Bladder obstruction in monochorionic monoamniotic twins

Posterior urethral valves (PUVs) may lead to complete urinary tract obstruction and profound oligohydramnios. This typically results in lethal pulmonary hypoplasia. We present a case of monoamniotic male twins discordant for urinary tract obstruction resulting from a PUV. After delivery, the twin with the PUV had no evidence of pulmonary hypoplasia. Previous reports of diamniotic twins discordant for complete urinary tract obstruction have described universal mortality from lung hypoplasia in the twin with the obstruction. We propose that, in our case, the normal amniotic fluid in the shared sac was protective of lung development in the affected fetus. This case may provide powerful insight into the pathophysiologic process of lung hypoplasia because it supports the concept that the lethal lung hypoplasia that occurs in PUVs results from oligohydramnios due to reduced fetal urinary production.