Monochorionic, diamniotic twins discordant for acardia are reported. A 775 g acardius was dead, generally hydropic and severely malformed, while the other twin with a weight of 1314 g was phenotypically normal. Both twins had a normal 46 XX karyotype. Genotyping at 24 autosomal polymorphic loci revealed that the twins were identical (monozygotic) and ruled out uniparental disomy for chromosome 2. X inactiva tion studies showed a skewed pattern in the acardiac twin and a normal random pattern in the co twin, suggesting that the underdevelopment of the acardiac twin may have reflected the small number of original cells that were separated from the inner cell mass.
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