AN IMPROVED METHOD OF FETAL WEIGHT ESTIMATION USING ULTRASOUND MEASUREMENTS OF FETAL ABDOMINAL CIRCUMFERENCE BY

V . POLL,Department of Clinical Physics Southampton General Hospital, Southampton AND

C . B. KASBY,Department of Human Reproduction and Obstetrics Southampton General Hospital, Southampton Summary A simple and accurate method is described for estimating fetal weight from a single fetal abdomen circumference measurement at the level of the umbilical vein. The abdominal circumference was converted into a weight centile for the maturity at the time of measurement so that the weight at delivery could be predicted. This method was tested on a population of fetuses presenting by the breech near term. Although the error between actual and predicted weights was 194 g (1 SD) for all the patients, for a large well-defined subgroup the error was much smaller, bzing only 104 g (1 SD).

BIRTH weight affects perinatal morbidity and mortality (Goldstein and Peckham, 1975). Preterm, and small-for-dates fetuses are especially at risk (Usher and McLean, 1974), and there could be a significant reduction in the perinatal death rate if fetal growth retardation could be diagnosed by 34 weeks gestation. Furthermore, in breech presentation, a knowledge of both fetal weight and maturity could significantly affect management of the delivery (Zatuchni and Andros, 1965). Maturity and weight estimations are therefore of considerable significance as is evident from the large number of ultrasound methods which have been developed to assess them. The crown-rump length (CRL) measurement (Robinson, 1973) and the biparietal diameter (BPD) measurement (Campbell and Newman, 1971) are both well established as indicators of fetal maturity. These techniques have been shown to be highly reliable in experienced hands (Robinson and Fleming, 1975; Poll, 1976).

Workers have used different methods to assess fetal weight and some of their results and ours are summarised in Table I while Figure 1 compares our results for fetal abdominal circumfercnce measurement at various stages of gestation with those obtained by others. Most of the methods listed in Table I give errors (expressed as one standard deviation) between predicted and actual weights of the order of k250 g. To put these errors into perspective, Lind (1970) pointed out that guesswork alone, based on the normal distribution of birth weight, would give an error of about 440 g (1 SD). The third sub-group of our data (see Table I) gave the most accurate weight prediction and only two other teams (Morrison and McLennan, 1976; Higginbottom et al, 1975) have achieved comparable results. However, the technique of Morrison and McLennan (1976) was very time-consuming because it involved calculations based on multiple closely spaced 922

FETAL ABDOMINAL CIRCUMFERENCE

923

TABLE I The error between predicted and actual birth weights for previously published methods, compared with the results of the present study

Author

Method

1 SD (9)

Suzuki et al, 1974 Cambell, 1974 Thompson and Makowski, 1971 Ylostalo and Jarvinen, 1974 Thompson, 1965 Thompson and Makowski, 1971 Campogrande et al, 1977 Campbell and Wilkin, 1975 Picker and Saunders, 1976 Lunt and Chard, 1976 Hansmann et al, 1973 Morrison and McLellan, 1976 Higginbottom et al, 1975 Poll, this study (1)

Fetal heart volume BPD BPD BPD, body length, mid-body dimensions BPD thorax circumference BPD +chest diameter BPD body area+ body circumference Abdominal circumference Trunk and limb volumes Skull area+ thorax area BPD transverse thoraxic diameter Fetal volume from multiple scans Abdominal circumference Abdominal circumference on all breech presentations using EDD alone to give fetal maturity Abdominal circumference on breech presentations using either: BPD before 30/52 to give fetal maturity EDD when BPD and EDD consistent at 37/52+

526 405 350 317 307 290 287 279 241 215 178 107 72

+ +

+

194 151 104

BPD = biparietal diameter. EDD = expected date of delivery.

scans. To compare our results with those of Higginbottom et aZ (1975) we converted their ‘fetal weight’ estimates to ‘maturity’ by using data from Thomson et aZ(1968) and the British Perinatal Survey (Butler and Alberman, 1969) for the weight of normal fetuses before 32 weeks. Figure 1 shows that the FAC growth curve we derived from their data was of markedly different shape to that of other workers. Most authors have concentrated on predicting birth weight from measurements taken as close to the date of delivery as possible. The ideal method would provide an estimate of birth weight at any time during the second half of pregnancy and could be used to detect fetal growth retardation by finding changes in weight centiles as pregnancy progressed. The method we now describe is simple, accurate, and can predict birth weight from 20 weeks gestation onwards. 10

I METHOD 30 40

20

Maturity (Weeks) FIG.1 The variation of fetal abdominal circumference with fetal maturity. Data derived from Campbell and Wilkin (1975); Higginbottom et a1 (1975); Meire (1977, personal communication); and Poll (this study).

A Nuclear Enterprises NE4102 ultrasound machine was used with a system velocity of 1540 m/second and with the electronic calipers adjusted for a velocity of sound of 1600 m/second* BPD measurements were made using routine

924 POLL AND

KASBY Normal Curve

methods (Campbell, 1968; Poll, 1976). We use our own BPD-maturity graph constructed from longitudinal data, obtained at four weekly intervals, from normal patients. The difference between our curve and that provided by Campbell and Newman (1971) is small; a maximum of one week's difference at 14 and 40 weeks with the curves crossing at 38 weeks. All fetal abdomen circumference (FAC) measurements were made at the level of the umbilical vein using a slight modification of the method of Campbell and Wilkin (1975). It was felt that a significant error could be introduced in the FAC if only a slight error was made in determining the position of the fetal spine, especially if this was markedly flexed. Consequently a series of scans were made at approximately right-angles to the fetal spine, and the position of the spine and approximate position of the umbilical vein marked on the maternal abdomen. These marks were then used to determine the exact lie of the spine to obtain the angles of tilt and rotation as described by Campbell and Wilkins (1975). Scans were routinely made at a magnification of 3/5 and photographed from the storage display by polaroid camera. The FAC was measured from the photograph using a map-measurer and this measurement was then multiplied by appropriate experimentally-determined correction factors to give a true full-size measurement. As a preliminary to the weight study we constructed a graph of the variation of the normal values of the FAC with fetal maturity and this graph is included in Figure 1. The method of estimating the delivery weight was as follows : (a) Both the fetal BPD and abdominal circumference were measured and the expected date of delivery (EDD) was noted. (b) The gestational age at the time of measurement was calculated to a precision of half a week from the current BPD, the EDD, or an earlier BPD or CRL. Figure 1 shows the estimated 2.3rd and 9 7 ~ 7 t hcentiles ( + 2 SD) of FAC values from 20 weeks gestation onwards. (c) The centile of the FAC measurement was calculated by noting the range in FAC between that actually measured and that corresponding to the - 2 SD centile, and

:

100 1

I

0.1

0.2 0.3 0.4 0.5 0.6 0.7

0.8 0.9 1-00

Fraction of Total Distribution I.e.

FIG.2 The curve used to convert fetal abdominal circumference measurement to a weight centile, given the values of abdominal circumference corresponding to the 2.3rd and 97.7th centiles. 2ox)

2500

3x0

3500

4wo

45w

2500

W O

3500 Bwlnwlgl-

4000

45w

1W

:

H2w(I )

FIG.3 Birth weight distributions for fetal maturities near term (Thomson et a/, 1968).

dividing this range by the range in values between the -2 SD and the f 2 SD centiles. The resultant ratio was converted to an equivalent centile using a table or graph (Fig. 2) of the Gaussian curve.

FETAL ABDOMINAL CIRCUMFERENCE

(d) To a first approximation the FAC centile so derived can be equated to the weight centile. The predicted weight at term (or earlier) can then be read from the data on birth weight at delivery given by Thomson et aZ(1968) (Fig. 3). (e) When the quantity minus ‘actual estimated birth weight’ was plotted against the fetal abdominal circumference centile for all patients, the distribution obtained was skewed, with an overprediction of weight at high FAC centiles. A regression analysis upon these data furnished a correction factor of the form ‘A+Bx(FAC centile) Cx(FAC Centile)2, to the weight prediction as determined in (d). The actual correction factors depended upon the criteria of fetal maturity used in (b), and were necessary because the method contained several assumptions and potential sources of error. These included the assumed equivalence of the FAC and weight centiles, the assumed Gaussian distribution of FAC values for any given maturity (the data is consistent with a Gaussian distribution), the difficulty of accurately determining the true + 2 SD ranges in FAC and also errors which will always tend to overestimate birth weight such as errors in obtaining the FAC and possible growth retardation between the times of scanning and delivery.

+

RESULTS Our results are from patients selected at a routine antenatal examination solely on the basis of the fetus presenting by the breech at a gestation of 37 weeks or more. For most patients there was an interval of up to three weeks between scanning and delivery. The weight prediction exercise was part of a survey to assess the significance of the BPD and weight measurements in the management of a pregnancy complicated by fetal breech presentation. It was noticed at the ultrasound examination that many of the patients had a fetal BPD which was smaller than one would expect from menstrual data (often by the equivalent of three weeks or more). Weight predictions were made using both the BPD and EDD maturities. Figure

925

4 shows the errors between the actual and predicted birth weight for both these maturities for the first few patients. The errors obtained using the EDD were approximately symmetrically distributed about the zero-error line but a large percentage of results using the BPD gave unacceptably high errors of more than 400 g. It was therefore concluded that the BPD near term was an unreliable indicator of fetal maturity especially when assessing fetal weight. If the EDD maturities were used, then the error between predicted and actual birth weights was 194 g (1 SD)-of the same order as found by other workers (Table 11). If, however, the maturity was based on a BPD measurement obtained before 30 weeks (as calculated by the EDD) then the error between predicted and actual birth weights was 151 g (1 SD) with 16 out of 22 patients within & 175 g. Our best results were consistently obtained in those patients in whom, at the time of FAC measurement, the BPD maturity was within one week of that calculated from menstrual data. For these patients the error was only 104 g (1 SD) with 22 out of 25 patients within & 175 g. This figure is considerably better than those obtained by most other workers.

DISCUSSION Our method possesses many of the characteristics needed if birth weight estimation by ultrasound is to be used routinely in antenatal care. The technique is simple, accurate and can give a longitudinal picture of fetal growth. A full analysis of statistical variation in our results using the above methodology is complex. There are not only simple measurement errors in the BPD and FAC themselves, but there are also normal ranges of BPD and FAC for any given maturity (Campbell and Newman, 1971) and weight (Campbell and Wilkin, 1975). The final complication is that a given error in maturity or FAC centile will produce different weight errors depending upon the combination of the absolute values of these factors. The accuracy of the technique seems to depend on the BPD at the time of the measurement of the FAC and on the availability of a maturity calculated from an early BPD. The choice of 30 weeks for one of the subgroups was based upon

926 POLL AND

+400

KASBY

400-300 300-200 200-100 100-0

0-100 100200 200-300 300-400 400+

Birth W t. Predicted w t. (9)

Birth Wt. c Predicted wt. (9)

FIG.4 Errors between actual and predicted fetal weights for the first few patients seen. TABLE I1 The accuracy of weight predictions in the present study Accuracy Maturity calculated using

Number of patients

2 SD (g)

Distribution of results

EDD alone

65

k 388

55/65 within & 300 g

EDD when BPD-EDD at 37 weeks plus

25

2 208

22/25 within & 175 g ~

CRL or BPD (measured before 30 weeks)

22

+

__ 302

~

16/22 within & 175 g

CRL = crown-rump length. EDD = expected date of delivery. BPD = biparietal diameter.

our clinical experience that up to this maturity BPD growth is usually normal (regardless of subsequent fetal growth patterns) and the normal range in BPD readings is relatively small. Small BPD measurement errors (equivalent to less than one week) are, in general, of little consequence. Thus an underestimate of fetal maturity will result in the fetus delivering at an

apparently lesser maturity but at an increased FAC centile (and vice versa). However, this is only a partial explanation of the accuracy of the weight estimations within the two data subgroups. A fuller explanation necessitates the reasonable hypothesis that there is some correlation between BPD and FAC, so that if the BPD, for example, is larger than average for a

FETAL ABDOMINAL CIRCUMFERENCE

given maturity (and hence the fetus is assigned an incorrect maturity), then the FAC will also be above average. Our greatest errors occurred when we estimated the abdominal circumference (and therefore the weight) as above the 80th centile. Although this overestimate could lead to unnecessary Caesarean section with fetal breech presentation, all these babies were heavier than 3.4 kg when delivered near term and were thus potentially unsuitable for vaginal delivery. The practical effect of such errors is thus small. Our smallest errors occurred with the small and average weight fetuses, and these, especially if within the two subgroups, can be identified with considerable precision (43 out of 47 within i200 g). It is thus possible to identify accurately the small-for-dates fetus. The two subgroups were chosen to test the hypothesis that most of the large errors in our original data were due to errors in estimation of fetal maturity. Our statistics show this can be only a partial explanation, and there are probably other factors, possibly associated with the absolute size and shape of the fetal skull. It should be added that none of the groups differ significantly (P < 0-05) from each other in terms of birth weights. There are several other points of importance relating to the FAC and BPD which arise from this study. Firstly, there is a considerable difference between our fetal abdominal circumference/maturity graph (Fig. l), that used by Campbell [1975, personal communication ; but approximately the same information can be obtained by combining the data given by Campbell and Wilkin (1975) with that of Thomson et al (1968)l and provisional data provided by H. Meire (1977, personal communication) after correcting for each systems magnification and linearity. Although the discrepancies are not readily explicable, all sets of information appear reliable in the hands of their respective authors. Thus ultrasound departments must not rely upon published data but should construct their own BPD and FAC curves. A BPD measurement before 30 weeks would now seem an absolute necessity for accurate diagnosis of maturity (Campbell, 1968) and hence a relatively accurate prediction of birth weight.

927

The interpretation of a BPD measurement near term in the fetus presenting by the breech should be regarded with some suspicion as these measurements are frequently below the accepted norms (even with a normally growing fetus). This may be due to differences between fetal skull shapes with breech and vertex presentations. Probably the greatest advantage of our method of fetal weight estimation is that it allows a longitudinal study of fetal weight throughout the third trimester. It is therefore possible that our method would detect fetal growth retardation better than would serial BPD measurements (Campbell, 1974) or head circumferencelabdominal circumference ratios (Campbell and Thorns, 1977) since confusion caused by symmetrical or asymmetrical growth retardation patterns would not exist. Our data are retrospective but a prospective study is in progress. REFERENCES Butler, N., and Alberman, E. (1969): Perinatal Problems, Livingstone, London. Campbell, S., and Newman, G. B. (1971): Journal of Obstetrics and Gynaecology of the British Commonwealth, 78,513. Campbell, S . (1974): Clinics in Obstetrics and Gynaecology, 1,41. Campbell, S . , and Wilkin, D. (1975): British Journal of Obstetrics and Gynaecology, 82,689. Campbell, S . , and Thorns, A. (1977): British Journal of Obstetrics and Gynaecology, 84,165. Campogrande, M., Todros, T., and Brizzolara, M. (1977): British Journal of Obstetrics and Gynaecology, 84, 175. Goldstein, H., and Peckham, C. (1975): Proceedings of the Symposium on Biology of Human Fetal Growth. Edited by A. Thomson and D. F. Roberts. Taylor and Frances, London, p 61. Hansmann, M., Voigt, U., and Baeker, H. (1973): Archiv fur Gynrikologie,214,194. Higginbottom, J., Slater, J., Porter, G., and Whitfield, C. R. (1975): British Journal of Obstetrics and Gynaecology, 82,698. Lind, T. (1970): British Journal of Hospital Medicine, 3, 501. Lunt, R., and Chard, T. (1976): British Journal of Obstetrics and Gynaecology, 83,l. Morrison, J., and McLellan (1976): British Journal of Obstetrics and Gynaecology, 83,833. Picker, R. H., and Saunders, D. M. (1976): American Journal of Obstetrics and Gynecology, 124,493. Poll, V. (1976): British Journal of Obstetrics and Gynaecology, 83,217.

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Robinson, H. (1973):British MedicalJournaI,4 2 8 . Robinson, H., and Fleming, J. (1975): British Journal of Obstetrics and Gynaecology, 82,702. Suzuki, K., Minei, L.,and Schnitzer,L. (1974): Obstetrics and Gynecology,43,867. Thompson, H. E. (1965): Diagnostic Ultrasound. Proceedings of the First International Conference, Pittsburgh, p 416. Thompson, H. E., and Makowski, E. L. (1971): Obstetrics and Gynecology, 31,44.

Thomson, A. M., Billewicz, W. Z., and Hytten, F. E. (1968): Journal of Obsretrics and Gynaecology of the British Commonwealth,75,903. Usher, R. H., and McLean, F. H. (1974): Scientific Foundations of Paediatrics. Edited by J. A. Davis and J. Dobbing. Heinemann, London, p 69. Ylostalo, P., and Jarvinen, P. A. (1974): Annales chirugiae et gynaecologiae Fenniae, 63,24. Zatuchni, G. I., and Andros, G. J. (1965): American Journal of Obstetrics and Gynecology,93,231.