CD4(+)CD25(high) regulatory T cells (Tregs) are implicated in the maintenance of murine pregnancy. However, reports regarding circulating Treg frequencies in human pregnancy are inconsistent, and the functionality and phenotype of these cells in pregnancy have not been clarified. The aim of this study was to determine the frequency, phenotype, and function of circulating Tregs in the second trimester of human pregnancy and the influence of progesterone and 17beta-estradiol on Treg phenotype and frequency. Based on expressions of Foxp3, CD127, and HLA-DR as determined by multicolor flow cytometry, we defined a proper CD4(dim)CD25(high) Treg population and showed, in contrast to most previous reports, that this population was reduced in second trimester of pregnancy. Unexpectedly, Foxp3 expression was decreased in the Treg, as well as in the CD4(+) population. These changes could be replicated in an in vitro system resembling the pregnancy hormonal milieu, where 17beta-estradiol, and in particular progesterone, induced, in line with the pregnancy situation, a reduction of CD4(dim)CD25(high)Foxp3(+) cells in PBMC from nonpregnant women. By coculturing FACS-sorted Tregs and autologous CD4(+)CD25(-) responder cells, we showed that Tregs from pregnant women still displayed the same suppressive capacity as nonpregnant women in terms of suppressing IL-2, TNF-alpha, and IFN-gamma secretion from responder cells while efficiently producing IL-4 and IL-10. Our findings support the view of hormones, particularly progesterone, as critical regulators of Tregs in pregnancy. Furthermore, we suggest that in the light of the results of this study, early data on circulating Treg frequencies in pregnancy need reevaluation.