BACKGROUND: Chromosomal abnormality in one fetus of a monozygotic twin pregnancy is rare, and discussion of prenatal detection of such a case offers some insight into this clinical problem. CASE: A 28-year-old gravida 1 had ultrasound screening at 11 weeks of gestation that revealed a monochorionic, diamniotic twin pregnancy with increased nuchal translucency (7.7 mm) in one fetus. Subsequent evaluation showed one 45,XY,- 21 karyotype and one normal male karyotype. The pregnancy was monozygous by DNA analysis. The affected neonate died 5 minutes after delivery of both twins by cesarean. CONCLUSION: Monochorionic twins discordant for fetal abnormalities can be evaluated with molecular analysis. Study of such cases may reveal the extent to which an early diagnosis can lead to therapeutic interventions to support survival of the viable twin.
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