- Email: [email protected]
The ultrasonographic estimation of fetal weight in the very low-birth weight infant
The ultrasonographic estimation of fetal weight in the very low-birth weight infant THOMAS
C.
BONNIE
J.
ROBERT San
KEY, DATTEL,
RESNIK,
Diego,
M.D. M.D. M.D.
California
The clinical estimation of fetal weight is notoriously inaccurate, especially when the fetus is very small. However, the ability to obtain easily an accurate estimate of the fetal weight is vital in the comprehensive evaluation and management of a pregnancy in which the delivery of a very low-birth weight infant is imminent. Recent ultrasonographic techniques have provided clinically useful assessments of fetal weight, but the ability of the techniques to estimate the birth weight of infants accurately in the highly critical, very low-birth weight range has not been established. In a prospective study of 50 consecutive pregnancies of women delivered prematurely of infants weighing 500 to 1,500 gm, estimates of fetal weight by means of ultrasonographic mensuration of the fetus and mathematical extrapolation to birth weight were compared with lieonatal weights. Each composite ultrasound measurement and mathematical relationship was compared statistically with neonatal measurements in 20 infants. The mean eiror of the estimated fetal weight was - 15.1 t 71.5 gm and the absolute error was 52.3 YC46.5 gm. The mean percentage error was -0.8 t 8.1. Estimates of birth weight were within 10% of the neonatal weight in 92% of the pregnancies studied, within 5% in 70%, and within 2.5% in 34%. The accuracy of the technique was uniform throughout the weight range studied with a mean absolute error of 5.0% +- 3.6%. All composite ultrasonographic measurements accurately predicted neonatal measurements. The accuracy of the technique was unaffected by the fetal presentation, the amount of amniotic fluid, or placental location. The technique is one that can be used by the obstetrician with basic real-time ultrasound skills on the labor and delivery service. (AM. J. OBSTET. GYNECOL. 145:574, 1983.)
AT THE CURRENT TIME great controversy exists over how aggressive the obstetric management should be when the delivery of a very low-birth weight infant is imminent.’ Currently, neonatal intensive care units are reporting improved neonatal survival rates and improved long-term prognosis for infants weighing between 750 and 1,500 gm, a fact which is forcing obstetricians to make management decisions based on expected fetal/neonatal weights.2 A major problem in the decision-making process is the inability to estimate fetal weight accurately prior to delivery. Efforts with clinical From the Department of Reproductive of Matewl-Fetal Medicine, University (San Diego). Received Revised Accepted
for publication June
26, 1982.
23, 1982.
September,
3, 1982.
Reprint requests: Thomas St., San Diego, Califrnia
574
March
Medicine, Division of Calz~ornia
C. Key, M.D., 92203.
225 Dickinson
parameters and the early ultrasonographic methods which were based on the measurements of the fetal biparietal diameter or abdominal circumference failed to predict fetal weight accurately.3-6 More recently, ultrasonographic techniques incorporating both the fetal biparietal diameter and abdominal circumference into a mathematical model or nomogram have provided useful and accurate information.7-s This report is a detailed account of our experience with estimating fetal weight in the very low-birth weight range by means of fetal measurements obtained with real-time ultrasonography. MhBthOdS Fifty women having high-risk pregnancies were delivered prematurely of infants weighing between 500 and 1,500 gm and were studied prospectively by means of real-time ultrasonography at the University of California (San Diego) Medical Center between Cktober 1, 1981, and May 31, 1982. Real-time ultra-
Ultrasonographic
sonography was performed with the use of an ADR Model 2 130 real-time scanner* coupled with a video monitor. All determinations were made by means of a 3.5 MHz transducer. Measurements were made with electronic calipers. All pregnancies were studied within 24 hours of delivery and no pregnancy was excluded from evaluation. All ultrasonograms were obtained by the authors. Biparietal diameters (BPDs) were measured at right angles to the longitudinal axis of the elliptical skull at a level at which a clear midline echo and easily discernible lateral ventricles could be visualized. Measurements from the outside of the anterior skull to the inner table of the posterior skull were made. The abdominal measurements were made at the level of the umbilical vein at right angles to the longitudinal axis of the fetus, from outer skin surface to outer skin surface. The transducer was carefully angled to exclude fetal heart and to ensure that the section was indeed at right angles to the longitudinal axis of the fetus. The anteroposterior abdominal diameter, FAD(AP), was measured through the spine posteriorly to the fetal abdominal surface anteriorly. The transverse abdominal diameter, FAD(TRANS), was measured at right angles to the FAD(AP) along the widest transverse abdominal plane. The fetal abdominal circumference (FAC) was mathematically calculated by means of formula 1: F.4C = ?‘FAD(AP)’
+ y(TRANS)’
estimation of tetal weight
575
Fig. 1. Real-time ultrasound-estimated fetal weight versus actual neonatal weight. The shaded arecl represents the 95% confidence interval for the ultrasound-predicted weight at a given actual neonatal weight.
performed on each measurement and calculation with each neonatal mensuration as the fixed variable. Differences between the ultrasonographically generated measurements and neonatal mensurations are reported in absolute, mean, and percentage error 2 standard deviation. Group and paired Student’s t tests were used for statistical comparisons as indicated.
1.
Results The formula for predicting fetal weight as reported by Warsof and associates7 was used to calculate the estimated fetal weight (formula 2) L.og,0 birth weight = - 1.599 + O.l44(BPD)
2.
+ 0.032 (FAC) - 0.11 (FAC)(BPD)* 1,000 An additional formula (formula 3), for determining fetal weights by means of ultrasonographically derived measures, reported by Shepard and co-workers,s was also evaluated. Log,, birth weight = -1.7492
+ O.l66(BPD) 3. 2.646(FAC)(BPD) + 0.046 (FAC) 1.ooo
All calculations were performed with a programmed electronic calculator. Following birth, anthropomorphic measurements of the first 20 infants studied were made in the nursery by the authors for comparison with those obtained with the use of ultrasound antenatally. Linear regression analysis by means of a method of least squares was *Advanced
Diagnostic Research, Tempe, Arizona.
By means of the formula of Warsof and colleagues (formula 2), the mean error in the estimated fetal weight was - 15.1 +- 71.5 gm. The absolute error was 52.3 IfI 48.5 gm. The estimation of fetal weight was within 10% of the neonatal weight in 92% of the pregnancies studied, within 5% in 70% of those studied, and within 2.5% in 34%. The mean percent error was -0.8% ? 6.1%. A high degree of correlation was established between the estimated fetal weight and the neonatal weight (r = 0.982, p < O.OOOl), as shown in Fig. 1. The slope of the regression line was 0.934 with an intercept of 50 gm. The estimated fetal weight was less than the actual weight of the infant in 28 cases and greater than the birth weight in 20 cases studied. The composite error analysis is shown in Table I. Although the mean absolute error was less in weight increments less than 1,000 gm, the mean absolute percentage errors in each weight increment were similar. The range of errors also was greater in the weight increments greater than 1,000 gm. This is graphically depicted in Fig. 1 which demonstrates the increase in rhe confidence interval with increasing fetal weight. When the formula of Shepard and associates (for-
576
Key,
Table
Dattel,
I. Composite
error
Weight range No.
h-N 500-750 751-1,000 l,OOl-1,250 1,251-1,500 Total
March 1, 1983 Am. J. Obstet. Gynecol.
and Resnik
analysis of ultrasonographically
Weight (mean 2 SD)
10
627
+- 108
+7.3
k 32.0
10
891 a 71
-2.1
2 35.2
BPD FAC FAD FAD
(AP) (TRANS)
A bsoltie error (mean t SD)
11 19
1116 1360
-+ 72 I? 131
-31.7 -31.9
t 61.5 2 98.1
25.7 28.0 59.5 70.4
50
1093
it 368
-15.1
“- 71.5
52.3
Table II. Composite measurement antenatally with ultrasonography Parameter
EwoT (mean C SD)
estimated
weights % Ewm (mean rt SD)
+ + + 5
18.7 18.1 27.1 45.4
+4.6
+ 0 f -3.7 2 -2.1 rt
8.5 4.1 4.4 6.5
+ 48.5
-0.8
+- 6.1
Absolute (mean
Mean 0.11 0.15 0.20 0.28
ewor + 2 r +
(mm) 2.12 10.5 4.61 12.1
Mean 0.39 0.56 0.71 0.91
% ewor f 2 2 2
3.72 5.68 3.68 4.10
E WIN range @4
2 r + 2
5.9 2.2 2.3 3.4
-60 -59 -115 -253
to 44 to 59 to 83 to 113
5.0 f
3.6
-253
to 113
7.1 3.2 5.2 4.5
errors and correlation statistics of 20 consecutive and by physical mensuration in the nursery
% error rt SD)
fetuses measured
Regression 0.977 0.832 0.785 0.835
slope
r 0.942* 0.885* 0.903* 0.829*
*p < 0.0001.
mula 3) was used to estimate fetal weight from the ultrasonographically derived measurements, the mean error was 40.2 + 109.5 gm and the absolute error was 79.7 2 84.6 gm. The correlation coefficient between the estimated fetal and actual neonatal weights was highly significant (r = 0.956, p < 0.0001). The slope of the regression line was 0.985 with an intercept of 49.5 gm. Thirty-two estimates were greater than the actual birth weight and 17 were less. The formula of Warsof and colleagues (formula 2), however, was more accurate in estimating fetal weight in 64% of the cases studied based on the absolute error of the individual predictions (t = 2.78, p < 0.01). An analysis of the composite measurement from our data and their errors are shown in Table II. Highly significant correlations existed between the ultrasonographic and neonatal measurements of the biparietal diameter, the FAD(AP) and the FAD(TRANS). Likewise, the calculated FAC (formula 1) correlated with the actual neonatal abdominal circumference. Table III lists the complications of pregnancy which necessitated premature delivery. The ultrasonographic findings in each pregnancy are listed in Table IV. The presence or absence of amniotic fluid, the presentation of the fetus, and the position of the placenta did not influence the accuracy of the technique. All pregnancies studied were singleton gestations.
Comment The decision to be aggressive in the obstetric management of a pregnancy complicated by the many disorders that eventuate in the delivery of a very low-
birth weight infant often revolves around a clinical estimation of fetal weight. Although neonatal intensive care units are reporting improved perinatal mortality rates and decreased long-term morbidity in infants whose birth weights are between 500 and 1,500 gm, significant differences exist in the perinatal outcomes between infants of each 100 gm increment within this weight range. lo The ability to adequately counsel the prospective parents concerning the potential benefits and risks of a specific perinatal decision in this birth weight range can be optimized with an accurate estimation of fetal weight. The time-honored tradition of abdominal palpation to estimate fetal weight is notoriously inaccurate, especially when the fetal weight is outside of normal ranges.3 Many investigators have used ultrasonographic measurements of the fetus to estimate fetal weight. Although ultrasonography can generate objective, reliable, and reproducible fetal measurements, attempts to relate single ultrasonographic measurements of fetal anatomy to birth weight have been inaccurate.4-6 Warsof and associates,’ using a computer-assisted statistical analysis, derived a mathematical formula (formula 2) which incorporated measurements of the fetal head (BPD) and the abdominal circumference (FAC) obtained at B-scan ultrasound to predict birth weight. The formula demonstrated that birth weight is best expressed as a logarithmic function of fetal body and head measurements. Ultrasonographic profiles of 85 fetuses over a wide weight range, 174 to 4,760 gm, were used in the derivation of the formula. The accuracy of the formula was then assessed prospectively in
Volume Number
Ultrasonographic
145 5
Table
Table III. Complications of pregnancy requiring premature deliverv Complication
Premature labor Severe pregnancy-induced hypertension Placenta previa Premature rupture of membranes: Occult cord prolapse Amnionitis Incompetent cervix
IV. Ultrasound
estimation of fetal weight
findings
I No.
28 4 4 24 : 1
32 patients who were delivered of infants weighing between 1,548 and 4,530 gm. Estimates of fetal weight were within 10% of birth weight in 78% of those pregnancies studies, and the absolute mean error was 228 gm, or 8.0% of birth weight. More recently, Shepard and colleagues,s who were from the same institution, reassessed the original formula of Warsof and co-workers.’ An additional formula (formula 3) was proposed, and the estimates of fetal weights generated from formula 2 and formula 3 were compared. They concluded that the estimates of fetal weight with formula 2 were more likely to be underestimates of actual neonatal weight. The mean error in birth weight prediction with the use of formula 3 was closer to zero than that with formula 2 (- 12.85 versus - 130.21 gm). They concluded that the estimates with formula 3 were more accurate predictions of the birth weight in 59% of the patients. They also noted that the error in the birth weight estimation was less in the subpopulation of infants weighing less than 2,500 gm (-71.1 t 212.4 gm with the use of formula 2 versus +24.7 2 218.8 gm). An analysis of the two formulas with our data confirms the findings of Shepard and associates in part. The estimates of fetal weight by means of formula 2 are more likely to be lower than neonatal weight. Conversely, the estimates of fetal weight with formula 3 were more likely to be overestimates of actual birth weight. However, the weight estimates with formula 2 were more accurate than those with formula 3. This observation is supported by an analysis of the individual regression equations generated by formula 2 and formula 3, which indicate that the two lines tend to converge in the low-birth weight ranges toward a common intercept. In the birth weight range less than 1,500 gm, formula 2 as originally set forth by Warsof and colleages appears to predict birth weight more accurately than does formula 3, although both formulas generate highly accurate estimations of fetal weight. The technique as described by Warsof and colleagues7 was modified by Ott’ for real-time ultrasound.
Presentation: Vertex Breech ‘Transverse lie Placental Location: Anterior Posterior Fundal Previa Amount of amniotic Normal Decreased Increased
577
‘VO
33 13 4 24 10 12
fluid: 14 32 4
Ott studied 101 fetuses including four sets of twins. Ultrasonographically estimated weights over a wide weight range were analyzed for accuracy. Incorporated within the study were 15 patients who were delivered of infants weighing less than 1,500 gm. The accuracy of the technique in this weight range appeared greater than that in larger weight ranges, and a higher degree of correlation between estimated and actual weight was suggested. Our study establishes the accuracy of this technique in estimating fetal weight in this critical birth weight range from 500 to 1,500 gm. the range at which obstetric decisions often have an impact on neonatal survival. Many factors contribute to the extreme accuracy of the technique in this limited weight range. The dynamics of the regression equations, the ability to identify body surfaces accurately with ultrasound in this weight range at which subcutaneous tissue deposition is minimal, and the relatively constant proportional contribution of head and body to fetal weight in the gestational ages commonly associated with weights less than 1,500 gm all contribute to the enhanced accuracy of the technique when applied to this very low-birth weight range. The technique appears uniformly accurate through the entire weight range studied. It can be universally applied in that the volume of amniotic fluid present, placental location, or presentation of the fetus did not influence the ability to accurately assess ieta1 weight. Additionally, the technology is available to the obstetrician with a modicum of real-time ultrasound experience and can be performed in the labor and deliver) area. The necessary calculations are easily performed by means of conventional calculators, or weights can be readily extrapolated from computer-derived tables.‘. n The information is available almost immediately for clinical use in the obstetric management plan when the delivery of the very low-birth weight infant threatens.
578
Key, Dattel, and Resnik
March Am. J.
REFERENCES
1. Dillon, W. P., and Egan, E. A.: Aggressive obstetric management in late second-trimester deliveries, Obstet. Gynecol. 58:685, 1981. 2. Stewart, A. L., Reynolds, E. 0. R., and Lipscomb, A. P.: Outcomes for infants of very low birth weights: Survey of world literature, Lancet 2: 1038, 1981. 3. Beazley, J. M., and Underhill, R. A.: Fallacy of fundal height, Br. Med. J. 4:404, 1970. 4. Kearney, K., Vigneron, N., Frischman, P., and Johnson, J. W. C.: Fetal weight estimated by ultrasonic measurement of abdominal circumference, Obstet. Gynecol. 51:156,
8.
9.
1978.
5. Kurjak, A., and Breyer, B.: Estimation of fetal weight by ultrasonic abdominometry, AM. J. OBSTET. GYNECOL. 125:962,
7.
1976.
6. Sabbagha, R. E., Hughey,
M., and Depp, R.: Method-
10.
1, 19x3
Obstet. Cynecol.
ology of B-scan sonar cephalometry with electronic calipers and correlation with fetal weight, Obstet. Gynecol. 51:383, 1978. Warsof, S. L., Gohari, P., Berkowitz, R. L., and Hobbins, J. C.: The estimation of fetal weight by computer-assisted analysis, AM. J. OBSTET. GYNECOL. 128:881, 1977. Shepard, M. J., Richards, V. A., Berkowitz, R. L., Warsof, S. L., and Hobbins, J. C.: An evaluation of two equations for predicting fetal weight by ultrasound, AM. J. OBSTET. GYNECOL. 142:47, 1982. Ott, W. J.: Clinical application of fetal weight determination by real-time ultrasound measurements, Obstet. Gynecol. 57:758, 1981. Bowes, W. A., Halgrimson, M., and Simmons, M. A.: Results of intensive perinatal management of very low-birth weight infants (501 to 1,500 grams), J, Reprod. Med. 23:245, 1979.
Bound volumes available to subscribers Bound volumes of the AMERICAN JOURNAL OF OBSTETRICS AND GYNECOLOGY are available to subscribers (only) for the 1983 issues from the Publisher, at a cost of $70.95 ($90.00 international) for Vol. 145 (J anuary-April), Vol. 146 (May-August), and Vol. 147 (September-December). Shipping charges are included. Each bound volume contains a subject and author index and all advertising is removed. Copies are shipped within 30 days after publication of the last issue in the volume. The binding is durable buckram with the JOURNAL name, volume number, and year stamped in gold on the spine. Payment must accompany all orders. Contact Mr. Deans Lynch at The C. V. Mosby Company, 11830 Westline Industrial Drive, St. Louis, Missouri 63141, USA. Subscriptions must be in force to qualify. Bound volumes are not available in place of a regular JOWAL subscription.
C.
BONNIE
J.
ROBERT San
KEY, DATTEL,
RESNIK,
Diego,
M.D. M.D. M.D.
California
The clinical estimation of fetal weight is notoriously inaccurate, especially when the fetus is very small. However, the ability to obtain easily an accurate estimate of the fetal weight is vital in the comprehensive evaluation and management of a pregnancy in which the delivery of a very low-birth weight infant is imminent. Recent ultrasonographic techniques have provided clinically useful assessments of fetal weight, but the ability of the techniques to estimate the birth weight of infants accurately in the highly critical, very low-birth weight range has not been established. In a prospective study of 50 consecutive pregnancies of women delivered prematurely of infants weighing 500 to 1,500 gm, estimates of fetal weight by means of ultrasonographic mensuration of the fetus and mathematical extrapolation to birth weight were compared with lieonatal weights. Each composite ultrasound measurement and mathematical relationship was compared statistically with neonatal measurements in 20 infants. The mean eiror of the estimated fetal weight was - 15.1 t 71.5 gm and the absolute error was 52.3 YC46.5 gm. The mean percentage error was -0.8 t 8.1. Estimates of birth weight were within 10% of the neonatal weight in 92% of the pregnancies studied, within 5% in 70%, and within 2.5% in 34%. The accuracy of the technique was uniform throughout the weight range studied with a mean absolute error of 5.0% +- 3.6%. All composite ultrasonographic measurements accurately predicted neonatal measurements. The accuracy of the technique was unaffected by the fetal presentation, the amount of amniotic fluid, or placental location. The technique is one that can be used by the obstetrician with basic real-time ultrasound skills on the labor and delivery service. (AM. J. OBSTET. GYNECOL. 145:574, 1983.)
AT THE CURRENT TIME great controversy exists over how aggressive the obstetric management should be when the delivery of a very low-birth weight infant is imminent.’ Currently, neonatal intensive care units are reporting improved neonatal survival rates and improved long-term prognosis for infants weighing between 750 and 1,500 gm, a fact which is forcing obstetricians to make management decisions based on expected fetal/neonatal weights.2 A major problem in the decision-making process is the inability to estimate fetal weight accurately prior to delivery. Efforts with clinical From the Department of Reproductive of Matewl-Fetal Medicine, University (San Diego). Received Revised Accepted
for publication June
26, 1982.
23, 1982.
September,
3, 1982.
Reprint requests: Thomas St., San Diego, Califrnia
574
March
Medicine, Division of Calz~ornia
C. Key, M.D., 92203.
225 Dickinson
parameters and the early ultrasonographic methods which were based on the measurements of the fetal biparietal diameter or abdominal circumference failed to predict fetal weight accurately.3-6 More recently, ultrasonographic techniques incorporating both the fetal biparietal diameter and abdominal circumference into a mathematical model or nomogram have provided useful and accurate information.7-s This report is a detailed account of our experience with estimating fetal weight in the very low-birth weight range by means of fetal measurements obtained with real-time ultrasonography. MhBthOdS Fifty women having high-risk pregnancies were delivered prematurely of infants weighing between 500 and 1,500 gm and were studied prospectively by means of real-time ultrasonography at the University of California (San Diego) Medical Center between Cktober 1, 1981, and May 31, 1982. Real-time ultra-
Ultrasonographic
sonography was performed with the use of an ADR Model 2 130 real-time scanner* coupled with a video monitor. All determinations were made by means of a 3.5 MHz transducer. Measurements were made with electronic calipers. All pregnancies were studied within 24 hours of delivery and no pregnancy was excluded from evaluation. All ultrasonograms were obtained by the authors. Biparietal diameters (BPDs) were measured at right angles to the longitudinal axis of the elliptical skull at a level at which a clear midline echo and easily discernible lateral ventricles could be visualized. Measurements from the outside of the anterior skull to the inner table of the posterior skull were made. The abdominal measurements were made at the level of the umbilical vein at right angles to the longitudinal axis of the fetus, from outer skin surface to outer skin surface. The transducer was carefully angled to exclude fetal heart and to ensure that the section was indeed at right angles to the longitudinal axis of the fetus. The anteroposterior abdominal diameter, FAD(AP), was measured through the spine posteriorly to the fetal abdominal surface anteriorly. The transverse abdominal diameter, FAD(TRANS), was measured at right angles to the FAD(AP) along the widest transverse abdominal plane. The fetal abdominal circumference (FAC) was mathematically calculated by means of formula 1: F.4C = ?‘FAD(AP)’
+ y(TRANS)’
estimation of tetal weight
575
Fig. 1. Real-time ultrasound-estimated fetal weight versus actual neonatal weight. The shaded arecl represents the 95% confidence interval for the ultrasound-predicted weight at a given actual neonatal weight.
performed on each measurement and calculation with each neonatal mensuration as the fixed variable. Differences between the ultrasonographically generated measurements and neonatal mensurations are reported in absolute, mean, and percentage error 2 standard deviation. Group and paired Student’s t tests were used for statistical comparisons as indicated.
1.
Results The formula for predicting fetal weight as reported by Warsof and associates7 was used to calculate the estimated fetal weight (formula 2) L.og,0 birth weight = - 1.599 + O.l44(BPD)
2.
+ 0.032 (FAC) - 0.11 (FAC)(BPD)* 1,000 An additional formula (formula 3), for determining fetal weights by means of ultrasonographically derived measures, reported by Shepard and co-workers,s was also evaluated. Log,, birth weight = -1.7492
+ O.l66(BPD) 3. 2.646(FAC)(BPD) + 0.046 (FAC) 1.ooo
All calculations were performed with a programmed electronic calculator. Following birth, anthropomorphic measurements of the first 20 infants studied were made in the nursery by the authors for comparison with those obtained with the use of ultrasound antenatally. Linear regression analysis by means of a method of least squares was *Advanced
Diagnostic Research, Tempe, Arizona.
By means of the formula of Warsof and colleagues (formula 2), the mean error in the estimated fetal weight was - 15.1 +- 71.5 gm. The absolute error was 52.3 IfI 48.5 gm. The estimation of fetal weight was within 10% of the neonatal weight in 92% of the pregnancies studied, within 5% in 70% of those studied, and within 2.5% in 34%. The mean percent error was -0.8% ? 6.1%. A high degree of correlation was established between the estimated fetal weight and the neonatal weight (r = 0.982, p < O.OOOl), as shown in Fig. 1. The slope of the regression line was 0.934 with an intercept of 50 gm. The estimated fetal weight was less than the actual weight of the infant in 28 cases and greater than the birth weight in 20 cases studied. The composite error analysis is shown in Table I. Although the mean absolute error was less in weight increments less than 1,000 gm, the mean absolute percentage errors in each weight increment were similar. The range of errors also was greater in the weight increments greater than 1,000 gm. This is graphically depicted in Fig. 1 which demonstrates the increase in rhe confidence interval with increasing fetal weight. When the formula of Shepard and associates (for-
576
Key,
Table
Dattel,
I. Composite
error
Weight range No.
h-N 500-750 751-1,000 l,OOl-1,250 1,251-1,500 Total
March 1, 1983 Am. J. Obstet. Gynecol.
and Resnik
analysis of ultrasonographically
Weight (mean 2 SD)
10
627
+- 108
+7.3
k 32.0
10
891 a 71
-2.1
2 35.2
BPD FAC FAD FAD
(AP) (TRANS)
A bsoltie error (mean t SD)
11 19
1116 1360
-+ 72 I? 131
-31.7 -31.9
t 61.5 2 98.1
25.7 28.0 59.5 70.4
50
1093
it 368
-15.1
“- 71.5
52.3
Table II. Composite measurement antenatally with ultrasonography Parameter
EwoT (mean C SD)
estimated
weights % Ewm (mean rt SD)
+ + + 5
18.7 18.1 27.1 45.4
+4.6
+ 0 f -3.7 2 -2.1 rt
8.5 4.1 4.4 6.5
+ 48.5
-0.8
+- 6.1
Absolute (mean
Mean 0.11 0.15 0.20 0.28
ewor + 2 r +
(mm) 2.12 10.5 4.61 12.1
Mean 0.39 0.56 0.71 0.91
% ewor f 2 2 2
3.72 5.68 3.68 4.10
E WIN range @4
2 r + 2
5.9 2.2 2.3 3.4
-60 -59 -115 -253
to 44 to 59 to 83 to 113
5.0 f
3.6
-253
to 113
7.1 3.2 5.2 4.5
errors and correlation statistics of 20 consecutive and by physical mensuration in the nursery
% error rt SD)
fetuses measured
Regression 0.977 0.832 0.785 0.835
slope
r 0.942* 0.885* 0.903* 0.829*
*p < 0.0001.
mula 3) was used to estimate fetal weight from the ultrasonographically derived measurements, the mean error was 40.2 + 109.5 gm and the absolute error was 79.7 2 84.6 gm. The correlation coefficient between the estimated fetal and actual neonatal weights was highly significant (r = 0.956, p < 0.0001). The slope of the regression line was 0.985 with an intercept of 49.5 gm. Thirty-two estimates were greater than the actual birth weight and 17 were less. The formula of Warsof and colleagues (formula 2), however, was more accurate in estimating fetal weight in 64% of the cases studied based on the absolute error of the individual predictions (t = 2.78, p < 0.01). An analysis of the composite measurement from our data and their errors are shown in Table II. Highly significant correlations existed between the ultrasonographic and neonatal measurements of the biparietal diameter, the FAD(AP) and the FAD(TRANS). Likewise, the calculated FAC (formula 1) correlated with the actual neonatal abdominal circumference. Table III lists the complications of pregnancy which necessitated premature delivery. The ultrasonographic findings in each pregnancy are listed in Table IV. The presence or absence of amniotic fluid, the presentation of the fetus, and the position of the placenta did not influence the accuracy of the technique. All pregnancies studied were singleton gestations.
Comment The decision to be aggressive in the obstetric management of a pregnancy complicated by the many disorders that eventuate in the delivery of a very low-
birth weight infant often revolves around a clinical estimation of fetal weight. Although neonatal intensive care units are reporting improved perinatal mortality rates and decreased long-term morbidity in infants whose birth weights are between 500 and 1,500 gm, significant differences exist in the perinatal outcomes between infants of each 100 gm increment within this weight range. lo The ability to adequately counsel the prospective parents concerning the potential benefits and risks of a specific perinatal decision in this birth weight range can be optimized with an accurate estimation of fetal weight. The time-honored tradition of abdominal palpation to estimate fetal weight is notoriously inaccurate, especially when the fetal weight is outside of normal ranges.3 Many investigators have used ultrasonographic measurements of the fetus to estimate fetal weight. Although ultrasonography can generate objective, reliable, and reproducible fetal measurements, attempts to relate single ultrasonographic measurements of fetal anatomy to birth weight have been inaccurate.4-6 Warsof and associates,’ using a computer-assisted statistical analysis, derived a mathematical formula (formula 2) which incorporated measurements of the fetal head (BPD) and the abdominal circumference (FAC) obtained at B-scan ultrasound to predict birth weight. The formula demonstrated that birth weight is best expressed as a logarithmic function of fetal body and head measurements. Ultrasonographic profiles of 85 fetuses over a wide weight range, 174 to 4,760 gm, were used in the derivation of the formula. The accuracy of the formula was then assessed prospectively in
Volume Number
Ultrasonographic
145 5
Table
Table III. Complications of pregnancy requiring premature deliverv Complication
Premature labor Severe pregnancy-induced hypertension Placenta previa Premature rupture of membranes: Occult cord prolapse Amnionitis Incompetent cervix
IV. Ultrasound
estimation of fetal weight
findings
I No.
28 4 4 24 : 1
32 patients who were delivered of infants weighing between 1,548 and 4,530 gm. Estimates of fetal weight were within 10% of birth weight in 78% of those pregnancies studies, and the absolute mean error was 228 gm, or 8.0% of birth weight. More recently, Shepard and colleagues,s who were from the same institution, reassessed the original formula of Warsof and co-workers.’ An additional formula (formula 3) was proposed, and the estimates of fetal weights generated from formula 2 and formula 3 were compared. They concluded that the estimates of fetal weight with formula 2 were more likely to be underestimates of actual neonatal weight. The mean error in birth weight prediction with the use of formula 3 was closer to zero than that with formula 2 (- 12.85 versus - 130.21 gm). They concluded that the estimates with formula 3 were more accurate predictions of the birth weight in 59% of the patients. They also noted that the error in the birth weight estimation was less in the subpopulation of infants weighing less than 2,500 gm (-71.1 t 212.4 gm with the use of formula 2 versus +24.7 2 218.8 gm). An analysis of the two formulas with our data confirms the findings of Shepard and associates in part. The estimates of fetal weight by means of formula 2 are more likely to be lower than neonatal weight. Conversely, the estimates of fetal weight with formula 3 were more likely to be overestimates of actual birth weight. However, the weight estimates with formula 2 were more accurate than those with formula 3. This observation is supported by an analysis of the individual regression equations generated by formula 2 and formula 3, which indicate that the two lines tend to converge in the low-birth weight ranges toward a common intercept. In the birth weight range less than 1,500 gm, formula 2 as originally set forth by Warsof and colleages appears to predict birth weight more accurately than does formula 3, although both formulas generate highly accurate estimations of fetal weight. The technique as described by Warsof and colleagues7 was modified by Ott’ for real-time ultrasound.
Presentation: Vertex Breech ‘Transverse lie Placental Location: Anterior Posterior Fundal Previa Amount of amniotic Normal Decreased Increased
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‘VO
33 13 4 24 10 12
fluid: 14 32 4
Ott studied 101 fetuses including four sets of twins. Ultrasonographically estimated weights over a wide weight range were analyzed for accuracy. Incorporated within the study were 15 patients who were delivered of infants weighing less than 1,500 gm. The accuracy of the technique in this weight range appeared greater than that in larger weight ranges, and a higher degree of correlation between estimated and actual weight was suggested. Our study establishes the accuracy of this technique in estimating fetal weight in this critical birth weight range from 500 to 1,500 gm. the range at which obstetric decisions often have an impact on neonatal survival. Many factors contribute to the extreme accuracy of the technique in this limited weight range. The dynamics of the regression equations, the ability to identify body surfaces accurately with ultrasound in this weight range at which subcutaneous tissue deposition is minimal, and the relatively constant proportional contribution of head and body to fetal weight in the gestational ages commonly associated with weights less than 1,500 gm all contribute to the enhanced accuracy of the technique when applied to this very low-birth weight range. The technique appears uniformly accurate through the entire weight range studied. It can be universally applied in that the volume of amniotic fluid present, placental location, or presentation of the fetus did not influence the ability to accurately assess ieta1 weight. Additionally, the technology is available to the obstetrician with a modicum of real-time ultrasound experience and can be performed in the labor and deliver) area. The necessary calculations are easily performed by means of conventional calculators, or weights can be readily extrapolated from computer-derived tables.‘. n The information is available almost immediately for clinical use in the obstetric management plan when the delivery of the very low-birth weight infant threatens.
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REFERENCES
1. Dillon, W. P., and Egan, E. A.: Aggressive obstetric management in late second-trimester deliveries, Obstet. Gynecol. 58:685, 1981. 2. Stewart, A. L., Reynolds, E. 0. R., and Lipscomb, A. P.: Outcomes for infants of very low birth weights: Survey of world literature, Lancet 2: 1038, 1981. 3. Beazley, J. M., and Underhill, R. A.: Fallacy of fundal height, Br. Med. J. 4:404, 1970. 4. Kearney, K., Vigneron, N., Frischman, P., and Johnson, J. W. C.: Fetal weight estimated by ultrasonic measurement of abdominal circumference, Obstet. Gynecol. 51:156,
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5. Kurjak, A., and Breyer, B.: Estimation of fetal weight by ultrasonic abdominometry, AM. J. OBSTET. GYNECOL. 125:962,
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6. Sabbagha, R. E., Hughey,
M., and Depp, R.: Method-
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1, 19x3
Obstet. Cynecol.
ology of B-scan sonar cephalometry with electronic calipers and correlation with fetal weight, Obstet. Gynecol. 51:383, 1978. Warsof, S. L., Gohari, P., Berkowitz, R. L., and Hobbins, J. C.: The estimation of fetal weight by computer-assisted analysis, AM. J. OBSTET. GYNECOL. 128:881, 1977. Shepard, M. J., Richards, V. A., Berkowitz, R. L., Warsof, S. L., and Hobbins, J. C.: An evaluation of two equations for predicting fetal weight by ultrasound, AM. J. OBSTET. GYNECOL. 142:47, 1982. Ott, W. J.: Clinical application of fetal weight determination by real-time ultrasound measurements, Obstet. Gynecol. 57:758, 1981. Bowes, W. A., Halgrimson, M., and Simmons, M. A.: Results of intensive perinatal management of very low-birth weight infants (501 to 1,500 grams), J, Reprod. Med. 23:245, 1979.
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