Prenatal diagnosis of renal developmental anomalies associated with an empty renal fossa

Fetal kidneys are evaluated on routine obstetric ultrasounds; therefore, fetal renal developmental anomalies are frequently encountered with prenatal screening. Absence of the fetal kidney from its expected location generates a differential diagnosis that originates from the knowledge of the developmental origins of the kidneys. Many of the renal developmental anomalies demonstrate overlapping features in both symptoms and imaging appearance. We illustrate the pertinent findings of selected fetal renal anomalies in the setting of a unilateral empty renal fossa and suggest an algorithm for differentiating between the diagnoses. Discussed anomalies include horseshoe kidney, pelvic kidney, crossed fused renal ectopia, and renal agenesis.