We present 2 cases in which the diagnosis of a unilateral nonfunctioning kidney was not anticipated. The first case appeared to be simple, antenatally diagnosed, unilateral hydronephrosis with adequate parenchyma. However, at 3 months postnatally, it was found to be nonfunctional, most likely secondary to early high-grade obstruction. The second case presented prenatally as a left hydronephrotic kidney with parenchymal cysts, and the right kidney showed only mild parenchymal cystic changes. By 6 months, however, the right kidney was nonfunctional. These cases emphasize the need for coordinated prenatal and postnatal care. The sonographic appearance of renal dysplasia is variable (in size, number of cysts, and degree of hydronephrosis), depending on the point at which it is evaluated during the disease course. Subtle signs of dysplasia, such as increased echogenicity, may be the only warning sign of a nonfunctioning kidney, for which one should have a high index of suspicion.
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