[Prenatal diagnosis of a probable human chimera after fertilization in vitro]

Chimerism is the coexistence of more than one cell line in an individual, due to the fusion of originally separate zygotes. It has been very rarely described in humans. A 36-year-old woman referred for in vitro fertilization (IVF) had three embryos transferred leading to a monofetal pregnancy. Ultrasound examination at 17 weeks showed severe intrauterine growth retardation. Amniocentesis revealed a mixture of 46,XY and 46,XX clones. Histopathologic examination showed a dysmorphic fetus with female phenotype and severe growth retardation. Fusion of two of the three embryos (one male and one female) seems to be the most probable mechanism that could explain both cytogenetic and histopathologic observations.