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Prediction of birth weight by ultrasound-estimated fetal weight: a comparison between single and repeated estimates
Journal of Obstetrics & Gynecology and Repr~uctive Biology 60 (1995) 37-40
European
Prediction of birth weight by ultrasound-estimated fetal weight: a comparison between single and repeated estimates Torben Larsen* a, Germ Greisenb, Sten Petersenb aDepartment of Ultrasound and Gynecology, Herlev Hospital, University of Copenhagen, DK-2730 Herlev, Denmark bDepartment of Neonatology. Rigshospitalet. University of Copenhagen, Copenhagen, Denmark
Accepted 20 January 1995
Abstract
Objectives: Ultrasound estimation of fetal weight is used for diagnosing intrauterine growth retardation. The aim of the present study was to assess the accuracy of birth weight prediction by use of a single or repeated estimations of fetal weight in the third trimester. Study Design: 1000 pregnant women considered at risk were scheduled to ultrasound estimation of fetal weight, using Warsofs formula, at 28, 31, 34 and 37 weeks of gestation. The 421 pregnancies with term delivery and complete set of ultrasound exa~nations and 57 pregnancies with preterm delivery with ultrasound examination at I6 and 28 weeks and once more before delivery were incfuded in the present analysis. Results: The accuracy of birth weight prediction improved sibilantly for every three weeks from 28 to 37 weeks of gestation in the term infants. Prediction based on the average of repeated weight estimates or linear extrapolation from two estimates or extrapolation by a second order ~l~omi~ fitted to four estimates did not improve accuracy compared to prediction based on the last estimate before delivery. Conclusion: When more than one ultrasound estimation of fetal weight are available, prediction of birth weight in relation to gestational age should be based on the last ultrasound examination only. Keywords: Ultrasound;
Fetal weight
1. Introduction Ultrasonography is regarded a valuable aid in diagnosing intrauterine growth retardation and is employed either on indication, if low fetal weight is suspected clinically in late pregnancy, or ag part of screening programmes [l-4]. In screening programmes or in settings with routine ultrasound examination of risk pregnancies, the ultrasound estimation of fetal weight is usually performed between 32 and 38 weeks of gestation [5-91. The reason for this is the assumption that growth retardation does not become apparent until late in pregnancy. In other words, we assume that in ~owth-re~rd~ fetuses the weight deviates negatively from the normal percentile during the third trimester of pregnancy. This is also the theoretical background for programmes of longitudinal measurements used in some
* Corresponding author. 002%2243/95/$09.50 0 1995 Elsevier SSDI 0028-2243(95)02079-S
institutions with the purpose of discriminating between biologically small fetuses growing along a low percentile (symmetrically growth-retarded) and fetuses with a growth velocity declining below the normal percentile (asymmetrically growth-retarded fetuses) [lo-121. The aim of the present study was to assess the accuracy of birth weight prediction based on a single ultrasound estimation of fetal weight at different times during the third trimester and to compare this with birth weight prediction based on longitudinal fetal weight estimation. 2. daters
and methods
The study ~p~ation was selected among 1000 highrisk pregnant women participating in a randomized study on detection of small-for-gestational-age (SGA) fetuses, described in detail previously [13]. The ultrasound examinations were scheduled with special refer-
Science Ireland Ltd. All rights reserved
38
T. Larsen et al. /European Journal of Obstetrics & Gynecology and Reproductive Biology 60 (1995) 37-40
ence to the above stated aims. Women who missed one or more exa~nations before the delivery were excluded from the analysis, leaving 478 women. This allowed us to evaluate the prediction of birth weight deviation at various times of ultrasound examination on an unvarying data set. The women were divided into two subgroups: (i) Women with term delivery (n = 421) in whom the first ultrasound examination was performed before 20 weeks of pregnancy and the subsequent ones at 28, 31, 34 and 37 weeks. The clinical characteristics of this group are presented in Table 1. (ii) Women with preterm delivery (n = 57) in whom the first ultrasound ex~ination was performed before 20 weeks of pregnancy and the subsequent ones at 28 weeks (in the period M-205 days) and at least once more before delivery. The clinical characteristics of this group are presented in Table 1. 2.1. Calculation of gestational age, estimated weight and weight deviation Estimate of actual gestational age was calculated from early biparietal diameter [ 141. In a pilot study we found acceptable agreement between actual birth weight and ultrasound estimated fetal weight, calculated according to Warsof et al. [4] from BPD and abdominal circumference. The mean deviation was 8.7%. Therefore, we used this equation in the present study. The inter- and intra-observer variations were 2.9% and 4.6%, respectively [15]. Normal fetal weight for actual gestational age was calculated using a reference curve based on longitudinal ultrasound weight estimates in normal pregnancies [IS] ‘. The ultrasound-estimated fetal weight and birth weight was expressed in percentage of deviation from expected mean weight for the day of gestation, as suggested by Eik-Nes et al. [ 161 2.2. Analysis of birth weight deviation based on a single estimation of fetal weight Birth weight deviation was predicted from ultrasound-estimated fetal weight deviation at 28, 31, 34 and 37 weeks of gestation assuming constant deviation from the gestational age specific reference. The mean difference between predicted fetal weight deviation at each gestational age and actual birth weight deviation and the mean errors of prediction were compared. 2.3. Analysis of birth weight deviation based on repeated estimations of fetal weight Birth weight deviation was also predicted from repeated estimations of fetal weight deviations at different gestational ages (a) as the average of two, three or four
’ Fetal weight = 2315.6
- 1.72679568-6 - GA4
- 26.46
* GA + 8.79618E-4
* GA3
Table 1 Clinical chara~te~stics of the pregnancies Term delivery n=421 Maternal age (years, mean SD.) Maternal height (cm, mean S.D.) Parity (median, range) Gestational age (days, mean S.D.) Birth weight (g, mean S.D.) Birth weight deviation (%, mean SD.)
Preteen delivery n = 57
27.6 * 4.9
26.9 f 5.2
167 zt 6
167 t 7
2 (l-8) 278 + 9
2 (I-6) 248 rt II
3367 f 438
2480 zt 322
-5 * 13
-6*
13
estimates of fetal weight deviations, and (b) by linear extrapolation from estimates on two occasions of rate of change of percentual deviation (trend) until delivery. The mean errors of prediction were compared. Finally, extrapolation by a second order polynomium fitted to four estimates was attempted. 2.4. Statistical methods The accuracy of prediction of weight deviation at birth by each single or combined ultrasound estimates was determined as the mean error, i.e. the root mean square of the difference between the predicted birth weight deviation and the actual birth weight deviation. The accuracy of the 14 different methods of prediction was compared pairwise and considered to differ significantly if the corresponding mean squares differed by the F-test at a level of 0.05 or less. 3. Results In the group with delivery at term, estimation of fetal weight deviation, in terms of mean error, significantly improved from 28 through 31, 34 to 37 weeks. Thus, prediction at 37 weeks was the most accurate (Table 2). Using two, three or four estimates of fetal weight deviation did not improve the prediction of birth weight deviation significantly, neither when average weight deviation nor when linear or second order polynomial extrapolation was used (Table 2). In the group with preterm delivery, the last estimate of fetal weight deviation before delivery was significantly more accurate for prediction of birth weight deviation than prediction based on an ultrasound scan at 28 weeks, or a linear extrapolation (trend) from estimates at 28 weeks and the last estimate before delivery, and as good as the average of estimated fetal weight deviation at 28 weeks and the last before delivery (Table 3).
T. Larsen et al. /European
Journal
of Obstetrics
Table 2 Accuracy of prediction of birth weight deviation in 421 term deliveries Basis of prediction of birth weight deviation
A single estimate of fetal weight deviation at: 28 weeks 31 weeks 34 weeks 37 weeks
Mean error of prediction (%)
11.7 (1) 10.7 (1) (2) 9.8 (3) (2) 8.8 (3)
The average of estimates of fetal weight deviation at: 10.4 28 and 31 weeks 28, 31, and 34 weeks 9.3 28, 31, 34, and 37 weeks 8.7 Linear change 28 and 31 and 28 and 34 and 31 and 28 and
extrapolation, from estimates on two occations, of rate of of percentual deviation: 31 weeks 30.9 34 weeks 22.9 34 weeks 15.8 37 weeks 13.2 37 weeks 11.2 37 weeks 10.3
Second order polynomial extrapolation, 28, 31, 34 and 37 weeks
10.9
Tbe accuracy based on a single estimate improved signi~cantly with increasing gestational age. Prediction based on more than one estimate was either significantly less accurate or no better. The numbers (l)-(3) relate to pairwise comparison, (t)-(3) P < 0.05.
4. Comment In clinical practice, great importance is attached to estimation of fetal weight by ultrasonography, especially when intrauterine growth retardation is suspected. On the basis of an ultrasound scan within 48 h before delivery, we found the error of birth weight prediction to be
Table 3 Accuracy of prediction of birth weight deviation in 57 preterm deliveries Basis of prediction of birth weight deviation
Mean error of prediction (%)
A single estimate of fetal weight deviation at: 28 weeks Last estimate before delivery
12.5 (1) 8.1 (1) (2) (3)
The average of estimates of fetal weight deviation at: 8.3 (3) 28 weeks and last estimate before delivery Linear extrapolation, from estimates on two occations, of rate of change of percentual deviation: 10.5 (2) 28 weeks and last estimate before delivery The numbers (l)-(3) relate to pairwise comparison (l)-(2) P < 0.05, (3) N.S.
& Gynecology
and Reproductive
Biology
60 (1995)
37-40
39
8.7% in a previous study [15]. The error in the present study was of the same order of magnitude when the ultrasound estimate at 37 weeks was used in term deliveries (8.8%), or when the ultrasound estimate nearest delivery was used in preterm deliveries (8.1%)).This is an important, and useful, reduction of uncertainty (variance) compared to the population variance of birth weight-for-gestational-age of 12-14% (Table 1). In settings where ultrasonography is offered as a routine, it is important to choose the optimal timing of examination in order to diagnose SGA. This implies that two opposite purposes should be taken into consideration. The ultrasound examination should be performed as late as possible to obtain the lowest error of prediction of birth weight but early enough for considering elective delivery, particularly in case of fetuses that are SGA early in the third trimester. To exemplify: at 37 weeks the error of prediction was as low as that 48 h before delivery and 71% of the term SGA fetuses were identified. A fetal weight estimate at this time makes it possible to decide whether to intervene or to remain expectant in the large majority of the pregnant women who will deliver at term. Of the 28 preterm infants in the present study who were SGA, 24 (86%) were diagnosed as SGA by ultrasound at 3 1 weeks. Furthermore, in 18 of these the ultrasound examination gave the first evidence of SGA. It has been proposed to use longitudinal fetal weight for direct diagnosis of subnormal intraute~ne growth [ 11,121 and extrapolation of the individual growth velocity until birth could thus be expected to improve precision of prediction. However, longitudinal ultrasound examinations with estimation of fetal weight at three-week intervals did not improve the prediction of birth weight. Also in pregnancies with preterm delivery, and thus with an assumed high incidence of subnormal rates of growth, a single fetal weight estimate shortly before delivery (in this study, the average was 11 days before) predicted birth weight better than longitudinal measurements during the third trimester. This means that the measurement error of ultrasound estimated fetal weight is too high for monitoring acceleration or deceleration in growth velocity in individual patients. From the present analysis, lon~tudinal ultrasound weight estimates (trend) seems of no use for prediction of subsequent fetal growth. It is reasonable to limit the number of ultrasound fetal weight estimations in the individual patient and use only one or two in the third trimester in an organised program: An early ‘dating scan’ and one or two later scans. One at 3 1 weeks of gestation to detect the small group of preterm SGA infants who are at great risk if undetected before term, and one at 37 weeks for detection of the much larger group of term SGA fetuses, so that elective delivery can be considered at term or before. Ultrasound biometry improves the detection rate of fetuses small-for-gestational-age
40
T. Larsen et al. /European Journal of Obstetrics & Gynecology and Reproductive Biology 60 (1995) 37-40
[5,6,8,13] but it does not necessarily identify only pathology. Additional ultrasound methods, i.e. fetal biophysical profile and Doppler ultrasound, may be helpful [17,18]. Acknowledgments The study was supported by grants from ‘Ville Heises Legat’ (26/89), ‘Sygekassemes Helsefond’ (37/89), ‘Dronning Louises Bornehospitals Forskningsfond’ (17/85 and 16/86) and ‘Lundbeckfonden’ (147/88). References 111Campbell S, Wilkin D. Ultrasonic measurement of fetal abdomen circumference in the estimation of fetal weight. Br J Obstet Gynaecol 1975; 82: 689-697. 121Sabbagha RE. Intrauterine growth retardation - antenatal diagnosis by ultrasound. Obstet Gynecol 1978; 52: 252-256. [31 Hadlock FP, Harrist RB, Sharman RS, Deter RS, Park SK. Estimation of fetal weight with the use of head, body, and femur measurement - A prospective study. Am J Obstet Gynecol 1985; 151: 333-337 141 Warsof SL, Gohari P, Berkowitz RL, Hobbins J. The estimation of fetal weight by computer-assisted analysis. Am J Obstet Gynecol 1977; 128: 881-892. 151 Eik-Nes SH, Okland 0, Aure JC. Ultrasound screening in pregnancy: A randomised controlled trial. Lancet 1984; 1347. 161 Nielson JP, Munjanja SP, Whitfield CR. Screening for small for dates fetuses: a controlled trial. Br Med J 1984; 289: 1179-l 182. 171 Secher NJ, Kern Hansen P, Lenstrup C, Sindberg Eriksen P. Controlled trial of ultrasound screening for light for gestational age (LGA) infants in late pregnancy. Eur J Obstet Gynecol Reprod Biol 1986; 23: 307-313.
PI Selbing A, Wichman K, RydCn G. Screening for detection of intrauterine growth retardation by means of ultrasound. Acta Obstet Gynecol Stand 1984; 63: 543-548. 191 Ewigman BG, Crane JP, Frigoletto FD, LeFevre ML, Bain RP, McNellis D, and the RADIUS Study Group. Effect of prenatal ultrasound screening on perinatal outcome. New Engl J Med 1993; 329: 821-827. [lOI Villar J, Belixan JM. The Timing Factor in the pathophysiology of the intrauterine growth retardation syndrome. Obstet Gynecol 1882; 37: 499-506. Hadlock FP, Deter RL, Harrist RB. Sonographic detection of abnormal fetal growth patterns. Clin Obstet Gynecol 1984; 27: 342-351. VI Seeds J. Impaired fetal growth: Ultrasound evaluation and clinical management. Obstet Gynecol 1984; 64: 577-584. u31 Larsen T, Falck Larsen J, Petersen S, Greisen G. Detection of small-for-gestational-age fetuses by ultrasound screening in a high risk population: a randomized controlled study. Br J Obstet Gynaecol 1992; 99: 469-474. iI41 Persson P-H, Weldner BM. Normal range growth curves for fetal biparietal diameter, occipital frontal diameter, mean abdominal diameters and femur length. Acta Obstet Gynecol Stand 1986; 65: 759-761. [I51 Larsen T, Petersen S, Greisen G, Falck Larsen J. Normal fetal growth evaluated by longitudinal ultrasound examinations. Early Hum Dev 1990; 24: 37-45. iI61 Eik-Nes SH, Grqttum P, Persson P-H, Marsal K. Prediction of fetal growth deviation by ultrasound biometry. Acta Obst Gynecol Scan 1982; 61: 53-58. 1171 Manning FA, Snijders R, Harman CR, Nicolaides K, Menticoglou S, Morrison I. Fetal biophysical profile score. Am J Obstet Gynecol 1993; 169: 755-763. WI Almstriim H, Axelsson 0, Cnattingius S et al. Comparison of umbilical-artery velocimetry and cardiotocography for surveillance of small-for-gestational-age fetuses. Lancet 1992; 340: 936-940.
European
Prediction of birth weight by ultrasound-estimated fetal weight: a comparison between single and repeated estimates Torben Larsen* a, Germ Greisenb, Sten Petersenb aDepartment of Ultrasound and Gynecology, Herlev Hospital, University of Copenhagen, DK-2730 Herlev, Denmark bDepartment of Neonatology. Rigshospitalet. University of Copenhagen, Copenhagen, Denmark
Accepted 20 January 1995
Abstract
Objectives: Ultrasound estimation of fetal weight is used for diagnosing intrauterine growth retardation. The aim of the present study was to assess the accuracy of birth weight prediction by use of a single or repeated estimations of fetal weight in the third trimester. Study Design: 1000 pregnant women considered at risk were scheduled to ultrasound estimation of fetal weight, using Warsofs formula, at 28, 31, 34 and 37 weeks of gestation. The 421 pregnancies with term delivery and complete set of ultrasound exa~nations and 57 pregnancies with preterm delivery with ultrasound examination at I6 and 28 weeks and once more before delivery were incfuded in the present analysis. Results: The accuracy of birth weight prediction improved sibilantly for every three weeks from 28 to 37 weeks of gestation in the term infants. Prediction based on the average of repeated weight estimates or linear extrapolation from two estimates or extrapolation by a second order ~l~omi~ fitted to four estimates did not improve accuracy compared to prediction based on the last estimate before delivery. Conclusion: When more than one ultrasound estimation of fetal weight are available, prediction of birth weight in relation to gestational age should be based on the last ultrasound examination only. Keywords: Ultrasound;
Fetal weight
1. Introduction Ultrasonography is regarded a valuable aid in diagnosing intrauterine growth retardation and is employed either on indication, if low fetal weight is suspected clinically in late pregnancy, or ag part of screening programmes [l-4]. In screening programmes or in settings with routine ultrasound examination of risk pregnancies, the ultrasound estimation of fetal weight is usually performed between 32 and 38 weeks of gestation [5-91. The reason for this is the assumption that growth retardation does not become apparent until late in pregnancy. In other words, we assume that in ~owth-re~rd~ fetuses the weight deviates negatively from the normal percentile during the third trimester of pregnancy. This is also the theoretical background for programmes of longitudinal measurements used in some
* Corresponding author. 002%2243/95/$09.50 0 1995 Elsevier SSDI 0028-2243(95)02079-S
institutions with the purpose of discriminating between biologically small fetuses growing along a low percentile (symmetrically growth-retarded) and fetuses with a growth velocity declining below the normal percentile (asymmetrically growth-retarded fetuses) [lo-121. The aim of the present study was to assess the accuracy of birth weight prediction based on a single ultrasound estimation of fetal weight at different times during the third trimester and to compare this with birth weight prediction based on longitudinal fetal weight estimation. 2. daters
and methods
The study ~p~ation was selected among 1000 highrisk pregnant women participating in a randomized study on detection of small-for-gestational-age (SGA) fetuses, described in detail previously [13]. The ultrasound examinations were scheduled with special refer-
Science Ireland Ltd. All rights reserved
38
T. Larsen et al. /European Journal of Obstetrics & Gynecology and Reproductive Biology 60 (1995) 37-40
ence to the above stated aims. Women who missed one or more exa~nations before the delivery were excluded from the analysis, leaving 478 women. This allowed us to evaluate the prediction of birth weight deviation at various times of ultrasound examination on an unvarying data set. The women were divided into two subgroups: (i) Women with term delivery (n = 421) in whom the first ultrasound examination was performed before 20 weeks of pregnancy and the subsequent ones at 28, 31, 34 and 37 weeks. The clinical characteristics of this group are presented in Table 1. (ii) Women with preterm delivery (n = 57) in whom the first ultrasound ex~ination was performed before 20 weeks of pregnancy and the subsequent ones at 28 weeks (in the period M-205 days) and at least once more before delivery. The clinical characteristics of this group are presented in Table 1. 2.1. Calculation of gestational age, estimated weight and weight deviation Estimate of actual gestational age was calculated from early biparietal diameter [ 141. In a pilot study we found acceptable agreement between actual birth weight and ultrasound estimated fetal weight, calculated according to Warsof et al. [4] from BPD and abdominal circumference. The mean deviation was 8.7%. Therefore, we used this equation in the present study. The inter- and intra-observer variations were 2.9% and 4.6%, respectively [15]. Normal fetal weight for actual gestational age was calculated using a reference curve based on longitudinal ultrasound weight estimates in normal pregnancies [IS] ‘. The ultrasound-estimated fetal weight and birth weight was expressed in percentage of deviation from expected mean weight for the day of gestation, as suggested by Eik-Nes et al. [ 161 2.2. Analysis of birth weight deviation based on a single estimation of fetal weight Birth weight deviation was predicted from ultrasound-estimated fetal weight deviation at 28, 31, 34 and 37 weeks of gestation assuming constant deviation from the gestational age specific reference. The mean difference between predicted fetal weight deviation at each gestational age and actual birth weight deviation and the mean errors of prediction were compared. 2.3. Analysis of birth weight deviation based on repeated estimations of fetal weight Birth weight deviation was also predicted from repeated estimations of fetal weight deviations at different gestational ages (a) as the average of two, three or four
’ Fetal weight = 2315.6
- 1.72679568-6 - GA4
- 26.46
* GA + 8.79618E-4
* GA3
Table 1 Clinical chara~te~stics of the pregnancies Term delivery n=421 Maternal age (years, mean SD.) Maternal height (cm, mean S.D.) Parity (median, range) Gestational age (days, mean S.D.) Birth weight (g, mean S.D.) Birth weight deviation (%, mean SD.)
Preteen delivery n = 57
27.6 * 4.9
26.9 f 5.2
167 zt 6
167 t 7
2 (l-8) 278 + 9
2 (I-6) 248 rt II
3367 f 438
2480 zt 322
-5 * 13
-6*
13
estimates of fetal weight deviations, and (b) by linear extrapolation from estimates on two occasions of rate of change of percentual deviation (trend) until delivery. The mean errors of prediction were compared. Finally, extrapolation by a second order polynomium fitted to four estimates was attempted. 2.4. Statistical methods The accuracy of prediction of weight deviation at birth by each single or combined ultrasound estimates was determined as the mean error, i.e. the root mean square of the difference between the predicted birth weight deviation and the actual birth weight deviation. The accuracy of the 14 different methods of prediction was compared pairwise and considered to differ significantly if the corresponding mean squares differed by the F-test at a level of 0.05 or less. 3. Results In the group with delivery at term, estimation of fetal weight deviation, in terms of mean error, significantly improved from 28 through 31, 34 to 37 weeks. Thus, prediction at 37 weeks was the most accurate (Table 2). Using two, three or four estimates of fetal weight deviation did not improve the prediction of birth weight deviation significantly, neither when average weight deviation nor when linear or second order polynomial extrapolation was used (Table 2). In the group with preterm delivery, the last estimate of fetal weight deviation before delivery was significantly more accurate for prediction of birth weight deviation than prediction based on an ultrasound scan at 28 weeks, or a linear extrapolation (trend) from estimates at 28 weeks and the last estimate before delivery, and as good as the average of estimated fetal weight deviation at 28 weeks and the last before delivery (Table 3).
T. Larsen et al. /European
Journal
of Obstetrics
Table 2 Accuracy of prediction of birth weight deviation in 421 term deliveries Basis of prediction of birth weight deviation
A single estimate of fetal weight deviation at: 28 weeks 31 weeks 34 weeks 37 weeks
Mean error of prediction (%)
11.7 (1) 10.7 (1) (2) 9.8 (3) (2) 8.8 (3)
The average of estimates of fetal weight deviation at: 10.4 28 and 31 weeks 28, 31, and 34 weeks 9.3 28, 31, 34, and 37 weeks 8.7 Linear change 28 and 31 and 28 and 34 and 31 and 28 and
extrapolation, from estimates on two occations, of rate of of percentual deviation: 31 weeks 30.9 34 weeks 22.9 34 weeks 15.8 37 weeks 13.2 37 weeks 11.2 37 weeks 10.3
Second order polynomial extrapolation, 28, 31, 34 and 37 weeks
10.9
Tbe accuracy based on a single estimate improved signi~cantly with increasing gestational age. Prediction based on more than one estimate was either significantly less accurate or no better. The numbers (l)-(3) relate to pairwise comparison, (t)-(3) P < 0.05.
4. Comment In clinical practice, great importance is attached to estimation of fetal weight by ultrasonography, especially when intrauterine growth retardation is suspected. On the basis of an ultrasound scan within 48 h before delivery, we found the error of birth weight prediction to be
Table 3 Accuracy of prediction of birth weight deviation in 57 preterm deliveries Basis of prediction of birth weight deviation
Mean error of prediction (%)
A single estimate of fetal weight deviation at: 28 weeks Last estimate before delivery
12.5 (1) 8.1 (1) (2) (3)
The average of estimates of fetal weight deviation at: 8.3 (3) 28 weeks and last estimate before delivery Linear extrapolation, from estimates on two occations, of rate of change of percentual deviation: 10.5 (2) 28 weeks and last estimate before delivery The numbers (l)-(3) relate to pairwise comparison (l)-(2) P < 0.05, (3) N.S.
& Gynecology
and Reproductive
Biology
60 (1995)
37-40
39
8.7% in a previous study [15]. The error in the present study was of the same order of magnitude when the ultrasound estimate at 37 weeks was used in term deliveries (8.8%), or when the ultrasound estimate nearest delivery was used in preterm deliveries (8.1%)).This is an important, and useful, reduction of uncertainty (variance) compared to the population variance of birth weight-for-gestational-age of 12-14% (Table 1). In settings where ultrasonography is offered as a routine, it is important to choose the optimal timing of examination in order to diagnose SGA. This implies that two opposite purposes should be taken into consideration. The ultrasound examination should be performed as late as possible to obtain the lowest error of prediction of birth weight but early enough for considering elective delivery, particularly in case of fetuses that are SGA early in the third trimester. To exemplify: at 37 weeks the error of prediction was as low as that 48 h before delivery and 71% of the term SGA fetuses were identified. A fetal weight estimate at this time makes it possible to decide whether to intervene or to remain expectant in the large majority of the pregnant women who will deliver at term. Of the 28 preterm infants in the present study who were SGA, 24 (86%) were diagnosed as SGA by ultrasound at 3 1 weeks. Furthermore, in 18 of these the ultrasound examination gave the first evidence of SGA. It has been proposed to use longitudinal fetal weight for direct diagnosis of subnormal intraute~ne growth [ 11,121 and extrapolation of the individual growth velocity until birth could thus be expected to improve precision of prediction. However, longitudinal ultrasound examinations with estimation of fetal weight at three-week intervals did not improve the prediction of birth weight. Also in pregnancies with preterm delivery, and thus with an assumed high incidence of subnormal rates of growth, a single fetal weight estimate shortly before delivery (in this study, the average was 11 days before) predicted birth weight better than longitudinal measurements during the third trimester. This means that the measurement error of ultrasound estimated fetal weight is too high for monitoring acceleration or deceleration in growth velocity in individual patients. From the present analysis, lon~tudinal ultrasound weight estimates (trend) seems of no use for prediction of subsequent fetal growth. It is reasonable to limit the number of ultrasound fetal weight estimations in the individual patient and use only one or two in the third trimester in an organised program: An early ‘dating scan’ and one or two later scans. One at 3 1 weeks of gestation to detect the small group of preterm SGA infants who are at great risk if undetected before term, and one at 37 weeks for detection of the much larger group of term SGA fetuses, so that elective delivery can be considered at term or before. Ultrasound biometry improves the detection rate of fetuses small-for-gestational-age
40
T. Larsen et al. /European Journal of Obstetrics & Gynecology and Reproductive Biology 60 (1995) 37-40
[5,6,8,13] but it does not necessarily identify only pathology. Additional ultrasound methods, i.e. fetal biophysical profile and Doppler ultrasound, may be helpful [17,18]. Acknowledgments The study was supported by grants from ‘Ville Heises Legat’ (26/89), ‘Sygekassemes Helsefond’ (37/89), ‘Dronning Louises Bornehospitals Forskningsfond’ (17/85 and 16/86) and ‘Lundbeckfonden’ (147/88). References 111Campbell S, Wilkin D. Ultrasonic measurement of fetal abdomen circumference in the estimation of fetal weight. Br J Obstet Gynaecol 1975; 82: 689-697. 121Sabbagha RE. Intrauterine growth retardation - antenatal diagnosis by ultrasound. Obstet Gynecol 1978; 52: 252-256. [31 Hadlock FP, Harrist RB, Sharman RS, Deter RS, Park SK. Estimation of fetal weight with the use of head, body, and femur measurement - A prospective study. Am J Obstet Gynecol 1985; 151: 333-337 141 Warsof SL, Gohari P, Berkowitz RL, Hobbins J. The estimation of fetal weight by computer-assisted analysis. Am J Obstet Gynecol 1977; 128: 881-892. 151 Eik-Nes SH, Okland 0, Aure JC. Ultrasound screening in pregnancy: A randomised controlled trial. Lancet 1984; 1347. 161 Nielson JP, Munjanja SP, Whitfield CR. Screening for small for dates fetuses: a controlled trial. Br Med J 1984; 289: 1179-l 182. 171 Secher NJ, Kern Hansen P, Lenstrup C, Sindberg Eriksen P. Controlled trial of ultrasound screening for light for gestational age (LGA) infants in late pregnancy. Eur J Obstet Gynecol Reprod Biol 1986; 23: 307-313.
PI Selbing A, Wichman K, RydCn G. Screening for detection of intrauterine growth retardation by means of ultrasound. Acta Obstet Gynecol Stand 1984; 63: 543-548. 191 Ewigman BG, Crane JP, Frigoletto FD, LeFevre ML, Bain RP, McNellis D, and the RADIUS Study Group. Effect of prenatal ultrasound screening on perinatal outcome. New Engl J Med 1993; 329: 821-827. [lOI Villar J, Belixan JM. The Timing Factor in the pathophysiology of the intrauterine growth retardation syndrome. Obstet Gynecol 1882; 37: 499-506. Hadlock FP, Deter RL, Harrist RB. Sonographic detection of abnormal fetal growth patterns. Clin Obstet Gynecol 1984; 27: 342-351. VI Seeds J. Impaired fetal growth: Ultrasound evaluation and clinical management. Obstet Gynecol 1984; 64: 577-584. u31 Larsen T, Falck Larsen J, Petersen S, Greisen G. Detection of small-for-gestational-age fetuses by ultrasound screening in a high risk population: a randomized controlled study. Br J Obstet Gynaecol 1992; 99: 469-474. iI41 Persson P-H, Weldner BM. Normal range growth curves for fetal biparietal diameter, occipital frontal diameter, mean abdominal diameters and femur length. Acta Obstet Gynecol Stand 1986; 65: 759-761. [I51 Larsen T, Petersen S, Greisen G, Falck Larsen J. Normal fetal growth evaluated by longitudinal ultrasound examinations. Early Hum Dev 1990; 24: 37-45. iI61 Eik-Nes SH, Grqttum P, Persson P-H, Marsal K. Prediction of fetal growth deviation by ultrasound biometry. Acta Obst Gynecol Scan 1982; 61: 53-58. 1171 Manning FA, Snijders R, Harman CR, Nicolaides K, Menticoglou S, Morrison I. Fetal biophysical profile score. Am J Obstet Gynecol 1993; 169: 755-763. WI Almstriim H, Axelsson 0, Cnattingius S et al. Comparison of umbilical-artery velocimetry and cardiotocography for surveillance of small-for-gestational-age fetuses. Lancet 1992; 340: 936-940.