Successful pregnancy depends on an adaptation of the maternal immune system that becomes tolerant to fetal antigens of paternal origin. The altered immune regulation induced by pregnancy occurs predominantly at the maternal-fetal interface, but it has also been observed in the maternal circulation. Th1/Th2 shift is one of the most important immunologic changes during gestation. It is due to the progressive increase of estrogens, which reach peak level in the third trimester of pregnancy. At these high levels, estrogens suppress the Th1-mediated responses and stimulate Th2-mediated immunologic responses. For this reason Th1-mediated diseases, such as rheumatoid arthritis, tend to improve, while Th2-mediated diseases, such as systemic lupus erythematosus (SLE) tend to worsen during pregnancy. However, in some recent studies SLE flare-ups were less frequently observed in the third trimester of gestation in comparison to the second trimester and postpartum period. These data are apparently in contrast to the Th2 immune-response polarization expected during pregnancy due to the progressive increase of estrogens. Some further data suggest that in SLE patients estradiol serum levels are surprisingly lower than expected during the third trimester of pregnancy, probably due to a placental compromise. This occurrence could lead to a lower-than-expected increase of IL-6, accounting for the low humoral immune response and the low disease activity observed in the third trimester of pregnancy in such patients.