A Report of Dizygous Monochorionic Twins

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Abstract
It is an accepted medical doctrine that monochorionic twins are exclusively monozygous. This doctrine is supported by two studies that assessed placental pathological features and multiple serologic markers of zygosity in a total of almost 800 monochorionic twins.1,2 Although the possibility that monochorionic twins are not invariably monozygous has been raised previously, deficiencies in the cytogenetic and pathological characterization of these reports have cast doubt on their reliability.3 We report a case of sex-discordant monochorionic twins conceived by in vitro fertilization. The twins also had blood chimerism (the presence of cells derived from more than one genetically distinct zygote); in both twins, there was a predominance of male-derived lymphocytes, and as a result, the initial findings of zygosity studies performed on DNA extracted from the blood were consistent with monozygosity. Subsequent detailed investigation showed the twins to be dizygous. The evidence from this case is inconsistent with the doctrine that all monochorionic twins are monozygous.