The Pathology of Monochorionic Monozygosity

Abstract

One of the major concepts espoused in this volume is that placental form can be as important to the outcome of a twin conception as the genetic derivation of the conceptuses. The most clear-cut evidence for this concept is in monochorionic monozygotic twinning. Except for genetic considerations, monozygotic twins with dichorionic placentas have similar developmental and gestational risks as dizygotic twins. Monochorionic monozygotic twins, however, have two additional potential sources of problems—vascular anastomoses between the fetuses and/or abnormalities of duplication.^{1, 2} The consequences of aberrations of monochorionic twinning may be a pair of remarkably different so-called identical twins—an argument for avoiding the term as it is not only inaccurate but potentially confusing when counseling affected families. Some of the most bizarre anomalies of human reproduction are seen in these cases—as Antonio asks of Sebastian “How have you made division of yourself?” (Shakespeare, Twelfth Night, V, i).