Single aberrant umbilical artery in a fetus with severe caudal defects: sirenomelia or caudal dysgenesis.

We describe a 1,000-g twin fetus with absent kidneys and ureters, anal atresia and minimal evidence of external genitalia, and hypoplastic lower limbs with absent feet. A postmortem arteriogram showed a large single umbilical artery in direct continuation with the abdominal aorta, a unique anomaly almost always related to sirenomelia. We discuss the possible diagnosis of this case as sirenomelia or caudal dysgenesis, and the controversy as to whether they are two related or separate entities.