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Background: Low birth weight is an important risk factor for hypertension and unfavorable prognoses of a number of renal diseases. It is also associated with reduced kidney size and nephron number. A differentiation between the effects of low birth weight versus being born premature or small for gestational age has, however, not been addressed., Methods: The influence of weight for gestational age (percentage deviation from expected mean), gestational age, birth weight, and early diet on kidney growth was studied in 178 children born pre- or postmature and/or small or large for gestational age, comparing them to 717 mature children, birth weight appropriate for gestational age. Kidney size was determined by bilateral ultrasonography measuring length, width and depth, using the equation of an ellipsoid for volume calculation. The examinations were performed at 0, 3, and 18 months of age together with measurements of body weight, height, and skinfold thickness., Results: Weight for gestational age had a significant, positive effect on combined kidney volume at all three ages (0 months, P < 0.001; 3 months, P < 0.001; and 18 months, P < 0.001). A slight catch-up growth in kidney size was seen in the most growth-retarded infants (<10th percentile) between 0 and 18 months of age (mean Deltaz score(0-18 mo)=+0.22 SD) (P= 0.037). Premature children had smaller kidneys compared to mature at all ages (0 months, P= 0.001; 3 months, P= 0.007; and 18 months, P= 0.042), without any significant catch-up with age. Relative kidney volume was inversely correlated with weight for gestational age at birth (P= 0.007) but positively at 18 months (P= 0.008). Relative kidney growth 0 to 18 months was positively correlated to weight for gestational age (P= 0.013). Low birth weight was associated with impaired relative kidney growth in response to formula feeding., Conclusion: Being small for gestational age is associated with small kidneys at birth and impaired kidney growth in early childhood. The present data suggest that intrauterine growth has a regulatory influence on nephron formation and renal function in humans reaching beyond the neonatal period.

A high protein intake results in increased kidney growth and glomerular filtration rate in human adults and young rats. It is unknown whether kidney size in young infants is influenced by increased protein intake in formula-fed compared with breast-fed infants. We investigated the effect of formula versus breast feeding on kidney growth in a cohort of 631 healthy children examined at birth, and at 3 and 18 months of age. Kidney size was determined by ultrasonography and related to gender, age, body size, and feeding category (fully breast fed, partially breast fed, or fully formula fed at 3 months). Serum urea nitrogen, serum creatinine, and estimated creatinine clearance were measured at 3 months of age. Kidney growth and serum urea nitrogen were significantly increased in partially or fully formula-fed 3-month-old infants. This effect was more pronounced in boys than in girls. The changes in relative kidney size were temporary, as they did not persist at 18 months of age, when all children received a normal mixed diet. The immediate renal effects of formula feeding should be taken into consideration for recommendations concerning infant feeding. Whether there are any long-term effects of early increased protein intake on later kidney function remains to be seen.

Kidney size is an important parameter in the evaluation of children with renal disease. However, reference materials for kidney size in healthy children have been limited beyond the neonatal period. We performed a longitudinal cohort study of 717 healthy children born at term with normal birth weight. Kidney size and shape were determined by ultrasonography and related to gender, age, and body size (weight, length, body surface area, skinfold thickness) at 0, 3, and 18 months of age. Gender-differentiated reference charts were established. Boys had significantly larger kidney volumes than girls ( P<0.001) and larger relative volumes (kidney volume/weight) at 0 and 3 months ( P<0.001), but not at 18 months of age. The best single predictor of gender-differentiated kidney volume was weight. Relative kidney volume changed with increasing age and height in a two-phase pattern: an initial decrease until a height of 65-70 cm was reached followed by a stable level. In conclusion, kidney size was significantly influenced by gender, age, and body composition. Relative kidney volume decreased with increasing age and height in a two-phase pattern. These characteristic changes in kidney volume indicated that infant kidney growth might be influenced by sex steroids and growth hormone in addition to body composition.