Monozygotic Monochorionic Twins Discordant for Trisomy 21: A Reason to Evaluate Both Fetuses: A Case Report

The occurrence of a discordant chromosomal abnormality in monozygotic twins is an extremely rare condition.

We report the prenatal sonographic findings and cytogenetic studies in a monochorionic twin pregnancy discordant for severe fetal anomalies. Amniocentesis was normal for both twins. The pregnancy was managed conservatively, resulting in the delivery of discordant twins at 28 weeks. Cytogenetic analysis performed on cultured lymphocytes from peripheral blood revealed a mosaic 47XY+21 (in 2% of the cells)/46XY (in 98%) in the structurally normal twin, and a mosaic 47XY+21 (4%)/46XY (96%) for the abnormal twin. The abnormal neonate died shortly after delivery. The structurally normal twin survived without sequelae and had a normal karyotype 2 years later.

This report adds to the literature a case of a monochorionic twin pregnancy with a mosaic fetus who gives his co-twin trisomic cells through placental vascular anastomoses, this twin being a chimera, highlighting the necessity of performing molecular genetics with polymorphic DNA markers to differentiate chimerism from mosaicism and define the origin of cell lines.