Tolerance to the developing fetus is thought to be accomplished through the action of several molecules that are able to modulate the maternal immune response. Among several mechanisms involved in pregnancy maintenance, progesterone-induced immunomodulation, asymmetric antibody (AAb) production, indoleamine 2,3-dioxygenase (IDO)-mediated tryptophan catabolism and Th1- to Th2-type cytokine balance have been particularly well studied. However, spontaneous abortions (SA) remain the most common complication of pregnancy, affecting 15% of women, primarily in the first trimester. Development of sensitive methods for the early diagnosis of this condition is therefore a matter of critical importance. In the present study, we investigated AAb production and IDO activity in pregnant women in order to assess their value as early markers for the diagnosis of pregnancy failure. Serum AAb percentages were significantly reduced in women who subsequently suffered from SA compared with controls (p<0.001). Levels of IL-10, IL-12 and IDO activity were also lower in the SA cases, although levels of significance were not reached. In view of these findings, low maternal serum AAb percentages during the first trimester of pregnancy may be indicative of a threat to pregnancy progression.