Sex‐Discordant Monochorionic Dizygotic Twins After Double Blastocyst Transfer: Blood Chimerism Demonstrated by Multimodal Genetic Testing

A 38-year-old woman conceived after intracytoplasmic sperm injection and frozen-thawed double blastocyst transfer. First-trimester ultrasonography suggested monochorionic diamniotic twins, but discordant external genitalia were identified at 21 weeks of gestation. Delivery at 37 weeks resulted in phenotypically normal female and male infants. Placental histopathology confirmed monochorionic diamniotic placentation. Postnatal multimodal genetic evaluation, including G-banding, single nucleotide polymorphism array, and buccal cell X/Y fluorescence in situ hybridization, demonstrated mixed 46,XX/46,XY cell lines in peripheral blood, consistent with predominantly blood-limited chimerism. Single nucleotide polymorphism array analysis demonstrated admixture of two independently derived genomes and findings inconsistent with a primary dispermic mechanism. These results supported a diagnosis of monochorionic dizygotic twinning with predominantly blood chimerism, most likely reflecting intertwin cell exchange. This case highlights the value of integrating tissue-specific genetic findings with postnatal phenotypic and clinical assessment to interpret predominantly blood-limited chimerism in MCDZ twins.