Estimation of Fetal Weight: Mean Value from Multiple Formulas

Estimation of Fetal Weight: Mean Value from Multiple Formulas Michael G. Pinette, MD, Yuqun Pan, MD, Sheila G. Pinette, RPA-C, Jacquelyn Blackstone, DO, John Garrett, Angelina Cartin Mean fetal weight value from multiple formulas was compared to fetal weight from single formulas. Data were collected on 975 fetuses who had estimation of fetal weight by ultrasonography within 1 week before birth. Improvement in estimation of fetal weight occurred using either the mean value of multiple formulas or the Hadlock BPD/FL/AC, in comparison to fetal volume, BPD/AC, or FL/AC. BPD/FL/AC appeared to provide the best estimate of true weight in terms of overall accuracy and in terms of not showing a trend in either overestimating or underestimating true weight. KEY WORDS: Weight, fetal, estimation; Fetus, weight; Formulas, fetal weight estimation. ABBREVIATIONS BPD, Biparietal diameter; AC, Abdominal circumference; FL, Femur length; EFW, Estimated fetal weight Received May 28, 1999, from the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Maine Medical Center, Scarborough, Maine. Revised manuscript accepted for publication September 5, 1999. Address correspondence and reprint requests to Michael G. Pinette MD, Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Maine Medical Center, 482 Payne Road, Scarborough, ME 04074. Avariety of formulas and parameters have been correlated with fetal weight. 1 4 Among them, the Shepard formula, which includes BPD and AC, 5 and the Hadlock formula using FL and AC 6 are widely accepted and commonly used for estimation of fetal weight. These parameters are considered to be more accurate and simpler than others. 7 9 The results from combining all three parameters (BPD, FL, and AC) for predicting fetal weight appear to be controversial in the literature. Hadlock and coworkers 10 and Rose and McCallum 11 found that combining all three of these parameters produced more accurate results than the use of only two parameters, but Woo and Wan 12 conversely found no improvement in predictive accuracy over that of formulas using two parameters. Although the Shepard and Hadlock formulas correlate well with fetal weight, significant errors in estimation still occur and have been attributed to biologic variation (difference of somatic types), pathologic variations (fetal dystrophy, hypertrophy, malformations), subjective operator errors of measurement, or any combination of these. Recently, a volume based model was reported to be superior to the biometric based formulas. 13 1999 by the American Institute of Ultrasound in Medicine J Ultrasound Med 18:813 817, 1999 0278-4297/99/$3.50

814 FETAL WEIGHT ESTIMATION J Ultrasound Med 18:813 817, 1999 This study was designed to test the hypothesis that application of the mean value of multiple formulas for predicting fetal weight may improve the predictive accuracy by decreasing the influence of biologic and pathologic variations. The Shepard BPD/AC, Hadlock FL/AC, and BPD/FL/AC models were chosen in the study because they are commonly used. The recently published fetal volume model of Combs and associates 13 also was chosen because of its reported superiority. These four models are representative of the models published in the literature. SUBJECTS AND METHODS We analyzed data on 975 fetuses who were born from January 1991 to August 1995 and who had had ultrasonographic examinations for EFW within 1 week before birth. Fetuses with congenital malformations were excluded. Sonographic evaluation was performed using any of the following machines: ATL-HDI Ultramark 9 or ATL Ultramark 9 (Advanced Technology Laboratories, Bothell, WA) or Acuson 128 (Acuson, Mountain View, CA). EFW was based on the Shepard BPD/AC formula, 5 the Hadlock FL/AC and BPD/FL/AC formulas, 6 the Combs formula using fetal volume, 13 and the fifth formula using mean EFW from the previous four formulas (multiple mean) (Table 1). The deviations between EFW and actual birth weight were considered as the estimating error, which was calculated on the basis of the following formula: Absolute Percentage Error = [(EFW Birth weight)/birth weight] 100 Mean absolute percentage errors were calculated and compared among all the formulas. Mean differences between EFW and actual birth weight for the formulas were compared. Statistical analysis was performed using a paired t-test provided by the StatView computer program (Abacus Concepts, Inc., Berkeley, CA). The fetuses were then divided into four groups on the basis of their actual weights in order to observe any possible effect of weight categories on estimating error. The four groups were birth weights of 500 g and 999 g (71 fetuses); 1000 g and 1999 g (184 fetuses); 2000 g and 3499 g (464 fetuses); and 3500 g (244 fetuses). The estimating errors were compared for each of the different fetal weight groups, using all of the five methods. A sign test for trend was performed. RESULTS Gestational age of the 975 fetuses at the time of delivery ranged from 20.3 to 42.0 weeks. Birth weights ranged from 260 g to 5368 g. All told, the new model using the mean value from multiple formulas and BPD/FL/AC showed statistically significantly improved accuracy with mean absolute percentage errors of 8.5% and 8.6%, respectively (Table 2). The Combs volume model had a mean error of 9.5%, the Shepard BPD/FL model showed a mean error of 9.7%, and the Hadlock FL/AC and BPD/FL/AC models resulted in mean errors of 9.1% and 8.7%, respectively. The volume based model, recently reported to have improvement in accuracy for EFW, 13 and BPD/AC were the least accurate overall. Table 1: Formulas Shepard BPD/AC (formula 1): Log 10 birth weight = 1.7492 + 0.166 (BPD) + 0.046 (AC) 2.646 (AC BPD) 1000 Hadlock FL/AC (formula 2): Log 10 birth weight = 1.3598 + 0.051 (AC) + 0.1844 (FL) 0.0037 (AC FL) Hadlock BPD/FL/AC (formula 3): Log 10 birth weight = 1.4787 + 0.001837 (BPD) 2 + 0.0458 (AC) + 0.158 (FL) 0.003343 (AC FL) Combs fetal volume (formula 4): EFW = (0.23718 AC 2 FL) + (0.03312 HC 3 ) Pinette mean fetal weight: Birth weight = Formula 1 + Formula 2 + Formula 3 + Formula 4 4

J Ultrasound Med 18:813 817, 1999 PINETTE ET AL 815 Table 2: Mean Absolute Percentage Errors: All Patients Multiple 8.5 7.4 -NS BPD/FL/AC 8.6 7.5 -P = 0.005* FL/AC 9.0 7.9 -P = 0.0121* Volume 9.5 8.4 -NS BPD/AC 9.7 8.3 In terms of the specific weight categories, interesting differences were noted. In the 500 999 g group (Table 3), BPD/FL/AC tended to be most accurate; however, no significant difference occurred between BPD/FL/AC, FL/AC, multiple formulas, and BPD/AC. Volume estimate was significantly less accurate. In fetuses weighing 1000 1999 g (Table 4), BPD/FL/AC tended to be most accurate; however, no significant difference was found between BPD/FL/AC, multiple formulas, and BPD/AC. BPD/FL/AC, when compared with FL/AC and BPD/AC, was significantly better for EFW. Once again the volume model was significantly less accurate. With fetuses weighing 2000 3499 g (Table 5), volume tended to be the best model of estimation; however, no significant difference occurred when compared to multiple formulas. BPD/FL/AC and FL/AC were significantly less accurate, and BPD/AC was significantly the least accurate model. In fetuses weighing 3500 g or more (Table 6), multiple formulas were significantly the most accurate predictor of fetal weight. BPD/FL/AC, FL/AC, volume, and BPD/AC were significantly less accurate to the same degree. The results of the sign test, each patient counting as +1 (if EFW is overestimated) or 1 (if EFW is underestimated), are reported in Table 7. As a whole, volume, FL/AC, and mean weight showed a significant trend toward overestimation of fetal weight. Likewise, BPD/AC and BPD/FL/AC did not show a trend to overestimate or underestimate fetal weight. BPD/FL/AC appeared to be best with regard to not showing a trend in either overestimating or underestimating fetal weight. In the data set taken as a whole (i.e., all patients), mean differences in grams from actual birth weights were similar, although FL/AC significantly underestimates and BPD/AC significantly overestimates the actual birth weight (Table 8). Looking at mean differences in specific weight categories, it became very apparent that significant interesting differences existed between weight categories. When looking at mean differences from birth weight, all formulas underestimate fetal weight in fetuses with birth weights under 1000 g (Table 9). For birth weights of 1000 1999 g (Table 10), the EFW tended to be underestimated, with the FL/AC, multiple formulas, and volume methods reaching statistical significance. Most methods, except FL/AC, showed no trend in overestimating or underestimating fetal weight in the 2000 3499 g group (Table 11). All methods except FL/AC tended to significantly overestimate fetal weight in the 3500 g or greater group (Table 12). DISCUSSION Many different formulas have been published in the literature for EFW, and various studies have been done to evaluate the accuracy of these formulas. 14 16 The meta-analysis performed by Medchill and coworkers 14 in 1991 with 20 published formulas for low birth weight fetuses, found no significant difference in accuracy. Combs and colleagues model Table 3: Mean Absolute Percent Error: 500 999g BPD/FL/AC 11.4 8.5 -NS FL/AC 11.8 9.0 -NS -NS Multiple 12.0 8.8 -NS BPD/AC 12.7 9.7 -P = 0.004* Volume 16.2 11.1 Table 4: Mean Absolute Percentage Error: 1000 1999g BPD/FL/AC 8.9 8.1 -NS Multiple 9.4 8.5 -NS -P = 0.002* FL/AC 9.8 9.1 -NS BPD/AC 10.4 9.3 -P = 0.002* Volume 12.5 10.3

816 FETAL WEIGHT ESTIMATION J Ultrasound Med 18:813 817, 1999 using fetal volume, which was reported recently as being accurate in a wide range of fetal weight, 13 was not investigated previously. Theoretically, an average of values predicted by different methods should yield the most accurate estimation of fetal weight by balancing each formula s limitation. The mean value of multiple formulas (Shepard BPD/AC, Hadlock FL/AC and BPD/FL/AC, and Combs fetal volume) in this study Table 5: Mean Absolute Percentage Error: 2000 3499g Volume 7.7 6.8 -NS Multiple 8.0 6.9 -P = 0.01* BPD/FL/AC 8.2 7.2 -NS FL/AC 8.3 7.4 -P = 0.008* BPD/AC 9.4 8.2 Table 6: Mean Absolute Percentage Error: 3500g Multiple 6.7 7.8 -P = 0.01* BPD/FL/AC 6.9 8.0 -NS FL/AC 7.1 8.5 -NS Volume 6.9 8.5 -NS BPD/AC 7.1 8.6 Table 7: Sign Test Volume BPD/AC FL/AC BPD/AC/FL Mean + 539 474 554 482 518 435 500 419 493 455 Difference +104 26 +125 11 +63 n 974 974 973 975 973 P <0.001 0.13 <0.001 0.26 <0.001 Table 8: Mean Difference from Actual Birth Weight: All Patients Multiple +8.1 NS -NS Volume +15 NS -NS BPD/FL/AC +17.6 NS -P = 0.0001* FL/AC 27.7 0.008* -P = 0.0001* BPD/AC +27.7 0.0112* appeared statistically superior to the BPD/AC, FL/AC, and fetal volume. However, altogether the multiple formulas were no better than BPD/FL/AC alone in predicting EFW. Table 9: Mean Difference from Actual Birth Weight: 500 999g Table 10: Mean Difference from Actual Birth Weight: 1000 1999g Table 11: Mean Difference from Actual Birth Weight: 2000 3499g Table 12: Mean Difference from Actual Birth Weight: 3500g FL/AC +26.0 NS -NS BPD/FL/AC +99.4 0.0002* -NS BPD/AC +112.8 0.0001* -NS Multiple +114.2 0.0001* -P = 0.0002* Volume +22.9 0.001* BPD/AC 31.1 0.02* -P = 0.02* BPD/FL/AC 49.1 0.0001* -NS FL/AC 52.1 0.0001* -NS Multiple 58.0 0.0001* -P = 0.0001* Volume 99.8 0.0001* BPD/FL/AC 23.6 NS -NS BPD/AC 29.6 NS -NS FL/AC 47.2 0.0013* -P = 0.03* Multiple 59.0 0.0001* -P = 0.0001* Volume 135.3 0.0001* BPD/FL/AC +2.9 NS -P = 0.001* Multiple 9.7 NS -P = 0.001* BPD/AC +15.9 NS -P = 0.0003* Volume 18.6 NS -P = 0.0008* FL/AC 44.0 0.003*

J Ultrasound Med 18:813 817, 1999 PINETTE ET AL 817 All formulas tested appeared to significantly underestimate fetal weight in the 500 999 g group on the average of 30 to 100 g. If viability is being judged on the basis of weight, this factor should be taken into account. In terms of predicting the weight of fetuses over 3500 g, the EFW formulas tested appeared to significantly overestimate fetal weight by 100 to 200 g on average, except for FL/AC. This difference may not be clinically significant since it is relatively small. In general, in terms of trends as well as accuracy of true weight, the BPD/FL/AC appeared to be the best estimate of fetal weight. REFERENCES 1. Ianniruberto A, Gibbons, JM: Prediction of fetal weight by ultrasonic B-scan cephalometry: An improved technique with disappointing results. Obstet Gynecol 37:689, 1971 2. Thompson HE, Makowski EL: Birth weight of fetal gestational age. Obstet Gynecol 37:44, 1971 3. Suzuki K, Minei LJ, Schnitzer LE, et al: Ultrasonography measurement of fetal heart volume for estimation of birth weight. Obstet Gynecol 43:867, 1974 4. Campbell S, Wilkin D: Ultrasonic measurement of fetal abdominal circumference in estimation of fetal weight. Br J Obstet Gynecol 82:689, 1975 5. Shepard MJ, Richards VA, Berkowitz RL, et al: An evaluation of two equations for predicting fetal weight by ultrasound. Am J Obstet Gynecol 142:47, 1982 6. Hadlock FP, Harrist RB, Carpenter RJ, et al: Sonographic estimation of fetal weight: The value of femur length in addition to head and abdominal measurements. Radiology 150:535, 1984 7. Sampson MB, Thomason JL, Kelly SL, et al: Prediction of intrauterine fetal weight using real-time ultrasound. Am J Obstet Gynecol 142:554, 1982 8. Timor-Tritsch JE, Itskovitz J, Brandes JM: Estimation of fetal weight by real-time sonography. Obstet Gynecol 57:653, 1981 9. Ott WJ: Clinical application of fetal weight determination by real-time ultrasound measurements. Obstet Gynecol 57:758, 1981 10. Hadlock FP, Harrist RB, Sharman RS: Estimation of fetal weight with the use of head, body and femur measurements: A prospective study. Am J Obstet Gynecol 153:333, 1985 11. Rose BI, McCallum WD: A simplified method for estimating fetal weight using ultrasound measurements. Obstet Gynecol 69:671, 1987 12. Woo JSK, Wan MCW: An evaluation of fetal weight prediction using a simple equation containing the fetal femur length. J Ultrasound Med 5:453, 1986 13. Combs AC, Jaekle RK, Rosenn B, et al: Sonographic estimation of fetal weight based on a model of fetal volume. Obstet Gynecol 82:365, 1993 14. Medchill MT, Peterson CM, Kreinick C, et al: Prediction of estimated fetal weight in extremely low birth weight neonates (500 1000 g). Obstet Gynecol 78:286, 1991 15. Pielet B, Sabbagha RE, MacGregor SN, et al: Ultrasonic prediction of birth weight in preterm fetuses: Which formula is best? Am J Obstet Gynecol 157:1411, 1987 16. Robson SC, Gallivan S, Walkinshaw SA, et al: Ultrasonic estimation of fetal weight: Use of targeted formulas in small for gestational age fetuses. Obstet Gynecol 82:359, 1993 17. Warsof S, Gohari P, Berkowitz R, et al: The estimation of fetal weight by computer-assisted analysis. Am J Obstet Gynecol 128:881, 1977