Discordant fetal anomalies in monochorionic multiple pregnancies

Abstract Monozygotic (MZ) twins sharing a placenta are known as monochorionic (MC) and account for approximately two-thirds of MZ twin pregnancies. MC twins discordant for a congenital anomaly are of particular interest as they challenge the common idea of “identical” monozygotic twins. We have performed a retrospective study including 51 monochorionic multiple pregnancies with discordant anomalies that received care at the University Hospital of Graz between 2013 and 2023, aiming to assess the accordance of the observed cases with the existing explanatory models in the literature. This study primarily identified structural discordance, while genetic insights were limited due to the rare decision of parents to undergo further genetic testing. The most frequent anomalies were hydrocephalus, neural tube defects, body-stalk anomaly and congenital heart defects. Acknowledging the limitations related to the sample size and incomplete cohort data, this study supports a growing awareness of the complexity underlying MC twin development, emphasizing the need for more longitudinal genetically and epigenetically focused studies to uncover the subtle and cumulative effects of early contributing factors that ultimately also determine phenotypic expression.