Summary CD4 + CD25 + T regulatory cells (T Reg ), suppress antigen‐specific immune responses and are important for allograft tolerance. During pregnancy the mother tolerates an allograft expressing paternal antigens (the fetus) requiring substantial changes in immune regulation over a programmed period of time. We analysed whether immune‐suppressive T Reg cells were altered during pregnancy and therefore might play a part in this tolerant state. The presence of T Reg cells was assessed in the blood of 25 non‐pregnant, 63 pregnant and seven postnatal healthy women by flow cytometry. We observed an increase in circulating T Reg cells during early pregnancy, peaking during the second trimester and then a decline postpartum. Isolated CD25 + CD4 + cells expressed FoxP3 messenger RNA, a marker of T Reg cells, and suppressed proliferative responses of autologous CD4 + CD25 – T cells to allogeneic dendritic cells. These data support the concept that normal pregnancy is associated with an elevation in the number of T Reg cells which may be important in maintaining materno‐fetal tolerance.

Normal human pregnancy is associated with an elevation in the immune suppressive CD25+ CD4+ regulatory T‐cell subset