The current study was aimed at developing a one-way mixed leucocyte culture–enzyme-linked immunospot (MLC-ELISPOT) assay for the study of CD4+ CD25+ regulatory T (Treg) cells and applying this method in the study of antifetal immune reactions during human pregnancy. Twenty-one pregnant women and the corresponding fathers-to-be, and 10 non-pregnant control women and men, participated in the study. CD4+ CD25+ cells were isolated from peripheral blood mononuclear cells (PBMC) by immunomagnetic selection. Maternal/control PBMC were stimulated with paternal or unrelated PBMC in MLC. Secretion of interleukin-4 (IL-4) and interferon-γ (IFN-γ) from responder cells, with or without the presence of autologous Treg cells, was analysed by ELISPOT. PBMC from pregnant women showed increased secretion of IL-4 compared to controls. In pregnant and non-pregnant controls, Treg cells suppressed IFN-γ reactivity against paternal and unrelated alloantigens. Interestingly, Treg cells suppressed IL-4 secretion against paternal but not unrelated alloantigens during pregnancy. We have successfully developed a model for studying Treg cells in antifetal cytokine reactions during pregnancy. Results indicate that Treg cells contribute to strict regulation of both T helper type 1-like and type 2-like antifetal immune reactions. Interestingly, T helper type 2-like cells specific to unrelated alloantigens are able to escape the suppression of Treg cells, which would allow for IL-4, alongside CD4+ CD25+ Treg cells, to control potentially detrimental IFN-γ reactions during pregnancy.