During pregnancy, the fetus that grows within the maternal uterus is not rejected by the maternal immune system. To enable both tolerance towards the fetus and defence against pathogens, modifications of the maternal immune system occur during gestation. These modifications are able to bring about a natural improvement in disease activity of some autoimmune diseases, such as rheumatoid arthritis (RA). Various mechanisms of the immune system contribute to the phenomenon of pregnancy-related improvement of RA, and the cessation of these immunomodulatory mechanisms after delivery correlates with postpartum disease flare. HLA disparity between mother and fetus, glycosylation of IgG, immunoregulatory pathways, and alterations in innate and adaptive immune cells and their cytokines have important roles in pregnancy and in pregnancy-related amelioration of RA.