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Head and abdominal circumference measurements of 166 fetuses (15-40 weeks) were made by three different examiners using two measurement techniques. In method 1 the actual circumferences were measured using an electronic digitizer, while in method 2 the circumference measurements were estimated using the formula for calculating the circumference for an ellipse. Evaluation of the entire data set as a whole demonstrated no significant mean errors (P > 0.05), and only a small degree of variability (2 SO = 6 per cent).Evaluation of the data for each examiner demonstrated no significant differences (P > 0.05) in the observed variability, but small statistically significant mean errors (1.7-4.1 per cent) were observed. The latter finding is thought not to be a limitation of the ellipse approximation technique, since similar findings have been observed when experienced sonographers make actual circumference measurements on the same fetus using the same equipment. (Key words: abdominal circumference; ellipse approximation technique; fetus; head circumference; measurement.) Sonographic methods for assessing the normalcy of fetal growth frequently utilize circumference measurements of the fetal head and abdomen. Reported values for these measurements at various points in gestation 1 -4 are based on measurements that were made with a hand-held map measurer (e.g., Dietzgen) or a more expensive electronic digital planimeter (e.g., Numonics Corporation). Some reports have suggested that satisfactory estimates of these measurements can be made using the formula for calculating the circumference of an ellipse; circumference = {short axis (d 1 } + long axis (d 2 )] x 1.57. However, there are few data to s upport this contention. 5 -7 The purpose of this investigation was to determine whether accurate measurements of the fetal head and abdominal circumferences can be obtained using this technique. MATERIALS AND METHODSSatisfactory images for measurements of the fetal head circumference and abdominal circumference were obtained on 166 fetuses (15-40 weeks) using methods and criteria previous ly described. 3 All examinations were performed by physicians using a commercially available linear array real-time ultrasound system (ADR, Model 2130). The images were recorded on Polaroid film using a minification factor of2.74 to 1. The two diameter measurements were made from outer margin to outer margin using hand-held calipers. For the head, the biparietal axis and occipital frontal axis were used as the two diameter measurements. For the abdo· men, the shortest and longest axes passing through the mid-abdomen were used as the two diameters. The next step in the procedure involved measuring the perimeters of the fetal head and abdomen images using a Numonics electronic digitizer (Model 1250-l). The minification fitctor (2.74 to I) was incorporated into the electronic digitizer, so that measurements were obtained in actual centimeters using this device. The last step involved converting the two diameters for both head and body to actual c...