British Journal of Obstetrics. and Gynaecology
I VOL. 82 No.9
NEW SERIES
SEPTEMBER 1975
ULTRASONIC MEASUREMENT OF FETAL ABDOMEN CIRCUMFERENCE IN THE ESTIMATION OF FETAL WEIGHT BY
STUARTCAMPBELL, Senior Lecturer AND
DAVIDWILKIN,Research Assistanr Institute of Obstetrics and Gynaecologj, Queen Charlotte’s Maternity Hospital Goldhawk Road, London W6 OXG Summary A method of estimating fetal weight by ultrasonic measurement of the fetal abdominal circumference is described. Assessment of birth weight predictions on 140 fetuses who were delivered within 48 hours of this measurement showed that the accuracy of predictions varied with the size of the fetus; at a predicted weight of 1 kg, 95 per cent of birth weights fell within 160 g, while at 2 kg, 3 kg and 4 kg the corresponding values were 290 g, 450 g and 590 g respectively. Expressed as a percentage of the predicted weight, confidence limits remained constant throughout the birth weight range. Extrapolation of these data to routine screening of the obstetric population showed that with a single measurement at 32 weeks menstrual age, 87 per cent of babies below the 5th centile would be detected by this method but that the diagnosis rate would fall to 63 per cent at 38 weeks. The false positive diagnosis rate would remain constant between 32 and 38 weeks at just over 1 per cent. A SIMPLE and accurate method of estimating fetal weight which could be applied to all pregnancies would be an important means of reducing perinatal mortality and morbidity. Goldstein and Peckham (1975) have shown that birth weight is the principle variable affecting late fetal and neonatal mortality and that low birth weight babies both pre-term and small-fordates are at greatest risk. The extent to which accurate identification of the small-for-dates fetus would contribute towards a reduction in perinatal mortality has been quantified by Usher and McLean (1974). In a series of 44256 consecutive births from 1958 to 1971 they found that there was a tenfold increase in perinatal
mortality in babies born more than two standard deviations under weight when compared with those whose birth weight was within two standard deviations of the mean and that 22 per cent of perinatal deaths were in the small-for-dates group. They calculated that 2.7 perinatal deaths per 1000 births were due solely to chronic fetal deprivation and that 70 per cent of these babies could be salvaged if the diagnosis was made by 34 weeks menstrual age. Accurate prediction of fetal size would also, if applied to all pregnancies, assist in identifying fetuses whose maturity had been wrongly estimated and hence reduce the number of pre-term perinatal deaths. Diagnostic ultrasound being a painless, non689
26
690
CAMPBELL AND WILKIN
invasive, inexpensive and apparently harmless technique, has the potential to be used to screen all patients. Routine single measurements of the fetal biparietal diameter early in the second trimester have been shown to be an accurate method of assessing the menstrual age of the fetus (Campbell, 1974) but single measurements in late pregnancy are not clinically useful in assessing birth weight (Campbell, 1973). Serial measurements have been used successfully to detect fetal growth retardation (Willocks et al., 1967; Campbell and Dewhurst, 1971; Varma, 1973), but this technique cannot be used to screen all obstetric patients due to the excessive work load which this would entail. Several workers have achieved greater success in predicting birth weight from a single examination by taking linear (Thompson and Makowski, 1971) or circumference (Levi, 1972; Hansmann et al, 1973) measurements of the fetal thorax. We have found that the greatest accuracy in prediction is achieved by taking circumference measurements of the fetal abdomen at the level of the umbilicaI vein and preliminary results with this technique have been described (Campbell, 1974). It is the purpose of this paper to assess the accuracy of birth weight predictions by measurement of the fetal abdomen circumference and to assess how many small-for-dates fetuses might be detected by this method if applied to the whole obstetric population. PATIENTS AND METHODS Ultrasonic examination was made within 48 hours of delivery in 138 pregnancies (140 fetuses). In 112 cases the examination was performed on the day prior to elective Caesarean section or induction of labour; in these patients the principle indication for these procedures was hypertensive disorders (30 per cent), fetal growth retardation (20 per cent) and post-maturity (23 per cent). In the remaining 26 cases (28 fetuses) spontaneous labour and delivery occurred within 48 hours of the examination. Circumference measurements of the fetal abdomen were obtained with the Diasonograph 4102 (Nuclear Enterprises, Edinburgh) by the technique described by Campbell (1974) and illustrated in Figure 1. Ultrasonic compound B-scans are first made at different angles to the midline of the maternal abdomen to identify the
position of the long axis of the fetal body; where there is marked flexion of the fetal body, it is helpful to identify a significant length of fetal abdominal aorta (Fig 2), or fetal dorsal spine. Scans are then made orthogonal to the long axis of the fetal body and a section across the upper abdomen selected; this is recognized by the typical appearance of the umbilical vein as it passes under the fetal liver (Fig 3). Usually the umbilical vein can be quickly and easily recognized from 24 weeks onwards except in about 5 per cent of cases when the fetal spine is directly anterior, which means that the walls of the umbilical vein are not orthogonal to the ultrasonic beam. Under these circumstances we have found the fetal stomach to be the most suitable reference point; it is not so precise a location as the umbilical vein for when distended it extends over a greater length of the fetal abdomen but it does lie in the upper abdomen to the left of the fetal liver and both umbilical vein and fetal stomach can usually be visualized on the same section (Figs 4 and 5). Circumference measurements were made to the nearest millimetre on a Polaroid photograph by means of a map measurer with appropriate correction for picture size. In general life size images were possible until 26 weeks; thereafter 4/5th and 3/5th size images had to be used because the Hewlett Packard 141 B oscilloscope could not contain the full image. Also the usual reduction in image size of 0.93 to 1 which is a feature of the oscilloscope was compensated for by altering the focal distance of the camera. In all cases an ultrasonic frequency of 2.5 MHz was used and the velocity calibration set to 1540 metres per second. RESULTS Of the 140 fetuses studied, 36 had a birthweight below 2-60 kg and 10 greater than 4 kg The birth weights ranged from 0.79 kg to 5.46 kg. It was found that if the birth weights were transformed to log, values the variations were approximately the same over the whole range of abdominal circumferences. A second degree polynomial regression of the form log, Y = a+bX+cX2 (where Y = birth weight and X = abdominal circumference) was fitted to the transformed data (Fig 6) using BMD X85 computer programme (Dixon, 1970). The 5th
FETAL ABDOMEN CIRCUMFERENCE
\
I
691
/
b \
S
FIG 1 The initial longitudinal scan determines the position of the long axis of the fetal body (L) in two dimensions: (a) relative to the midline of the maternal abdomen, and (b) relative to the vertical axis. The transverse measuring scan (S) is then made orthogonal to the long axis of the fetus in both dimensions.
692
CAMPBELL AND WILKIN
FIG2 In difficult cases, visualization of a significant length of abdominal aorta will aid in determining the long axis of the fetus.
FIG 3 Transversesection of the fetal abdomen showing umbilical vein. The sacrospinalismuscle and the fetal spine should be visualized. The circumference is determined by means of a map measurer.
FETAL ABDOMEN CIRCUMFERENCE
FIG4 Transverse section of the fetal abdomen showing the fetal stomach and umbilical vein on the same section.
FIG5 Transverse section of the fetal abdomen when the fetal spine is lying directly anterior; the umbilical vein cannot be visualized when the fetus is in this position and the fetal stomach is used as the reference point. The sacrospinalis muscle, fetal spine, abdominal aorta and inferior vena cava can be identified in this section.
693
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CAMPBELL AND WILKIN
and 95th centile limits were determined by multiplying the median by e&1.64s where S is the standard error for a given circumference. In Figure 7 birth weights have been converted back to normal values ; the skewed distribution of the confidence limits is a result of this conversion. The confidence limits for predictions depend on the size of the measurement, the greater the measurement, the wider the limits. At a predicted weight of 1 kg, 95 per cent of birth weights fell within 160 g of the estimation while at 2, 3 and 4 kg the corresponding values were 290 g, 450 g and 590 g respectively. From Table I, however, it is clear that if these confidence limits are expressed as a percentage of the median birth weight, then the ultrasonic “error” is constant throughout the birth weight range. Extrapolation to routine fetal weight prediction. The above data were used to assess the probable value of this technique when applied to a whole TABLEI Relationship between fetal abdominal circumference measurements from 21 to 40 cm and birth weight centiles (140 estimations) Abdominal circumference (cm) 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Estimated birth weight centiles (kg) 5 (*)
0.78 0.90 1.03 1.17 1.32 1.47 1.64 1.81 1.99 2.17 2.35 2.53 2.71 2.88 3.03 3.18 3.31 3.42 3.51 3.57
(86.4) (86.7) (86.9) (87.1) (87.2) (87.3) (87.3) (87.3) (87.3) (87.4) (87.4) (87.4) (87-4) (87.4) (87.4) (87.4) (87.3) (87.3) (87.2) (87.0)
50
0.90 1.03 1.18 1.34 1.51 1.69 1.88 2.09 2.28 2.49 2.69 2.90 3.10 3.29 3.47 3.64 3.79 3.92 4.02 4.10
95 1.04 1.19 1.36 1.54 1.73 1.94 2.15 2.38 2.61 2.85 3.08 3.32 3.55 3.76 3.97 4.16 4.33 4.49 4.61 4.72
(*I (1 15.7) (1 1 5.3) (1 15.0) (114.8) (114.7) (114.6) (114.5) (114.5) (1 14.5) (114.5) ( I 14.5) (114.4) (1 14.4) (1 14.4) (1 14.4) (114-4) (1 14.5) (114.6) (1 14.7) (115.0)
* Figures in brackets represent the centile limit expressed as a percentage of the median birth weight.
TABLEI1 Correct classification of babies above and below the 5th centile weight at 32, 34, 36 and 38 weeks menstrual age using data shown in Table I (simulated populations of 10,000 births derived from Thomson et a1 (1968))
Menstrual age (weeks)
32 34 36 38
Correct classification from 5th centile* fetal abdominalcircumference limit measurements (kg) > 5th centile < 5th centile 1.11 1.68 2.13 2.47
Per cent 98.8 98.9 98.7 98.7
Per cent 86.7 81.8 74.9 63.2
* From Table XI of Thomson ef a1 (1968).
hospital obstetric population. “Actual” birth weights were generated at random from the distributionof birthweights for a given menstrual age, characterized by the centiles published by Thomson et a1 (1968). The equation, circumference = 21.79+6.56 log weight+3.25 (log weight)2, derived from the original data, was used to compute an abdominal circumference for each of the actual weights generated and added to this was a variable component (a function of the standard deviation of the circumference conditional on the computed value). This corresponds to the observed circumference and from this value the predicted weight was computed. This gave a pair of values, the actual birthweight and the predicted birthweight. These were then classified into one of four groups based on whether one or both fell above or below the 5th centile limit given by Thomson et al. (1968). Populations of 10 000 births were simulated for 32, 34, 36 and 38 weeks menstrual age. Table I1 shows that the maximum yield (i.e. 87 per cent) of small-for-dates fetuses would be achieved at 32 weeks and that the successful diagnosis of babies below the 5th centile fell with increasing maturity to 63 per cent at 38 weeks. The false positive diagnosis rate remained constant between 32 and 38 weeks at just over 1 per cent. DISCUSSION We have concentrated on a simple technique Involving a single measurement in order that
’1
FETAL ABDOMEN CIRCUMFERENCE
695
1-6
i -02-
routine screening of a whole hospital obstetric population could be a feasible proposition. The technique described is simple, can be quickly learned and is usually completed in under five minutes. From our studies (Wilkin and Campbell, unpublished data), we have found that the fetal abdomen circumference measurement at the level of the umbilical vein is the one most representative of the total fetal weight. Because a single section is being taken as representative of the total fetal body mass, it is not surprising that the “error” as expressed as a percentage of the median birth weight is constant throughout the birth weight range; this means that in terms of actual weight predictions the smaller the fetus the narrower will be the confidence limits which acts in favour of the diagnosis of the small-for-
dates fetus. Our simulation studies suggest that routine screening at 32 weeks and at 36 weeks would successfully detect 87 per cent and 75 per cent of small-for-dates fetuses respectively and importantly a negligible number of false positive diagnoses would be made. This is a higher pickup rate than is achieved by any other technique and significantly better than the 30 per cent successful diagnosis rate of small-for-dates fetuses which is achieved by routine abdominal palpation (Campbell, 1974). A single measurement of the fetal abdomen circumference would also appear to be more efficient at diagnosing the small-for-dates fetus than serial cephalometry and have a lower incidence of false positive diagnoses (Campbell and Dewhurst, 1971). Furthermore, it can be applied to the
696
CAMPBELL AND WILKIN 548
--
centile
--
50th centfie
90th
46 44 42
4- -3s 3e34 3.2 -
5th
centile
3-26
--
24
-
28
h
2 .-
g .m
2.2
-
2-1.8 1.6 1.4
1.2
-
I 1
1--.8 -
-4 -6
2 -
Abdominal circumference (cm)
FIG7 Relationship between the fetal abdominal circumference measurement and birth weight in 140 fetuses who delivered within 48 hours of the ultrasound measurement; 2nd degree polynomial regression, 95th and 5th centile confidence limits.
whole obstetric population, whereas it is impracticable to apply serial cephalometry for this purpose. Unlike serial cephalometry, however, single measurements of the fetal abdomen circumference cannot distinguish between the small fetus which is the result of growth retardation from that which is due to a mistaken maturity estimation, as our data indicate that birth weight predictions from a particular circumference measurement are independent of menstrual age. We have been endeavouring to achieve this distinction from the measurement of the fetal head to abdomen circumference ratio (Campbell, 1974) but present evidence indicates that the optimal method of screening out the small-for-dates fetus would be the combination of an early determination of fetal maturity, combined with a late measurement of fetal size.
This would entail the measurement of an embryonic crown-rump length between 6 and 12 weeks (Robinson, 1973) or the biparietal between 13 and 20 weeks (Campbell, 1974), combined with a late measurement of the fetal abdomen circumference between 32 and 36 weeks menstrual age. By this means we believe that a much higher diagnosis rate of the small-fordates fetus is possible than is at present being achieved and consequently that the perinatal loss from this condition can be significantly reduced.
ACKNOWLEDGEMENT We would like to thank Mr. Peter Thomas, BSc, for statistical help. The Portsmouth Polytechnic Computer ICL 4130 was used for the statistical analysis.
FETAL ABDOMEN CIRCUMFERENCE
REFERENCES Campbell, S. (1 973): Clinics in Obstetrics and Cynaecology Vol. 1. “Fetal Medicine”. Edited by R. W. Beard. W. B. Saunders and Co. Ltd. London, p. 41. Campbell, S . (1974): Clinics in Perinatology. Vol. 1, No. 2. “The Pregnancy at Risk.” Edited by A. Milunsky. W. B. Saunders and Co. Ltd. Philadelphia, p. 507. Campbell, S., and Dewhurst, C. J. (1971): Lancet, 2, 1002. Dixon, W. J. (1970): Biomedical Computer Program, X series, supplement. University of California Press. 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 (in press). Hansmann, M., Voigt, U., and Baeker, H. (1973): Archiv fur Gynakologie, 214, 194.
697
Levi, S. (1 972) : Diagnostic par ultrasons en gynecologieobstetrique. Masson et Cie, Paris. p. 62. Robinson, H. (1973): British Medical Journal, 4, 28. Thompson, H. E., and Makowski, E. L. (1971): Obstetrics and Gynecology, 37, 44. Thomson, A. M., Billewicz, W. Z., and Hytten, F. E. (1968): Journal of Obstetrics 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. Varma, T. R. (1 973) : Australia and New Zealand Journal of Obstetrics and Gynaecology, 13, 191. Willocks, J., Donald, I., Campbell, S., and Dunsmore, I. R. (1967): Journal of Obstetrics and Gynaecology of the British Commonwealth, 74, 639.
I VOL. 82 No.9
NEW SERIES
SEPTEMBER 1975
ULTRASONIC MEASUREMENT OF FETAL ABDOMEN CIRCUMFERENCE IN THE ESTIMATION OF FETAL WEIGHT BY
STUARTCAMPBELL, Senior Lecturer AND
DAVIDWILKIN,Research Assistanr Institute of Obstetrics and Gynaecologj, Queen Charlotte’s Maternity Hospital Goldhawk Road, London W6 OXG Summary A method of estimating fetal weight by ultrasonic measurement of the fetal abdominal circumference is described. Assessment of birth weight predictions on 140 fetuses who were delivered within 48 hours of this measurement showed that the accuracy of predictions varied with the size of the fetus; at a predicted weight of 1 kg, 95 per cent of birth weights fell within 160 g, while at 2 kg, 3 kg and 4 kg the corresponding values were 290 g, 450 g and 590 g respectively. Expressed as a percentage of the predicted weight, confidence limits remained constant throughout the birth weight range. Extrapolation of these data to routine screening of the obstetric population showed that with a single measurement at 32 weeks menstrual age, 87 per cent of babies below the 5th centile would be detected by this method but that the diagnosis rate would fall to 63 per cent at 38 weeks. The false positive diagnosis rate would remain constant between 32 and 38 weeks at just over 1 per cent. A SIMPLE and accurate method of estimating fetal weight which could be applied to all pregnancies would be an important means of reducing perinatal mortality and morbidity. Goldstein and Peckham (1975) have shown that birth weight is the principle variable affecting late fetal and neonatal mortality and that low birth weight babies both pre-term and small-fordates are at greatest risk. The extent to which accurate identification of the small-for-dates fetus would contribute towards a reduction in perinatal mortality has been quantified by Usher and McLean (1974). In a series of 44256 consecutive births from 1958 to 1971 they found that there was a tenfold increase in perinatal
mortality in babies born more than two standard deviations under weight when compared with those whose birth weight was within two standard deviations of the mean and that 22 per cent of perinatal deaths were in the small-for-dates group. They calculated that 2.7 perinatal deaths per 1000 births were due solely to chronic fetal deprivation and that 70 per cent of these babies could be salvaged if the diagnosis was made by 34 weeks menstrual age. Accurate prediction of fetal size would also, if applied to all pregnancies, assist in identifying fetuses whose maturity had been wrongly estimated and hence reduce the number of pre-term perinatal deaths. Diagnostic ultrasound being a painless, non689
26
690
CAMPBELL AND WILKIN
invasive, inexpensive and apparently harmless technique, has the potential to be used to screen all patients. Routine single measurements of the fetal biparietal diameter early in the second trimester have been shown to be an accurate method of assessing the menstrual age of the fetus (Campbell, 1974) but single measurements in late pregnancy are not clinically useful in assessing birth weight (Campbell, 1973). Serial measurements have been used successfully to detect fetal growth retardation (Willocks et al., 1967; Campbell and Dewhurst, 1971; Varma, 1973), but this technique cannot be used to screen all obstetric patients due to the excessive work load which this would entail. Several workers have achieved greater success in predicting birth weight from a single examination by taking linear (Thompson and Makowski, 1971) or circumference (Levi, 1972; Hansmann et al, 1973) measurements of the fetal thorax. We have found that the greatest accuracy in prediction is achieved by taking circumference measurements of the fetal abdomen at the level of the umbilicaI vein and preliminary results with this technique have been described (Campbell, 1974). It is the purpose of this paper to assess the accuracy of birth weight predictions by measurement of the fetal abdomen circumference and to assess how many small-for-dates fetuses might be detected by this method if applied to the whole obstetric population. PATIENTS AND METHODS Ultrasonic examination was made within 48 hours of delivery in 138 pregnancies (140 fetuses). In 112 cases the examination was performed on the day prior to elective Caesarean section or induction of labour; in these patients the principle indication for these procedures was hypertensive disorders (30 per cent), fetal growth retardation (20 per cent) and post-maturity (23 per cent). In the remaining 26 cases (28 fetuses) spontaneous labour and delivery occurred within 48 hours of the examination. Circumference measurements of the fetal abdomen were obtained with the Diasonograph 4102 (Nuclear Enterprises, Edinburgh) by the technique described by Campbell (1974) and illustrated in Figure 1. Ultrasonic compound B-scans are first made at different angles to the midline of the maternal abdomen to identify the
position of the long axis of the fetal body; where there is marked flexion of the fetal body, it is helpful to identify a significant length of fetal abdominal aorta (Fig 2), or fetal dorsal spine. Scans are then made orthogonal to the long axis of the fetal body and a section across the upper abdomen selected; this is recognized by the typical appearance of the umbilical vein as it passes under the fetal liver (Fig 3). Usually the umbilical vein can be quickly and easily recognized from 24 weeks onwards except in about 5 per cent of cases when the fetal spine is directly anterior, which means that the walls of the umbilical vein are not orthogonal to the ultrasonic beam. Under these circumstances we have found the fetal stomach to be the most suitable reference point; it is not so precise a location as the umbilical vein for when distended it extends over a greater length of the fetal abdomen but it does lie in the upper abdomen to the left of the fetal liver and both umbilical vein and fetal stomach can usually be visualized on the same section (Figs 4 and 5). Circumference measurements were made to the nearest millimetre on a Polaroid photograph by means of a map measurer with appropriate correction for picture size. In general life size images were possible until 26 weeks; thereafter 4/5th and 3/5th size images had to be used because the Hewlett Packard 141 B oscilloscope could not contain the full image. Also the usual reduction in image size of 0.93 to 1 which is a feature of the oscilloscope was compensated for by altering the focal distance of the camera. In all cases an ultrasonic frequency of 2.5 MHz was used and the velocity calibration set to 1540 metres per second. RESULTS Of the 140 fetuses studied, 36 had a birthweight below 2-60 kg and 10 greater than 4 kg The birth weights ranged from 0.79 kg to 5.46 kg. It was found that if the birth weights were transformed to log, values the variations were approximately the same over the whole range of abdominal circumferences. A second degree polynomial regression of the form log, Y = a+bX+cX2 (where Y = birth weight and X = abdominal circumference) was fitted to the transformed data (Fig 6) using BMD X85 computer programme (Dixon, 1970). The 5th
FETAL ABDOMEN CIRCUMFERENCE
\
I
691
/
b \
S
FIG 1 The initial longitudinal scan determines the position of the long axis of the fetal body (L) in two dimensions: (a) relative to the midline of the maternal abdomen, and (b) relative to the vertical axis. The transverse measuring scan (S) is then made orthogonal to the long axis of the fetus in both dimensions.
692
CAMPBELL AND WILKIN
FIG2 In difficult cases, visualization of a significant length of abdominal aorta will aid in determining the long axis of the fetus.
FIG 3 Transversesection of the fetal abdomen showing umbilical vein. The sacrospinalismuscle and the fetal spine should be visualized. The circumference is determined by means of a map measurer.
FETAL ABDOMEN CIRCUMFERENCE
FIG4 Transverse section of the fetal abdomen showing the fetal stomach and umbilical vein on the same section.
FIG5 Transverse section of the fetal abdomen when the fetal spine is lying directly anterior; the umbilical vein cannot be visualized when the fetus is in this position and the fetal stomach is used as the reference point. The sacrospinalis muscle, fetal spine, abdominal aorta and inferior vena cava can be identified in this section.
693
694
CAMPBELL AND WILKIN
and 95th centile limits were determined by multiplying the median by e&1.64s where S is the standard error for a given circumference. In Figure 7 birth weights have been converted back to normal values ; the skewed distribution of the confidence limits is a result of this conversion. The confidence limits for predictions depend on the size of the measurement, the greater the measurement, the wider the limits. At a predicted weight of 1 kg, 95 per cent of birth weights fell within 160 g of the estimation while at 2, 3 and 4 kg the corresponding values were 290 g, 450 g and 590 g respectively. From Table I, however, it is clear that if these confidence limits are expressed as a percentage of the median birth weight, then the ultrasonic “error” is constant throughout the birth weight range. Extrapolation to routine fetal weight prediction. The above data were used to assess the probable value of this technique when applied to a whole TABLEI Relationship between fetal abdominal circumference measurements from 21 to 40 cm and birth weight centiles (140 estimations) Abdominal circumference (cm) 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Estimated birth weight centiles (kg) 5 (*)
0.78 0.90 1.03 1.17 1.32 1.47 1.64 1.81 1.99 2.17 2.35 2.53 2.71 2.88 3.03 3.18 3.31 3.42 3.51 3.57
(86.4) (86.7) (86.9) (87.1) (87.2) (87.3) (87.3) (87.3) (87.3) (87.4) (87.4) (87.4) (87-4) (87.4) (87.4) (87.4) (87.3) (87.3) (87.2) (87.0)
50
0.90 1.03 1.18 1.34 1.51 1.69 1.88 2.09 2.28 2.49 2.69 2.90 3.10 3.29 3.47 3.64 3.79 3.92 4.02 4.10
95 1.04 1.19 1.36 1.54 1.73 1.94 2.15 2.38 2.61 2.85 3.08 3.32 3.55 3.76 3.97 4.16 4.33 4.49 4.61 4.72
(*I (1 15.7) (1 1 5.3) (1 15.0) (114.8) (114.7) (114.6) (114.5) (114.5) (1 14.5) (114.5) ( I 14.5) (114.4) (1 14.4) (1 14.4) (1 14.4) (114-4) (1 14.5) (114.6) (1 14.7) (115.0)
* Figures in brackets represent the centile limit expressed as a percentage of the median birth weight.
TABLEI1 Correct classification of babies above and below the 5th centile weight at 32, 34, 36 and 38 weeks menstrual age using data shown in Table I (simulated populations of 10,000 births derived from Thomson et a1 (1968))
Menstrual age (weeks)
32 34 36 38
Correct classification from 5th centile* fetal abdominalcircumference limit measurements (kg) > 5th centile < 5th centile 1.11 1.68 2.13 2.47
Per cent 98.8 98.9 98.7 98.7
Per cent 86.7 81.8 74.9 63.2
* From Table XI of Thomson ef a1 (1968).
hospital obstetric population. “Actual” birth weights were generated at random from the distributionof birthweights for a given menstrual age, characterized by the centiles published by Thomson et a1 (1968). The equation, circumference = 21.79+6.56 log weight+3.25 (log weight)2, derived from the original data, was used to compute an abdominal circumference for each of the actual weights generated and added to this was a variable component (a function of the standard deviation of the circumference conditional on the computed value). This corresponds to the observed circumference and from this value the predicted weight was computed. This gave a pair of values, the actual birthweight and the predicted birthweight. These were then classified into one of four groups based on whether one or both fell above or below the 5th centile limit given by Thomson et al. (1968). Populations of 10 000 births were simulated for 32, 34, 36 and 38 weeks menstrual age. Table I1 shows that the maximum yield (i.e. 87 per cent) of small-for-dates fetuses would be achieved at 32 weeks and that the successful diagnosis of babies below the 5th centile fell with increasing maturity to 63 per cent at 38 weeks. The false positive diagnosis rate remained constant between 32 and 38 weeks at just over 1 per cent. DISCUSSION We have concentrated on a simple technique Involving a single measurement in order that
’1
FETAL ABDOMEN CIRCUMFERENCE
695
1-6
i -02-
routine screening of a whole hospital obstetric population could be a feasible proposition. The technique described is simple, can be quickly learned and is usually completed in under five minutes. From our studies (Wilkin and Campbell, unpublished data), we have found that the fetal abdomen circumference measurement at the level of the umbilical vein is the one most representative of the total fetal weight. Because a single section is being taken as representative of the total fetal body mass, it is not surprising that the “error” as expressed as a percentage of the median birth weight is constant throughout the birth weight range; this means that in terms of actual weight predictions the smaller the fetus the narrower will be the confidence limits which acts in favour of the diagnosis of the small-for-
dates fetus. Our simulation studies suggest that routine screening at 32 weeks and at 36 weeks would successfully detect 87 per cent and 75 per cent of small-for-dates fetuses respectively and importantly a negligible number of false positive diagnoses would be made. This is a higher pickup rate than is achieved by any other technique and significantly better than the 30 per cent successful diagnosis rate of small-for-dates fetuses which is achieved by routine abdominal palpation (Campbell, 1974). A single measurement of the fetal abdomen circumference would also appear to be more efficient at diagnosing the small-for-dates fetus than serial cephalometry and have a lower incidence of false positive diagnoses (Campbell and Dewhurst, 1971). Furthermore, it can be applied to the
696
CAMPBELL AND WILKIN 548
--
centile
--
50th centfie
90th
46 44 42
4- -3s 3e34 3.2 -
5th
centile
3-26
--
24
-
28
h
2 .-
g .m
2.2
-
2-1.8 1.6 1.4
1.2
-
I 1
1--.8 -
-4 -6
2 -
Abdominal circumference (cm)
FIG7 Relationship between the fetal abdominal circumference measurement and birth weight in 140 fetuses who delivered within 48 hours of the ultrasound measurement; 2nd degree polynomial regression, 95th and 5th centile confidence limits.
whole obstetric population, whereas it is impracticable to apply serial cephalometry for this purpose. Unlike serial cephalometry, however, single measurements of the fetal abdomen circumference cannot distinguish between the small fetus which is the result of growth retardation from that which is due to a mistaken maturity estimation, as our data indicate that birth weight predictions from a particular circumference measurement are independent of menstrual age. We have been endeavouring to achieve this distinction from the measurement of the fetal head to abdomen circumference ratio (Campbell, 1974) but present evidence indicates that the optimal method of screening out the small-for-dates fetus would be the combination of an early determination of fetal maturity, combined with a late measurement of fetal size.
This would entail the measurement of an embryonic crown-rump length between 6 and 12 weeks (Robinson, 1973) or the biparietal between 13 and 20 weeks (Campbell, 1974), combined with a late measurement of the fetal abdomen circumference between 32 and 36 weeks menstrual age. By this means we believe that a much higher diagnosis rate of the small-fordates fetus is possible than is at present being achieved and consequently that the perinatal loss from this condition can be significantly reduced.
ACKNOWLEDGEMENT We would like to thank Mr. Peter Thomas, BSc, for statistical help. The Portsmouth Polytechnic Computer ICL 4130 was used for the statistical analysis.
FETAL ABDOMEN CIRCUMFERENCE
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