Background Nephron number in humans is determined during fetal life. The objective of this study was to investigate the effects of preterm birth on nephron number using renal volume as a surrogate for nephron number. Methods This observational study was conducted over 12 months in a tertiary perinatal center. Preterm babies less than 32 weeks of gestation were recruited and followed until discharge. Term infants were recruited for comparison. The babies underwent renal sonography and renal function measurements at 32 and 38 weeks corrected age. The primary outcome measurement was total kidney volume at 38 weeks and the secondary outcome was estimated glomerular filtration rate (eGFR). Results Forty-four preterm infants and 24 term infants were recruited. At 38 weeks corrected age, premature infants had lower total kidney volume than term infants (21.6 ± 5.7 vs. 25.2 ± 5.7 ml; p = 0.02) and a significantly lower eGFR (73.6 [IQR 68.1-77.6] vs. 79.3 [IQR 72.5-86.6] ml·min−1·1.73 m−2; p = 0.03). There was a significant correlation between total kidney volume and eGFR in premature and term babies. Conclusions Premature infants have smaller kidney volume and likely decreased nephron number and lower estimated glomerulofiltration rate relative to infants born at term.