The influence of pregnancy on the occurrence and evolution of maternal tumors has been long debated. Breast carcinomas or melanomas have been suspected to be more severe during gestation. Recently, many investigators have described the transfer and persistence of fetal cells in maternal circulation and tissues during and after pregnancy. These fetal microchimeric cells have been described in a variety of maternal injured tissues where they displayed the host-tissue phenotype. Given the wide variety of injury and tissue types described, cancer has appeared as a potential situation that could be influenced by fetal microchimeric cells. This new unexplored effect of gestation on tumor course has been hypothesized as either protective against cancer, via the activity of allogenic fetal cells, or as promoting cancer, via a supportive role of fetal microchimeric cells in the tumor stroma. In this review, we will detail recent data supporting these hypotheses.