ESTIMATION OF FETAL WEIGHT FROM ULTRASONIC MEASUREMENT OF TRUNK CIRCUMFERENCE BY

J. HIGGINBOTTOM, Clinical Tutor and Senior Registrar

J. SLATER, Senior Radiographer G . PORTER, Senior Radiographer AND

C. R. WHITFIELD, Professor Department of Obstetrics and Gynaecology University of Manchester and University Hospital of South Manchester

Summary A new method of estimating fetal weight from the circumference of the fetal trunk, measured by ultrasound, is described. Comparison of weights estimated in this way with the actual birth weights of 50 fetuses, delivered not more than 48 hours later, shows a mean error in prediction of only 75 g, and in 94 per cent the error was less than 145 g. With such accuracy, the method is of clinical value, particularly in anticipating difficult delivery and in monitoring growth of the fetus at risk.

FOR the reliable estimation of fetal weight, a more precise method than palpation through the maternal abdomen is needed for this purpose, increasing attention is being paid to ultrasonic measurements, including the biparietal diameter ofthe fetal skull (Inniruberto and Gibbons, 1971), the diameter of the fetal thorax (Thompson and Makowski, 1971), the circumference of the fetal trunk and the ratio between it and the biparietal diameter (Campbell, 19743), and the fetal heart volume (Kotaro Susuki et al, 1974), but with each of these methods errors of more than 300 g occur. Unfortunately, the more accurate method derived from measurement of fetal volume (Morrison, 1974) is too complex and too slow for routine use. On theoretical grounds, fetal volume rather than a diameter provides the most logical approach to the problem. As volume is a cubic measurement, a linear relationship between it and any fetal dimension is improbable; instead,

a cubic relationship is much more likely and may be expressed in the following formula: y

= mx3,

Where y is the fetal volume for weight, since fetal density is virtually one), x is the dimension concerned and m is an expression of unknown (assumed constant) factors related to the shape of the fetus. The main practical consideration is selecting a suitable fetal dimension for ultrasonic measurement; it must be directly measurable in the newborn so that m can be calculated and a theoretical cubic curve, from which the fetal weight is readily determined, can be drawn. The circumference of the fetal trunk fulfils these criteria, and it can be measured at any level in the thorax or abdomen. In practice the abdominal circumference at the level of the liver, using the ductus venosus as a marker as suggested by Campbell (1974a and 1974c), is very suitable. 698

FETAL WEIGHT AND TRUNK CIRCUMFERENCE

METHOD With standard B mode sonar, using a Kretz IM 4100 M.G.S., the position of the fetus was determined and the fetal heart was located. A series of scans, transverse to the fetus, were made caudal to this point until the ductus venosus was seen crossing the fetal abdomen towards the inferior vena cava, as shown in Figures 1 and 2. The fetal spine and aorta were also seen. It was sometimes necessary to wait a few minutes, until the fetus became still, and, to make certain that the correct angulation for a true transverse fetal scan was obtained, as nearly circular an outline of the trunk as possible was obtained. A Polaroid photograph was taken, the Circumference was measured with a map measurer or similar device, and this measurement was corrected for the known magnification factor used. The fetal weight was then read off the graph illustrated

m FIGI Diagram of transverse scan through fetal abdomen at level of ductus venosus.

FIG2 Transverse ultrasound scan through fetal abdomen at level of ductus venosus.

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(Fig 3) which was based on the formula described, using a value of 0.0816 for m that had been established from circumferential measurements at this level on the newborn. Between observer variations in circumference were extremely small and rarely more than 2 mm Satisfactory Polaroid photographs were obtained at the first attempt in about nine patients out of ten. If difficulties were encountered, a repeat examination after an interval of a few days was nearly always successful.

RESULTS Figure 4 shows the close correlation between the predicted birth weights and the actual birth weights of 50 fetuses, including two pairs of twins, all of whom were born within 48 hours after the ultrasonic examination. The mean prediction error was 75 - 4 g, the standard error of the estimate was 97 ’3, and 94 per cent of the birth weights were within 145 g of the predicted values. The maximum error was 225 g, and this occurred twice in the series. DISCUSSION Reliable estimation of fetal weight would be helpful in deciding the method of delivery when the breech presents, when a previous delivery was by Caesarean section, or when disproportion is suspected. Serial estimations are of special value in monitoring fetal growth, defined as increase in weight per unit time, particularly when dysmaturity or placental insufficiency are suspected. The method has therefore been established as a routine procedure in our hospital. The ultrasound technique is based on that of Campbell (1974b and 1974c), and we have found no difficulty in identifying the ductus venosus which ensures that the scan is made at the correct level. Occasionally the edges of the fetal trunk appear slightly blurred, and this may be due to fetal “breathing” movements; if this does not resolve within a few minutes, the circumference is measured round the outside edge of the fetal abdominal outline. In predicting birth weight, measurements should be made as near as possible to the time of delivery, preferably the previous day. For this purpose even our maximum prediction error of 225 g would probably be of little practical importance.

700 HIGGINBOTTOM,

SLATER, PORTER AND WHlTFlELD

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FIG3 Theoretical cubic curve used to estimate fetal weight.

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FIG4 Correlation between actual birth weight and predicted birthweight.

In measuring growth of the small-for-dates baby by serial estimations of its weight at 7 or 14 day intervals, accuracy within 150 g is desirable. This was achieved in 94 per cent of

cases using the mathematical formula described, and this compares with prediction within 280 g in 68 per cent of cases reported by Campbell (1974a), who did not make use of such a formula.

FETAL WEIGHT AND TRUNK CIRCUMFERENCE

ACKNOWLEDGEMENTS thank D ~ G . . Hartley and D ~ .D. L. Asbury for encouragement and advice; and Dr. K. W. Burslem and Mr. J. B. Jones whose patients were included in this study.

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REFERENCES Campbell, S. ( 1 9 7 4 ~ )Clinics : in Perimtology, 1, 507. Campbell, S. (19746): Clinics in Obstetrics and Gynnecology, 2, 41.

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Campbell, S. (1 974c): Personal communication. Inniruberto, A., and Gibbons, J. M. (1971): Obstetrics and Gynecology, 37, 689. Kotaro Susuki, M. L. J., and Schnitzer, L. E. (1974): Obstetrics and Gynecology, 43, 867. Morrison, J. (1974): William Blair Bell Memorial Lecture, 31st October, 1974. Royal College of Obstetricians and Gynaecologists, London. Thompson, H. E., and Makowski, E. L. (1971):Obstetrics and Gynecology, 37, 44.