The ratio between sonographically measured fetal femur length to abdominal circumference (F/A) was studied in 471 consecutive normal singleton pregnancies from 18 to 40 weeks' gestation. The 90th (0.2715) percentile of this ratio was evaluated as a predictor for intrauterine growth retardation (IUGR) in an additional 1,502 pregnant women; 146 fetuses were found to be above the 90th percentile, of which 20 were born growth retarded. A total of 69 fetuses was found post partum to be small for date; 44 were asymmetrically growth retarded and 25 were symmetrically growth retarded. Of the 20 growth-retarded fetuses, which were correctly diagnosed in utero, 19 were asymmetrically growth retarded. The additional 49 growth-retarded newborns were not detected using the F/A ratio as a diagnostic parameter. Low sensitivity (29.98%) together with low positive predictive value (13.69%) rendered the parameter unsuitable for screening IUGR. Nevertheless, the high specificity (91.20%), together with the high negative predictive value (96.38%), makes it a useful age-independent parameter in high risk pregnancies, where asymmetric IUGR is suspected.