ABSTRACT
MZ twins are often presumed to be ‘genetically identical’ at birth and to have lived their fetal life in a common intrauterine environment, which will have affected them equally. Furthermore, MZ twins are often presumed to have originated from equal numbers of multipotential ‘founder cells’, and to have enjoyed equal relationships with their sources of placental nutrition and gas exchange. Unfortunately, all of these assumptions are untrue, albeit to varying extents, and more so in some MZ twin pregnancies than in others. The complexity and variety of early MZ twin development lend a degree of sophistication and fascination to the understanding of MZ twinning that also vitiates many ‘twin studies’ that do not take account of the degree to which MZ twins may be discordant at birth as a result of a variety of environmental, genetic and epigenetic events that take place after conception.
