[Tetragametic chimerism: Case report].

We report the third case of spontaneous monochorionic dizygous pregnancy, discovered on foetal sex discordance. Blood group testing on the female twin revealed a hematopoietic chimera. The mechanism of monochorionic dizygous formation could be the fusion of two independent zygotes at a late morula stage. A single placental mass with vascular anastomosis then develops. Stem cells exchanged during early foetal life can thus lead to chimeras, in similar conditions to stem cell transfusion in adults. Immaturity of the foetal immune system allows cell graft in the other twin's marrow. Assisted reproductive procedures are believed to promote such pregnancies.