Monochorionic Twin Discordance for Horseshoe Lung and Tricuspid Atresia.

The horseshoe lung is a congenital malformation in which the bases of the right and the left lung are fused. We describe a monochorionic twin gestation with malformation discordance. The abnormal twin had a horseshoe lung with hypoplasia of the right lung, tricuspid atresia, cleft lip, and a pelvic right kidney. The discordance of anomalies in this monochorionic twin suggests that a postzygotic mutation, epigenetic change, or environmental factors may be responsible for these malformations.