As several cases of chimerism have been reported in pregnancy by assisted reproductive technology (ART), ART-related problems include not only an increased risk of multiple pregnancy but also a possibility of chimerism. In particular, confinedblood chimerism (CBC), which is attributable to placental vessel anastomosis between twins, has been detected in 11 cases of monochorionic dizygotic twins. It is possible that three or four alleles could be detected in patients with CBC, misleading physicians at the time of blood transfusion or genotyping for transplantation of an allograft. Therefore, sufficient informed consent and genetic counseling prior to ART are absolutely necessary for patient quality of life. ART may increase the chance of cell fusion (or cell cleavage) during embryogenesis, causing a increased risk of chimerism. Elective single-embryo transfer could avoid chimerism caused by cell-fusion after embryo transfer, but not chimerism generated before embryo transfer. To clarify the problem of chimerism in pregnancy by ART and determine solutions for this problem, both long-term follow-up of cases of chimerism and molecular genetic analysis using DNA polymorphic markers are essential.