AMERICAN JOURNAL OF PERINATOLOGY/VOLUME 7, NUMBER 2

April 1990

ULTRASOUND DETERMINATION OF FETAL ABDOMINAL CIRCUMFERENCE: A COMPARISON OF MEASUREMENT METHODS William J. Watson, M.D., Nancy C. Chescheir, M.D., and John W. Seeds, M.D.

ABSTRACT

Determination of the fetal abdominal circumference (AC) is an important part of every basic obstetric ultrasound examination. This measurement is used in assessment of gestational age and estimation of fetal weight. Technically, the AC can be measured directly, using a digitizer to outline the perimeter of the imaged fetal abdomen, or it can be calculated from the mean of two abdominal diameters (MAD). In a recent report, Tamura et al1 studied the two methods and showed a discrepancy. They concluded that a measurement determined by one method should not be compared with normative data derived by the other method. The purpose of this prospective investigation was to compare both the statistical and clinical significance of differences in direct versus calculated AC measurements to determine if, in fact, a sufficient discrepancy exists to preclude the use of either method. MATERIALS AND METHODS

Two hundred and thirty-five patients referred for obstetric ultrasound examination were included in the study. The gestational age ranged from 16 to 40 weeks. The AC was measured from a scan plane perpendicular to the fetal spine, at the level of the fetal stomach and umbilical vein branching. The AC was measured directly using a digitizer to outline the

abdominal perimeter and was also calculated by multiplying the MAD times pi (3.14). MAD was determined by averaging the anteroposterior and transverse fetal abdominal dimensions, measured from the outer to the outer surfaces of the same scan image. The two measurement methods were compared using the Wilcoxson paired sign rank test for nonparametric data. A two-tailed p value of =£0.05 was considered significant. RESULTS

Direct measurement of the fetal abdominal perimeter using a digitizer resulted in a value that was significantly greater than the AC determination by calculation from MAD (Z = 3.82, p = 0.00014, n = 223 untied pairs). The directly measured AC was greater than the calculated AC by a mean difference of+1.3 ± 2.2% (Fig. 1). DISCUSSION

These data show that directly measured fetal AC by the perimeter method is statistically significantly greater than the diameter calculated AC. This is in technical agreement with the findings of Ta-

Department of Obstetrics and Gynecology, Maternal Fetal Medicine Division, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina Reprint requests: Dr. Watson, Department of Obstetrics and Gynecology, Maternal Fetal Medicine, UNC Chapel Hill, CB #7570 MacNider, Chapel Hill, NC 27599

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Copyright © 1990 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.

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The measurement of fetal abdominal circumference by the direct perimeter method using a digitizer was compared with calculation of this measurement using the fetal mean abdominal diameter in 235 cases. The directly measured abdominal circumference was found to be greater than the calculated value (p = 0.00014). The magnitude of the difference, however was only 1.3 ± 2.2%, which is smaller than the average interobserver measurement error. Although statistically significant, the difference between these two measurement methods is not clinically significant, and these data and discussion show that either method is acceptable to determine fetal abdominal circumference.

FETAL ABDOMINAL CIRCUMFERENCE/Watson, Chescheir, Seeds

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£ 250 o "5 200 £ o X> -9 150

• Directly measured AC o Calculated AC

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Gestational Age in Weeks Figure 1 . Differences in these two methods of fetal AC measurement are very small when compared with the 95% confidence intervals shown.

depending on which formula for fetal weight estimation is used. Since sonographic fetal weight estimation methods result in estimates with an error of plus or minus 8 to 10% (1 SD), discrepancies on the order of magnitude observed here would not result in significant clinical error. A second important factor to consider is the interobserver difference in AC determination. Deter et al2 found that under optimal measuring conditions, the interobserver error in AC determination was 2.4 ± 1.6%. This is similar to the magnitude of the difference in measurement methods found in the present study. It is theoretically ideal to compare sonographic dimensions to normative tables derived in a similar fashion. The minor differences found in fetal AC measurement by two methods in this study, however, will not be a source of erroneous clinical care. These data suggest that the common clinical practice of measuring average fetal abdominal diameters to assess fetal growth while using normative data derived from perimeter methods is quite acceptable. Downloaded by: NYU. Copyrighted material.

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REFERENCES

mura and coworkers.* Although this difference was statistically significant, however, a mean difference of 1.3% between these methods of determination is not likely to be clinically significant. A difference of this magnitude would result in a fetal weight estimation error of only 0.3 to 0.8%,

Tamura RK, Sabbagha RE, Wen-Harn P, Vaisrub N: Ultrasonic fetal abdominal circumference: Comparison of direct versus calculated measurement. Obstet Gynecol 67:833-835, 1986 Deter RL, Harrist RB, Hadlock FP, Carpenter RJ: Fetal head and abdominal circumferences: I. Evaluation of measurement errors. JCU 10:357-363, 1982

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Ultrasound determination of fetal abdominal circumference: a comparison of measurement methods.

The measurement of fetal abdominal circumference by the direct perimeter method using a digitizer was compared with calculation of this measurement us...
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