A monozygotic twin pregnancy discordant for acardia and X-inactivation pattern.

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-inactivation 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.