During normal pregnancy, the fetus continues to mature inside the uterus without rejection. Inherited paternal antigens could be targeted by the maternal immune system. These reactions are believed to play a role in a number of habitual abortions. However, the precise maternal mechanisms preventing fetal tissue rejection are not well understood. Maternal T cells should recognize fetal antigens, so it is conceivable that antigen-specific T cell response to fetal antigens would occur by proliferation and accumulation of certain T cell clones in the pregnant mother. To elucidate the maternal immune response to the fetus we investigated the clonality of expanded T cells in peripheral blood lymphocytes in ten normal pregnant women. We employed reverse transcriptase-polymerase chain reaction for T cell receptor beta chain gene and subsequently analyzed the PCR product by single-strand conformation polymorphism analysis. A large number of distinctly expanded T cell clones were detected during pregnancy. These accumulations were observed as early as the ninth to tenth week post-conception and reached a maximum during the second trimester, suggesting the existence of dynamic antigen-specific T cell responses in the pregnant mother. However, after the 30th week of gestation, nearly all expanded T cell clones disappeared before parturition and the degree of clonality reached almost normal levels. Our results clearly indicate the existence of dynamic maternal T cell responses during pregnancy.