Three-Dimensional Ultrasound-Assessed Fetal Thigh Volumetry in Predicting Birth Weight FONG-MING CHANG, MD, REN-ING LIANG, MD, HUEI-CHEN KO, PhD, BOR-LIN YAO, MD, CHIUNG-HSIN CHANG, MD, AND CHEN-HSIANG YU, MD Objective: To compare the accuracy of three-dimensional ultrasound-assessed fetal thigh volumetry in predicting birth weight with that of other commonly used formulas composed of biparietal diameter (BPD), abdominal circum- ference (AC), and femur length (FL) by two-dimensional ultrasound. Methods: We assessed the thigh volume of 100 fetuses using three-dimensional ultrasound. Meanwhile, their BPD, AC, and FL were measured by two-dimensional ultrasound. All infants were delivered within 48 hours after the ultra- sound examinations. From polynomial regression analysis, we generated a best-fit formula for the thigh volume to predict birth weight. The accuracy of this thigh-volume formula was compared with those of three formulas com- monly used in the United States. In addition, another group of 50 fetuses was measured for prospective validation. Results: The thigh volume assessed by three-dimensional ultrasound was highly correlated with birth weight (r  0.89, n  100, P < .0001). The best-fit formula for thigh volume to predict birth weight was linear, and it was superior to the other commonly used two-dimensional formulas in predict- ing birth weight. The predicting error (0 g), percent error (0.7%), absolute error (176.1 g), and absolute percent error (5.8%) of the thigh-volume formula were all smaller than those of the other formulas (n  100, all P < .05). In addition, the thigh-volume formula predicted birth weight more ac- curately than the other two-dimensional formulas in the prospective-validation group. The three-dimensional for- mula had smaller mean values of predicting error (38.6 g), percent error (1.5%), absolute error (160.0 g), and absolute percent error (5.1%) than the two-dimensional formulas (n  50, all P < .001), as well as the smallest variances of the above errors (178.1 g, 5.6%, 84.3 g, and 2.9%, respectively). Conclusion: The three-dimensional ultrasound-assessed thigh volume has better accuracy in predicting birth weight than the commonly used formulas by two-dimensional ul- trasound, and it may improve fetal weight prediction in clinical practice. However, a large-scale prospective valida- tion study may be needed to confirm our conclusions. (Obstet Gynecol 1997;90:331–9. © 1997 by The American College of Obstetricians and Gynecologists.) Fetal weight estimation is one of the major tasks in modern prenatal care. Accurate assessment may be related to critical points of decision making and proper management. To date, ultrasound is the main diagnos- tic tool for fetal weight evaluation. 1 With computer- assisted analysis, many fetal weight estimating formu- las, consisting of common fetal growth indices such as biparietal diameter (BPD), abdominal circumference (AC), and femur length (FL), assessed by two- dimensional ultrasound, have been used as a routine examination. 2–6 However, the accuracy in predicting fetal weight remains to be improved. 1 Although fetal limb volume has been proposed to be associated with fetal growth and nutrition, 7 many re- searchers have attempted to use limb circumference to predict fetal weight, yet their results have been unsat- isfactory. 7–11 With the erroneous assumption that the fetal thigh is a cylinder, 7 or using the thigh circumfer- ence indirectly to replace the real thigh volume, 8 –11 inaccurate results could be expected. With the advent of three-dimensional ultrasound, we showed its primary use in obstetrics in 1991. 12 In addition, the accuracy of three-dimensional ultrasound in volumetry has been validated extensively in many organ systems, either in vitro or in vivo. 13–20 Riccabona et al 20 concluded that three-dimensional ultrasound provides accurate volume measurements of regular and irregular objects and can offer improved accuracy com- pared with two-dimensional methods. Our initial stud- ies evaluating fetal heart and liver volume also proved the better precision of three-dimensional volumetry. 18,19 Hence, the accurate assessment of limb volume be- comes feasible using three-dimensional ultrasound From the Departments of Obstetrics and Gynecology and Public Health, National Cheng Kung University Medical College and Hospital, Tainan, Taiwan. This study is supported in part by a grant from the National Science Council, Taiwan, Republic of China. 331 VOL. 90, NO. 3, SEPTEMBER 1997 0029-7844/97/$17.00 PII S0029-7844(97)00280-9