To develop an improved means for evaluation of the outcome of fetal growth in utero, a growth study of 37 fetuses at risk for intrauterine growth retardation was undertaken. Intrauterine growth was evaluated with ultrasonography at 2- to 3-week intervals beginning at 15 weeks' gestation. Measurements obtained in the second trimester were used to specify Rossavik growth models for various anatomic parameters. Values for weight, head circumference, abdominal circumference, and thigh circumference at birth were predicted with the use of these models. Growth potential realization index values for these four parameters were calculated from actual and predicted birth measurements. The growth potential realization index values were combined to form a neonatal growth assessment score. With the use of cluster analysis and neonatal growth assessment score values, the 37 infants were separated into 24 group I (neonatal growth assessment score values between 3.7 and 18.6) and 13 group II (neonatal growth assessment score values between 19.4 and 50.0) infants. In group I, 66.7% of the infants had no growth abnormalities; a single abnormality, usually borderline, was found in an additional 20.8% (total = 87.5%). In group II all infants had growth abnormalities (range, 2 to 6) although only 46.2% had birth weights below the 10th percentile. No differences in risk factors (except for the incidence of smoking) or second trimester growth were found in these two groups. The neonatal growth assessment score appears to be a sensitive indicator of third trimester growth retardation, is not affected by differences in growth potential, can separate normal infants from those with evidence of intrauterine growth retardation, and provides a quantitative assessment of growth problems in individual fetuses.