Background <p>Only ten patients with both trisomy 21 and normal karyotypes have been documented, including four singleton patients and three sets of twins. Among these twins, two were monochorionic dizygotic: one pair was terminated at 18 weeks, and the other was followed for two years.</p> Case presentation <p>A 38-year-old woman conceived through in vitro fertilization. Ultrasound examinations confirmed monochorionic twins. Standard karyotyping of peripheral blood cells from the twins revealed 47,XY,+21/46,XX chimerism, with a predominance of the 46,XX line. Single nucleotide polymorphism microarray analysis of oral mucosa cells identified a single cell line (46,XX) in the twin girl and a single cell line (47,XY,+21) in the twin boy. Over a decade of follow-up, the twins exhibited normal development, including hormonal values and external genitalia. While the twin boy exhibited a typical Down Syndrome phenotype.</p> Conclusions <p>Monochorionic dizygotic (MCDZ) twins, though rare, can occur in assisted reproductive technology (ART) pregnancies, possibly due to blastocyst fusion or other mechanisms. These cases may be associated with confined blood chimerism (CBC) and monochorionic complications, despite genetic discordance between the twins. MCDZ twinning presents unique diagnostic and management challenges. For monochorionic dizygotic twins, early determination of zygosity and chorionicity is essential. Timely diagnosis, multidisciplinary care—including maternal-fetal medicine, medical genetics, and neonatology—and individualized counseling are critical components of management. Additionally, the mechanisms and long-term effects of intrauterine cell trafficking—especially concerning gonadal development—remain areas requiring further research.</p>