Monochorionic dizygotic twins with discordant genetic findings and congenital malformations.

OBJECTIVE: To describe prenatal evaluation and postnatal outcome of monochorionic diamniotic twins with discordant anomalies and genetic findings after intracytoplasmic sperm injection and double-embryo transfer. METHODS: A 36-year-old gravida 3 para 0 woman conceived after transfer of two blastocysts. Ultrasound identified monochorionic diamniotic twins and a liver-containing omphalocele in fetus B. Both sacs underwent separate amniocentesis. Testing included karyotyping, chromosome microarray, Beckwith-Wiedemann syndrome analysis, trio whole-exome sequencing, short tandem repeat analysis, postnatal peripheral blood fluorescence in situ hybridization, and placental histopathology. RESULTS: Fetus A had mosaic 45,X[9]/46,XY[91]; fetus B had a 46,XY karyotype. Other prenatal genetic tests were unremarkable. Discordance at 11 of 21 short tandem repeat loci supported dizygosity. Following prelabor rupture of membranes at 34 + 5 weeks, two male infants were delivered by cesarean section. Twin A had hypospadias and bilateral cryptorchidism; twin B had giant omphalocele requiring surgical repair. Postnatal fluorescence in situ hybridization detected no sex-chromosome mosaicism. Placental histopathology confirmed monochorionicity. CONCLUSION: Monochorionicity does not exclude dizygosity after assisted reproduction. Separate sampling of both sacs may inform discordant cases, and prenatal-postnatal discrepancies in 45,X/46,XY mosaicism warrant cautious longitudinal interpretation.