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| Journal of the New Zealand Medical Association,
08-September-2006, Vol 119 No 1241
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Reliability of ultrasound estimation of fetal weight in term
singleton pregnancies
Atalie Colman, Dushyant Maharaj, John Hutton, Jeremy
Tuohy
The ultrasound estimation of fetal weight in term
pregnancies is used to determine growth, and this may affect the timing and
route of delivery.1–4 Although antenatal care has focused more on the
diagnosis of fetal growth restriction, the delivery of macrosomic infants is
associated with higher rates of adverse outcomes for both mother and infant in
comparison to the delivery of normal weight infants. Increased risks to the
large infant include shoulder dystocia, brachial plexus injury, perinatal
asphyxia, and neonatal death.5–7 Adverse maternal outcomes include
prolonged labour, genital tract trauma, postpartum haemorrhage, and a higher
rate of caesarean delivery.6–8
Whilst early delivery is the obvious management option for
growth-restricted term or near term infants,9 management of suspected fetal
macrosomia is less certain.2,10 Macrosomia has variously been defined as birth
weight >4000 g, >4500 g or >90th centile for weight by gestation.11 One
of the causes of fetal macrosomia is maternal diabetes.7,8 Ultrasound fetal
weight estimations are undertaken as part of the routine management of pregnant
women with diabetes. Ultrasound estimations of fetal weight are also undertaken
in cases where there is a clinical suspicion of abnormal growth. The appropriate
clinical response to an ultrasound diagnosis of macrosomia is unclear, in part
because the predictions have been considered unreliable.2,10,12,13
The aim of this study was to determine the reliability of
ultrasound estimation of fetal weight performed antenatally at Wellington
Hospital in singleton term pregnancies delivered within 7 days of the ultrasound
assessment, including the particular accuracy of the diagnosis of fetal
macrosomia when defined as >4000 g or >4500 g.
MethodData were collected retrospectively for the 7-year
period from 1 July 1998 to 30 June 2005. The study cohort consisted of term
infants (≥37 weeks gestation) who had undergone an ultrasound estimation
of fetal weight at the Wellington Perinatal Ultrasound Unit, and who delivered
<7 days after the measurement. All multiple pregnancies and stillbirths were
excluded. Estimated fetal weight data as recorded in the ultrasound database
were matched with data derived from the hospital’s Perinatal Information
Management System, in which maternal and infant information, including the
diagnosis of diabetes, was recorded perinatally. Where electronic information
was incomplete or uncertain, individual patient records were obtained and viewed
to affirm accuracy.
All ultrasound measurements were performed by trained
sonographers or obstetric specialists. Estimation of fetal weight was determined
using measurements of biparietal diameter (BPD), abdominal circumference (AC),
and femur length (FL), which were applied within the formula:
Estimated fetal weight = EXP [2.3026*(1.385 +
0.06739*BPD + 0.03591*AC – 0.00006883*BPD*AC + 0.1312*FL –
0.002675*AC*FL)].
This is a local modification of a formula described by
Woo14 and used at Wellington Hospital since 1996. It was developed after
analysis of 581 cases between 1990–1995, which showed this formula to have
the best prediction of birth weight (Personal Communication, Paula Carryer,
2002).
Statistical analyses were performed using the
Student’s t-test and linear regression analysis for parametric data and
the non-parametric Kruskal-Wallis, Mann-Whitney U tests, and Chi-squared tests
with p<0.05 considered significant. Data are presented as mean ±
standard deviation (SD).
ResultsOf the 20,649 term live-born singleton infants delivered at
Wellington Hospital during the study period, 1177 (5.7%) had undergone
ultrasound estimation of fetal weight <7 days preceding birth.
The mean actual birth weight within the study cohort was
3325 g (range 1620–5580 g). Ninety-eight infants (8%) weighed less than
2500 g, and 170 (14%) weighed more than 4000 g. Of the 170, 36 weighed more than
4500 g and three babies weighed more than 5000 g.
Forty-eight (4.1%) women in the study cohort had diabetes in
pregnancy. The birth weight of the infants born to these women was significantly
higher (3603±629 g) than in non-diabetic women (3314±604 g, n=1129)
(p=0.001).
The mean time interval between ultrasound estimation of
fetal weight and delivery was 2.9±1.8 days (n=1177) and did not differ
significantly between diabetic and non-diabetic women, nor amongst infants in
different birth weight categories. Within each birth weight category (<2500
g, 2500–4000 g and ≥4000 g), birth weight did not significantly
differ between diabetic and non-diabetic women.
The ultrasonic estimation of fetal weight significantly
correlated with actual birth weight for all infants (R=0.879, p<0.001)
(Figure 1). Seventy-five percent of all fetal weight estimations were within 10%
of actual birth weight; in one out of four women, the error was >10% (Table
1). The difference was >20% in 3% of the weight estimations.
Figure. 1 Scatter plot showing correlation between
birth weight and the ultrasound estimation of fetal weight (R=0.879, R2=0.772,
p<0.001, n=1177)
 The mean absolute error of fetal weight estimations was
7.0±5.7% (n=1177). This did not differ significantly between infants of
different birth weights (<2500 g, 7.3±6.4%, n=98; 2500–3999 g,
6.9±5.6%, n=909; ≥4000 g, 7.3±5.8%, n=170) nor between diabetic
and non-diabetic pregnancies (diabetic 8.3±6.5%, n=48; non-diabetic
6.9±5.6%, n=1129).
Table 1. Error distribution of estimated fetal
weights
Although the absolute percent errors for fetal weight
estimation were similar in each of the birth weight categories, the direction of
the error (i.e. underestimation vs overestimation) differed. The percentage of
infants whose birth weight was underestimated by more than 10% rose from 3% of
babies in the low birth weight group to 21% in the macrosomic group. Conversely,
the incidence of ultrasound overestimation of weight by more than 10% dropped
from 24% of infants in the low birth weight group to 5% in the macrosomic group.
Infants in the normal weight group were equally likely to have their weight
underestimated (12%) or overestimated (13%) by >10%. These trends remained
the same even when non-diabetic pregnancies were analysed separately (Table 1).
Although the absolute percent errors for fetal weight
estimation were similar for diabetic and non-diabetic pregnancies, infants of
diabetic mothers showed a tendency towards marked underestimation of weight (27%
of infants) rather than overestimation (2% of infants), and this trend was seen
in both normal weight infants and macrosomic infants (Table 1). This
underestimation of weight of infants of diabetic pregnancies (27% [CI:
17–41%]) was significantly more common in comparison to infants of
non-diabetic pregnancies (12% [CI: 10–14%]) (p<0.01). Only one infant
of low birth weight was born to a diabetic mother; its fetal weight estimation
was within 10% of birth weight.
These trends were also evident when signed percent errors
were examined in the different groups. The mean signed error for all fetal
weight estimations was
-0.2±9.0%—but when infants were analysed by weight category, the mean signed errors were as follows: <2500 g, +3.5±9.1% (n=98); 2500–3999 g, 0.0±8.8% (n=909); ≥4000 g, -3.3±8.7% (n=170). Thus the calculation of weight based on ultrasound
measurements tended to overestimate the weight of low birth weight infants while
underestimating the birth weight of large babies. Mean signed error was
0.0±8.0% in non-diabetic women (n=1129) but was -5.1±9.2% in diabetic
women (n=48), indicating that ultrasound tended to underestimate fetal weight in
women with diabetes in pregnancy. Linear regression analysis showed that birth
weight and diabetic status each had a significant and independent influence on
mean signed error (birth weight, p<0.001; diabetes, p=0.001).
The data were examined to determine the influence of time
interval between the ultrasound scan and delivery on accuracy of the fetal
weight estimation. There was no significant difference in the absolute error
between estimations made ≤3 days prior to delivery (7.0%±5.7, n=724)
and those performed 4–6 days before delivery (7.0%±5.7, n=453).
Ultrasound measurements carried out 4–6 days prior to delivery tended to
result in a slight underestimation of fetal weight (mean signed error =
-1.3%±8.9, n=453, p<0.01) whereas
ultrasound examination performed ≤3 days before delivery resulted in a
mean signed error that was not significantly different from zero (mean signed
error = +0.5%±9.0, n=724).
The ability of ultrasound fetal weight estimation to predict
fetal macrosomia in non-diabetic women when defined as ≥4000 g or
≥4500 g is shown in Tables 2 and 3, respectively. The cohort contained
only 48 diabetic pregnancies, of which only 13 resulted in macrosomic
deliveries; thus it was not possible to perform a meaningful analysis of
macrosomia prediction in pregnant women with diabetes.
Table 2. Ultrasound prediction of birth weight greater
than 4000 g in pregnancies of non-diabetic women
Sensitivity: 61% (CI:
53–68%), specificity: 96% (CI: 94–97%), positive predictive value:
69% (CI: 61–76%), negative predictive value: 94% (CI: 92–95%);
CI=confidence interval.
Table 3. Ultrasound prediction of birth weight greater
than 4500 g in pregnancies of non-diabetic women
Sensitivity: 50% (CI:
34–66%), specificity: 98% (CI: 97–99%), positive predictive value:
47% (CI: 32–63%), negative predictive value: 98% (CI: 97–99%);
C=confidence interval.
There were no cases of macrosomia in infants when the
estimated fetal weight was <3000 g. In the non-diabetic women, 3% (10/349) of
infants with fetal weight estimations of 3000–3499 g had actual birth
weights of more than 4000 g and 19% (52/273) of infants with fetal weight
estimations of 3500–3999 g had actual birth weights of more than 4000 g.
Conversely, 41% (41/100) of infants of non-diabetic women with fetal weight
estimations in the range 4000–4499 g actually weighed less than 4000 g at
birth. Sixteen percent (16/100) of estimated fetal weights in the range
4000–4499 g resulted in infants weighing more than 4500 g, and 53% (18/34)
of estimated fetal weights in the range 4500–4999 resulted in infants
weighing less than 4500 g.
DiscussionThis analysis of 1177 pregnancies is the largest study of
the reliability of ultrasound fetal weight estimation in New Zealand. The
ultrasound estimation of fetal weight at Wellington Hospital was associated with
a mean absolute error of 7%, a figure that compares favourably with other
published data.15–17
Three out of four (75%) fetal weight estimations were within
10% of actual birth weight—this rate is as good or better than in most
published studies (63%,15 74%,16 23–78%,17 52%,18 60%,19 and 74%20).
Although the accuracy of our estimations was comparatively
good, one out of every four fetal weight estimations was more than 10% different
from actual birth weight. Ultrasound measurements give the appearance of
precision, but the accuracy of ultrasonic estimations of fetal weight is limited
by the fact that the mature fetus is an irregular, three dimensional structure
of varying density, the weight of which cannot be calculated with certainty from
biometric measurements.12 It is therefore not surprising that the Australasian
Society for Ultrasound in Medicine states that “No formula for estimating
fetal weight has achieved an accuracy which enables us to recommend its
use,”21 despite the large number of formulae available.17,22
In our study there was an association between fetal size and
the direction of the weight estimation error. Thus, for the one in four infants
whose fetal weight estimation was more than 10% different from actual birth
weight, the error was generally one of overestimation in the case of the small
infants and an underestimation in the case of the macrosomic infants. These
trends have previously been documented in a systematic review of ultrasonic
estimation of fetal weight.22
However, our study did not confirm the findings of
others15,18 that the ultrasound estimation of fetal weight was less accurate in
macrosomic infants than in non-macrosomic infants. In our study both the mean
absolute percent error and the percentage of infants whose estimated fetal
weight was within 10% of birth weight were similar in all three weight groups.
The tendency of the ultrasound estimation of fetal weight to
err towards normal when the infant was subsequently found to be either <2500
g or ≥4000 g is important because the estimation of fetal weight is of
relevance in clinical decision-making at these extremes. The relationship
between birth weight and the direction of the estimation error was not due to a
bias in the time interval between ultrasound and delivery (as might occur if
smaller infants were scanned more regularly) as there was no relationship
between infant birth weight and the time interval between ultrasound and
delivery.
The reliable estimation of fetal weight is especially
important in diabetic pregnancies because these pregnancies are at greater risk
of macrosomia.7,8 Amongst pregnancies complicated by fetal macrosomia, shoulder
dystocia occurs more commonly in diabetic than non-diabetic women.5 Fetal weight
estimations have been reported to be less accurate in women with diabetes by
some authors,23,24 whereas others have found no difference in accuracy.15
In our study there was no significant difference in the
accuracy of fetal weight estimation between women with diabetes and non-diabetic
women. There was, however, a systematic underestimation (-5%) of fetal weight in
the women with diabetes. This underestimation was also noted by Wong et al
(2001),24 who attributed it to the greater liver size and the increased
subcutaneous fat that commonly occurs in fetuses of women with diabetes not
being reflected in the formulae used in ultrasound fetal weight estimation.
In this study, the ultrasound estimations of fetal weight
were performed <7 days prior to delivery. Although some authors studying
reliability of ultrasound estimation of fetal weight have included estimations
performed up to 14 days prior to delivery,25 others have restricted their data
to estimations performed within 7 days24 or 3 days,15,20 or have attempted to
correct for the time elapsed between the ultrasound and delivery by the addition
of 25 g per day26 or 12.4 g or 13.0 g per day (Nahum et al, 2003).17 Although
fetal weight estimations made 4–6 days before delivery tended to slightly
underestimate birth weight in our study, the error was small
(-1.3±8.9%).
Amongst the non-diabetic cohort, ultrasound estimation of
fetal weight detected only three out of every five infants weighing more than
4000 g and only half of the infants weighing more than 4500 g. Our findings thus
confirm those of others,10,13,20 that ultrasound does not reliably detect
macrosomia, at least in non-diabetic mothers. Until more reliable methods are
developed to determine fetal macrosomia, the use of ultrasound to assess fetal
weight in singleton term pregnancies must be interpreted with caution.
Author information:
Atalie Colman, Medical Student; Dushyant Maharaj, Senior Lecturer in Obstetrics
and Gynaecology; John Hutton, Professor of Reproductive Medicine; Wellington
School of Medicine and Health Sciences, Otago University, Wellington; Jeremy
Tuohy, Clinical Leader, Perinatal Ultrasound Unit, Wellington Hospital,
Wellington
Acknowledgements: We
are grateful to Keith Fisher for data retrieval and to Gordon Purdie for
statistical advice.
Correspondence:
Dushyant Maharaj, Senior Lecturer, Department of Obstetrics and Gynaecology,
Wellington School of Medicine and Health Sciences, Otago University, PO Box
7343, Wellington South. Fax: (04) 385 5943; email: dmaharaj@wnmeds.ac.nz
References:
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