Pregnancy is associated with transfer of maternal cells to the fetus and fetal cells to the mother. In both cases, the transferred cells are described as microchimeric. Fetal microchimeric cells include semi-allogeneic stem cells, which are few in number and are capable of long-term survival in the "foreign" host. They are recognized by the maternal immune system but not rejected or attacked. These cells appear to survive and even thrive for years in a mother's body, perhaps for her lifetime. Previously regarded as potentially dangerous interlopers that might propagate autoimmune and even malignant disease, fetal microchimeric cells are now increasingly being recognized and analyzed for their healing, reparative, and perhaps regenerative roles. Fetal microchimerism (MC) may make significant and previously unknown positive contributions to women's health, longevity, and risk of disease. This article reviews the history, major discoveries, and current concepts and gaps in knowledge in the field of fetal MC.