rate returns to pre-pregnancy baseline. The protective effects of pregnancy on MS disease activity are thought in part to reflect the anti-inflammatory effects of sex hormones. Sex hormones such as estrogen and progesterone are increased during pregnancy, with the highest levels reported in the third trimester, and a sharp drop post-partum. There is a sex hormone mediated shift in immune response during pregnancy, which is characterized by a decrease in pro-inflammatory TH1 responses and an increase in anti-inflammatory TH2 responses, intended to promote fetal survival.