The concept of normal contralateral kidney growth rate was introduced as a parameter to consider in conjunction with others in the evaluation of unilateral hydronephrosis in asymptomatic neonates and infants. Historically measurements of renal size obtained at different times from different patients have been relied on to construct normal renal growth charts. We report normal kidney growth rates in year 1 of life derived from interval ultrasound measurements of renal size in the same newborns and infants longitudinally with time.

We reviewed 333 normal renal ultrasound studies performed between 1991 and 1995 in 55 patients during year 1 of life. These longitudinal measurements were made bilaterally 2 to 8 times (median 3) per patient. Growth curve models that consider random patient effects were used to estimate the mean growth curve and ascertain its form. We also reviewed an additional 91 ultrasound studies in 10 patients with myelomeningocele without hydronephrosis or reflux. We analyzed the kidney growth rate in a similar manner and compared the results with those in the normal patients.

Analysis of the normal cases revealed a changing growth rate during year 1 of life estimated to be 3.1 mm. per month at birth, decreasing gradually during the first 7 months of life to 0.25 mm. per month and remaining constant thereafter (p = 0.0064). Mean kidney size at birth was estimated to be 42.8 mm. (95% confidence limits 27.0 to 58.5). At age 7 months mean size was 60.7 mm. (95% confidence limits 40.1 to 81.3).

These data demonstrate that normal kidney growth is age dependent. A rapid but slowly decreasing growth rate during the first 7 months of life is followed by a more constant and lower rate. The data also suggest that unusually rapid growth, such as that which may occur in the normal contralateral kidney in unilateral obstruction, may also be age dependent, exceeding 5.0 mm. per month during the first 7 months of life and 2.0 mm. per month thereafter.