[4] It is defined as abnormally small kidneys, where the size is less than two standard deviations below the expected mean for the corresponding demographics, and the morphology is normal.

[4][1] The severity of the disease depends on whether hypoplasia is unilateral or bilateral, and the degree of reduction in the number of nephrons.

The classification of renal hypoplasia establishes four types, termed simple, oligomeganephronic, segmental, and cortical.

[6][additional citation(s) needed] Oligomeganephronia (or oligomeganephronic hypoplasia) is a rare pediatric renal disease where the number of nephrons is reduced but is distinctly enlarged.

The number of renal lobes is reduced, and the kidney size is less than two standard deviations from the average, with the weight often being over 50g in adults and 12–25g in children.

Vesicoureteral reflux and urinary tract infections are commonly present, while cysts, dysplasia, and proteinuria are absent.

The surface of the kidney has a pelvic recess and one or more transverse cortical grooves, presumed to be a failed formation of a renal lobe.

The renal parenchyma next to the hypoplastic part of the kidney is seemingly normal; however, it can undergo hypertrophy, making the grooves more noticeable.

On the other hand, the medullary rays and renal pyramids have decreased in size with the radial glomerular count reduced to eight or less.

However, environmental factors like maternal diseases such as diabetes, hypertension, and intoxication via smoking and alcohol are linked to renal hypoplasia.