A composite curve of biparietal diameters at various gestational ages was constructed from previous reports from five large centers. Estimates of the mean gestational age using the composite curve compared favorably with other reporting institutions. This suggests that the newly established echography department may employ the composite curve of mean values and need not develop its own biparietal growth curve. The clinical application of these mean values however, should recognize a considerable variance from the mean. These variations represent normal growth differences in the biparietal dimension, errors inherent in the ultrasonic methodology and apparent differences in the populations examined. A method is suggested by which variations from the mean may be identified for each echographic facility.