Prenatal Diagnosis and Individualized Perinatal Care in Apparently Nonmosaic Trisomy 9.

Background: Nonmosaic Trisomy 9 is a rare chromosomal abnormality, accounting for approximately 2.2%-2.7% of spontaneous abortions caused by autosomal trisomies, and is characterized by a heterogeneous multisystem phenotype and an extremely poor prognosis. Prenatal imaging may demonstrate severe multisystem abnormalities; however, these findings are not specific to Trisomy 9, making cytogenetic testing essential for definitive diagnosis. We report a case of prenatally diagnosed apparently nonmosaic Trisomy 9 in which diagnosis enabled individualized perinatal care. Case: A 44-year-old multiparous woman was referred to our hospital at 25 weeks' gestation because of severe fetal growth restriction and multiple structural anomalies. Amniocentesis at 28 weeks' gestation demonstrated 47,XY,+9 in all 100 examined colonies, consistent with apparently nonmosaic Trisomy 9. Following multidisciplinary counseling and shared decision-making, an individualized birth plan was developed. The family wished to avoid aggressive life-prolonging interventions while prioritizing the opportunity to meet their infant alive; therefore, fetal monitoring, cesarean delivery for fetal indications, and mask ventilation without further resuscitation were incorporated into the plan. Emergency cesarean delivery was performed at 36 weeks and 2 days for nonreassuring fetal status. The infant died approximately 9 h after birth while receiving family-centered palliative care. Conclusion: This case highlights the clinical importance of definitive prenatal cytogenetic diagnosis in apparently nonmosaic Trisomy 9, enabling phenotype-informed prognostic counseling and individualized perinatal management.