The time-dependent changes in the antero-posterior (A-P) diameter and length (L) of fetal kidneys has been evaluated by means of statistical analysis of measurements made during the third trimester in a normal population. Two hundred eighty pregnant women were examined between 22 and 40 weeks of gestation. Differences between operator-scanner combinations were evaluated. The relationships of the A-P diameter and length to menstrual age were determined in cross-sectional and longitudinal studies of kidney growth. The results obtained confirmed that measurements of fetal kidney can be used as an additional parameter in the routine assessment of fetal well being and to rule out kidney malformations characterized by changes in kidney size.