Chapter Three
THIRD TRIMESTER MEASUREMENTS:
An Inter/Intra Sonographer Evaluation of Accuracy
3.1
Introduction
“From twenty five weeks to term, measurements do not necessarily relate to the period of
gestation but simply give a comparison to the mean values for the population at large.
The variation among normal fetuses is considerable, and in the third trimester a variation
of three weeks size on either side of the mean value may be seen.”
Garrett and Robinson
Third trimester pregnancy ultrasound assessment of fetal size is a common component of
obstetric care. Of particular importance are those pregnancies deemed to be at greater
risk with gestational diabetes, maternal hypertension, macrosomia, or intra uterine growth
retardation.
Although the accuracy of ultrasonic fetal measurements for weight
estimation was questioned by, amongst others, Sarmandal et al (1989) and also the
British Medical Ultrasound Society (BMUS) Fetal Measurements Working Party (1990),
many clinicians compare their clinical estimation with the ultrasonic result.
As
ultrasound practitioners it is important to perform fetal measurements accurately and
reproducibly so that when the figures are used for serial assessment of growth, or are
applied to a fetal weight formula, the results may be more reliable. These measurements
may impact on the outcome of a pregnancy (Dudley and Chapman 2002) with the
decision to intervene often based on the ultrasound findings. It is therefore essential to
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perform fetal parameter measurements, at all stages of pregnancy, in a consistent and
reproducible way. Anderson et al (1995) assessed ultrasonic prenatal diagnosis of fetal
anomalies, concluding that 2% of all pregnancies had an anomaly. Dudley and Chapman
noticed that over 90% of all antenatal ultrasound journal articles were written about these
2% of anomalies and that fetal measurements and quality control, which affects every
ultrasound examination, was almost neglected.
It has only been as recently as May 2001 that the Australasian Society for
Ultrasound in Medicine (ASUM) recommended the use of ultrasonic fetal measurement
charts based on an Australian population, to help standardize fetal size/growth reporting.
Previously a combination of different charts by, amongst others, American sonologists
Hadlock and Deter (1982) were used which made it difficult for an obstetric patient being
scanned at different practices, to get the most reliable assessment of fetal growth.
In Australia sonographers perform fetal measurements for gestational age and
fetal growth and weight evaluation. All sonographer training establishments in Australia
require a high standard of obstetric scanning ability, with particular reference to obtaining
the correct imaging plane and measuring of the individual fetal parameters before
registration as an accredited medical sonographer (AMS) with the Australasian
Sonographer Accreditation Register (ASAR). To remain registered sonographers must
undertake ongoing continuing professional development via conferences, seminars,
workshops or individual journal reading with the aim of maintaining an acceptable skill
level. Of particular importance is that their ability to competently scan and measure a
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fetus for size and growth remains, with a high inter and intra sonographer measuring
accuracy level.
As discussed in Chapter Two the relevant head image required for the
measurement of the biparietal diameter (BPD), occipito-frontal diameter (OFD) and head
circumference (HC) is a transverse axial plane, which includes the falx cerebri anteriorly
and posteriorly, cavum septum pellucidum anteriorly in the midline, and the thalami
(Image 3/1). The BPD should be measured at the widest point of the head from the outer
edge of the nearest parietal bone to the inner edge of the more distant parietal bone and
the OFD perpendicular to the BPD from mid to mid occipital bones.
The head
circumference should be traced either with an ellipse mode or manually around the outer
perimeter of the skull.
In late pregnancy it can be difficult to obtain the ideal imaging
plane due to the head lying low in the pelvis.
The imaging plane for the abdominal circumference (AC) is a true transverse cut
at the level of the fetal liver and stomach, including the left portal vein at the umbilical
region. Although the AC can be measured using the ellipse mode, in the third trimester it
is usually more precise to manually trace the perimeter of the abdomen, including the fat
layer. Long bones should be imaged in the axial plane to achieve the longest length, with
clean blunt ends and a strong acoustic shadow behind the bone (Image 3/5). Measuring
must be along the diaphyseal shaft, excluding the epiphysis.
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3.2
Aims of Objective Two
The aim of this study was to assess inter and intra-sonographer measurement
reproducibility of fetal variables in late pregnancy and to detect any common pitfalls of
measuring that could affect the report on fetal size.
3.3
Methodology
Using an HDI 5000 Ultrasound system (Philips Medical Systems), images were captured
for measurements of the BPD, OFD, HC, AC and long bones. Three images of a head,
abdomen and femur, were deliberately suboptimal, taken at awkward imaging planes to
reflect the difficulty of always obtaining ideal images to measure in late pregnancy. Two
pregnancies in the final 6 weeks of gestation were chosen for imaging and the dates were
confirmed based on the LMP and scans at both 12 and 19 weeks gestation. The ASUM
2001 Fetal Measurement Charts were used to estimate size.
Eighteen images from these two pregnancies were archived twice on to an optical
disc. This gave a total of 36 images to be measured. The optical disk was copied and
distributed to 20 Medical Imaging Australasia (MIA) practices throughout Australia
where HDI 5000 or 3000 systems were installed. Images were downloaded from the
optical disc to the hard drive of each ultrasound machine, and the systems recalibrated for
measuring imported images.
Following a point-by-point list of system instructions
(Appendix 5), sonographers scrolled through the collection of images, measured the
variables as prompted, and recorded the results both onto film and data sheets.
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Sonographers were asked to measure all images, even those deliberately taken at
awkward angles that they may not normally image. This was important, as difficult fetal
positions and perceiving edges for measuring are a reality in obstetric ultrasound.
To assess inter & intra-sonographer reproducibility, sonographers, both qualified
and unqualified (students), sequentially measured the 36 images.
Intra-sonographer
reproducibility was achieved, as the initial 18 images were each measured twice. The
images were randomly measured and no indication was given to the sonographers that
any of the images were in fact duplicated. No measuring instructions were given to
sonographers in regard to fetal variables, as it was important to determine whether correct
measuring procedures, as suggested by ASUM, were routinely followed in clinical
practice.
Assessment of calliper placement was by viewing the recorded images.
Sonographers were not given the option to refuse to measure the suboptimal images.
3.4
Results
The 26 sonographers from MIA practices in NSW who participated in the study had
scanning experience ranging from 6 months to 21 years with a mean of 8.9 years. Five of
the more senior sonographers no longer routinely perform obstetric scans and at their
own admission said they were out of practice in correct calliper placement. All 26
sonographers were disadvantaged as they did not physically perform the scans and this
can make image edge perception difficult.
The resultant measurements therefore,
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particularly those from the images at awkward angles, had to be assessed with this
perception problem noted.
3.4.1 BPD, OFD and Head Circumference
Image 3/1: Head imaged at an acceptable angle
Image 3/2: Head at difficult angle
Table 3/1: Range of head measurements.
Actual dates: 34wks – BPD 86 +/- 6mm(2SD), OFD 108+/- 5.5mm, HC 305 +/- 25mm.
BPD range
OFD range
Image 3/1
Image 3/2
83 – 88mm (mean 84mm)
84 – 88mm (mean 86mm)
32.5 to 35 weeks
33 to 35 weeks
100
-
107mm
(mean 97
104mm) 31 to 33 weeks
HC range
294
–
319mm
–
107mm
(mean
102mm) 29 to 33 weeks
(mean 295
–
328mm
(mean
305mm) 32.5 to 36.5 weeks 305mm) 32.5 to 38 weeks
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Image 3/1 represents an acceptable head image for measuring the biparietal diameter,
occipito-frontal diameter and head circumference in the third trimester. Twenty-one
sonographers measured the BPD at the widest section of the image and were within a
3mm range. The differences occurred with calliper placement from outer to inner and
outer to outer parietal bones.
The occipito-frontal diameter range was wider due to a poorly defined occipital
bone and this also translated to the head circumference differences.
The inter-
sonographer variability for the head circumference on this image was 8.1%. Image 3/2 is
a technically unacceptable image but was taken at an awkward angle to reflect the
problems often occurring with third trimester measuring. Once again the OFD and HC
range was due to perception of the occipital edge. All but one of the sonographers used
the ellipse-measuring mode for the head circumference which means that the
circumference didn’t always follow the outline of the skull, thus making a few
millimetres difference.
Comparing the measurements obtained from the optimum image (3/1) and the
suboptimal image (3/2) with a one-tailed t-test showed no statistically significant
difference.
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3.4.2 Abdominal Circumference
Image 3/3: Typical image
Image 3/4: Spine anterior
Table 3/2: Range of abdominal circumference measurements
Actual dates
Image 3/3
Image 3/4
34 weeks. AC 305mm
36 weeks. AC 317mm
Abdominal circumference 295
range
–
308mm
(mean 314
301mm) 33 to 34.5 weeks
–
338mm
(mean
321mm) 35 to 37.5 weeks
Image 3/3 abdominal circumference was performed well by all sonographers, even
considering the ill defined posterior edge. The inter-sonographer variability for this
image was 4.3%. Image 3/4, an often-typical 3rd trimester abdominal image, had poorly
defined abdominal wall yet the AC measurements were within normal range for the
gestation. Twenty-two of the 26 sonographers utilized the ellipse measuring facility for
both images which can be a source of error due to the often irregular shape of the
abdomen in late pregnancy, particularly when the cross section is non-elliptical.
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3.4.3 Femur Length
Image 3/5: Correct imaging plane
Image 3/6: Acute angle – unacceptable.
Table 3/3: Range of Femur Length Measurements
Actual dates
Image 3/5
Image 3/6
34 weeks FL 67mm
34 weeks
+/- 5.5mm (2SD)
Femur length range
66 – 71mm (mean 68mm)
66 – 76mm (mean 70mm)
34 to 36.5 weeks
34 to 40 weeks
Image 3/5 represents the ideal long bone imaging plane.
Twenty-three of the
measurements were within a 2mm range. The higher figures were due to measurement of
artefact. Image 3/6 was a technically unacceptable image plane for measuring long
bones, which was reflected in the wide range of results. The majority of sonographers
included the epiphysis and/or artefacts in their measurements. This fetal parameter in
particular emphasises the importance of a correct imaging plane for accurate assessment
of size. A long bone more than 15 degrees off the horizontal plane should not be used.
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3.4.4 Fetal Weight Estimation
The following Hadlock fetal weight formula incorporating the BPD, HC, AC and FL was
used for this study.
Log10 (EFW in grams) = 1.3596 – (0.00386 AC x FL) + (0.0064 HC) + (0.00061 BPD x AC) +
(0.0424 AC) + (0.174 FL). Standard deviation (EFW) = +/- 7.5% of EFW (predicted mean).
Table 3/4: Fetal Weight Variations
Measurements used.
Weight
+/- standard deviation
Lowest measurement
2212 grams
166 grams
For each variable.
(4lbs 14ozs)
Highest measurement
2657 grams
For each variable
(5lbs 14ozs)
Individual sonographer -
2243 grams
168 grams
2578 grams
190grams
199 grams
Mean of each variable (lowest).
Individual sonographer Mean of each variable (highest).
The weight range seen in Table 3/4 highlights the problem of predicting fetal weight.
Individual sonographers can vary slightly in their measuring techniques for each fetal
variable, leading to a difference of over 300 grams in weight estimation. The most
commonly used fetal weight formulas are those of Hadlock et al, using different
combinations of fetal parameters. All of these formulas have a standard deviation of
seven to ten percent. The sub-optimal images were not used in this analysis.
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3.4.5 Intra-Sonographer Results
Statistical analysis for this sub study was performed using SPSS for Windows Percentage
Exact Agreement (PEA) and Percentage Close Agreement (PCA) 1mm & 3mm. For
PEA, repeat measurements must be exactly the same to achieve 100%. PCA 1mm means
that the repeat can be within 1mm of the first measurement and PCA 3mm within a 3mm
range to achieve 100%. Intra-sonographer results were divided into 3 groups. Group (A)
had less than 4 years experience and included the student sonographers, Group (B) had 4
to 10 years experience and Group (C) with greater than 10 years experience. These
groups were further divided into the best and worst results, which helped distinguish
those sonographers without much obstetric experience.
Only this later group are
represented in the table below.
Table 3/5: Intra-Sonographer Results
Group (number)
Years Experience
Percentage
A. < 4years (6)
B. 4 – 10years (11)
C. > 10years (9)
Best
Worst
Best
Worst
Best
Worst
6mths
3yrs
6yrs
7yrs
12yrs
15yrs
16.7%
50%
16.7%
41.7%
16.7%
91.7%
25%
66.7%
41.7%
66.7%
33.3%
91.7%
58.3%
75%
66.7%
75%
41.7%
Exact 66.7%
Agreement
Percentage Close
Agreement (1mm)
Percentage Close
Agreement (3mm)
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3.5
Discussion
In this study, with few exceptions, sonographers measured the fetal variables in an
acceptable way. The only variables where some of the 52 measurements were not within
the accepted range of two standard deviations were the OFD in both image 3/1and 3/2
and the femur at the acute angle in image 3/6. Although the OFD on its own is not
utilised in any fetal weight formula it is important in the third trimester when head shape
can change due to moulding thus affecting the BPD and consequently the estimated fetal
weight. The OFD measurement is used in the calculation of the cephalic index (CI =
(BPD/OFD)x100%) and the BPD should only be used in a fetal weight formula if the CI
is within the normal range.
The abdominal circumference grows by approximately 10mm a week and so any
measuring error can place an IUGR fetus at risk of non-detection. Dudley and Chapman
(2002) found in their study that 53% of abdominal circumference errors were 5mm or
greater and 16% greater than 10mm, or a one-week error. Gull et al (2002) also found
the abdominal circumference to have the largest variation. This type of clinical error,
over or under measuring, could impact on clinical management decisions for intra-uterine
growth restriction (IUGR) or the macrosomic fetus and was also one of the components
of the study by Mongelli et al (1998) when they contrived a mathematical model of the
effect of time interval and ultrasound error when screening for IUGR.
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Inter/intra observer variability was an important component of the works of the
prominent sonologists Hadlock and Deter. Using the optimum head and abdominal
images (images 3/1, 3/3) and the measurements of all 26 sonographers, this study had an
inter-sonographer error of 8.1% for the head circumference and 4.3% for the abdominal
circumference compared with Hadlock’s study, with only two operators, of 1.2% for the
head circumference and 2.4% for the abdominal circumference. Measuring suboptimal
images may be a factor in inter-observer variability, as could the averaging of
measurements taken from both optimal and suboptimal images. The range of results for
the individual fetal variables, using the optimal images 3/1, 3/3 and 3/5, made a 445gram
or 17% difference in the fetal weight estimation. Using the individual sonographers
mean set of variables on these same images the difference was 335 grams or 12%. The
intra-observer error with 26 sonographers was 83gms, which compared favorably with
the study of Chang who concluded that ultrasound for fetal weight assessment was
reproducible with an intra-observer difference of less than 75gms. Gull et al (2002)
assessed the inter and intra- sonographer contribution to discrepancies in fetal weight
estimation. Using only three sonographers the BPD, HC, AC and femur length were
measured in 39 fetuses at term and the resultant individual measurements and weight
estimations were compared with the abdominal circumference showing the largest
discrepancy. There was a difference of 5.8% in estimated fetal weight between the three
sonographers.
One of the aims was to detect any common pitfalls of measuring that could affect
the report on fetal size. The most common problem was on images that were not
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standard measuring planes due to angulation. The best example was the long bones,
where both experienced and inexperienced sonographers measured the diaphysis,
epiphysis and/or artefact. This may have been due to the fact that the sonographers did
not personally perform the scans and so end point/edge perception was difficult to
evaluate. In the third trimester it is not always possible to capture the recommended
image plane for measuring a fetal variable. In this instance it may be necessary to accept
a sub-optimal image for measuring.
Experience with obstetric measurements was reflected in the final analysis, with
incorrect calliper placement more obvious by those sonographers who did not routinely
perform obstetric scans. Five of the sonographers involved in the study either did not
routinely perform third trimester scans, or had limited obstetric ultrasound exposure. It
was important to include this group of sonographers as they were qualified to perform
such examinations, regardless of how often they actually scanned pregnancies. This lack
of obstetric scanning experience was reflected in their measurements of both the
acceptable and sub-optimal images, with calliper placements less accurate than those
sonographers with more obstetric exposure. Three of these sonographers were the worst
in their respective group for intra-sonographer evaluation. Years of experience made no
difference to the accuracy of the measurements, with the best result obtained by a student
with six months of clinical practice and the worst by a fifteen-year veteran. All of the
student sonographers involved in the study were very precise with calliper placement,
which may reflect the constant supervision and reinforcement of correct measuring
procedure and practice in implementing what they have been taught during the obstetric
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component of their ultrasound course.
More veteran sonographers may become
complacent with measuring procedures.
The more accurate and reproducible measurements were achieved when the
recommended imaging planes were used as in images 3/1, 3/3 and 3/5. Although the
difference in using ellipse and tracing method for the head and abdominal circumference
was not statistically significant in this study, Chitty et al (1994) believed that a difference
of 3.5% in the two methods justify separate charts. Dudley and Chapman’s 2002 study
showed a difference of 1.3% and concluded that this may have been due to the
improvement in measuring facilities on the newer ultrasound machines.
One of the statistical reviewers for this article was critical of the simplistic
approach to the statistical analysis. When the paper was resubmitted it was thought
appropriate to explain why a more complicated approach was not taken. For the purpose
of this study, adjustment for correlations was not necessary and so a principal
components analysis to correlate the various measures was not performed. The aim was
to establish the agreement between sonographers, for each of the measures separately, as
these are the measures routinely taken by sonographers. Reducing the various measures
to a principal component would not shed light on the sonographers' agreement on any one
of them.
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3.6
Quality Control in the Third Trimester
Quality control in obstetric ultrasound is essential if clinically significant errors are to be
avoided. Obstetric ultrasound will always be limited by maternal body habitus, position
of the fetus, liquor volume, gestational age, the physical principles of ultrasound and the
time constraints of scanning.
Equipment maintenance, with particular reference to
calliper calibration, is an important aspect of quality control. By implementing ongoing
sonographer training, adherence to scanning protocols and audits for imaging planes and
measurement accuracy, improvements can be made in fetal growth assessment. The
failure to meet quality criteria results in errors.
In Australia ASUM has attempted to standardise ultrasonic fetal assessment of
size/growth by recommending scanning protocols, the use of particular measuring planes
and fetal measurement charts for reporting. It is hoped that sonographers and sonologists
adopt this policy so that all practices are taking measurements in a similar way and as a
result pregnant women can confidently move inter-practice throughout Australia for
ultrasonic examinations without jeopardising the report on fetal size.
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