Renal hypoplasia is a congenital abnormality in which one or both of the kidneys are smaller than normal, resulting in a reduced nephron number but with normal morphology. 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. The severity of the disease depends on whether hypoplasia is unilateral or bilateral, and the degree of reduction in the number of nephrons..