Natural history of fetal and neonatal hydronephrosis.

Since pyelocalicectasis alone is common in fetuses, we reviewed reports of fetal hydronephrosis that resolved spontaneously or at birth. Severe fetal hydronephrosis with calicectasis or parenchymal thinning rarely resolves spontaneously before or after birth. We also reviewed the clinical and experimental literature on renal hypertrophy. After unilateral nephrectomy in neonatal animals or after birth with congenital absence of one kidney in humans, the remaining kidney hypertrophies very quickly. In infants and young animals, the eventual size of the remaining kidney is inversely proportional to the age at which one kidney is lost. This improvement in residual renal function seen after renal loss in infancy, compared with older children, itself constitutes a strong argument for early relief of obstruction. If contralateral renal hypertrophy has occurred, the treated damaged kidney may resume growth in parallel with its hypertrophied mate but does not become as large or recover normal potential for growth. In other words, if correction of a unilateral obstruction is deferred until contralateral hypertrophy occurs, the obstructed kidney then has less potential for recovery of function.