Monochorionic twins discordant for mosaic trisomy 14.

Monochorionic-diamniotic twins are usually monozygotic twins and known to be associated with adverse obstetric and perinatal outcomes. Cases of discordant karyotype of monozygotic twins are rare and most involves sex chromosomes. We present the first case of monochorionic twins with discordant karyotype manifested as mosaic trisomy 14 in one twin (B) and a normal karyotype in the other (A). We describe the postmortem pathological and imaging findings of the trisomic twin and for the first time neuropathological findings of this entity. Metaphase chromosome analysis of twin B using fetal tissue showed a 47,XX, +14 karyotype. Chromosomal microarray analysis (CMA) using fetal tissue revealed 38% mosaicism. CMA with placental tissue from both sides demonstrated normal karyotype and confirmed monozygosity, highlighting the value of array based testing on diagnosing mosaicism and zygosity.