We agree with Rao et al that careful placental examinations are a desirable feature of any twin study, and it is deplorable how infrequently pathologic documentation of the disposition of the fetal membranes is included as a routine part of the management of twin births. However, placentation is not an infallible criterion of zygosity, since approximately 25% of all monozygotic twins are dichorionic and may be indistinguishable from dizygotic pairs by examination of the afterbirths.1 Misinformation about twin placentation contributes to errors in self-perceived zygosity. In the present series, the only twins previously considered to be fraternal were a pair whose parents had been erroneously told at the time of birth that their twins must be dizygotic because there were two placentas. Although skin transplantation has been accepted as proof of monozygosity in a court of law,2 even this criterion may fail in the case of
Miller JZ, Nance WE. Diagnosis of Twin-Zygosity by Dermatoglyphics-Reply. JAMA. 1977;237(25):2718–2719. doi:10.1001/jama.1977.03270520028005
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