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318 E.P. Gaziano, U. F. Harkness
Fig. 20.5. Discordant head size in a monochorionic twin
gestation
femur length and abdominal circumference are obtained
for 96% and 99% of fetuses, respectively [45];
thus, the ultrasonographically determined EFW is superior
to the BPD or femur length for predicting discordance
[46]. No apparent advantage is anticipated
using abdominal circumference alone for predicting
growth discordance or restriction, as it is part of
most formulas for estimating fetal weight. In addition,
multiple measurements allow potential classification
into symmetric and asymmetric patterns of restricted
growth.
Sensitivity for SGA prediction is reported as high
as 58% in the presence of an abnormal pulsed-wave
Doppler result from the umbilical artery, while Doppler
abnormality precedes ultrasonographic diagnosis
of FGR by an average of 3.7 weeks [32]. Combining
sonographic measurements with an abnormal Doppler
result may improve the sensitivity to 84% [32].
Both Hastie et al. [30] and Gaziano et al. [33] found
relatively low sensitivities (29% and 44%, respectively)
for SGA prediction in multiple gestations using
abnormal Doppler results in the umbilical artery. Gaziano
et al. [33] compared ultrasonographically determined
EFW (expressed as a percentile for the gestational
age at which it was obtained) with abnormal
umbilical artery velocimetry and found biometry to
be superior to abnormal Doppler results for SGA prediction.
The overall sensitivity for SGA prediction
with an ultrasonographic EFW of < 10th percentile
was 50%, and the best positive predictive value (PPV)
was 87.5%, achieved with an ultrasonographic EFW
of 35th percentile. Other authors have demonstrated
no difference in diagnostic accuracy for twin discordance
between Doppler velocimetry and ultrasonography
[29].
When Doppler waveform indices and serial measurements
of EFW and abdominal circumference were
compared in singletons, serial biometric measurements
were superior to a number of Doppler indices
for FGR prediction [47]. Similar findings are suggested
from the limited data on twin pregnancies.
Deter et al. [40, 41] developed a detailed ultrasound
biometric assessment technique applicable to
the twin fetus. It included measurement of head
circumference (HC), abdominal circumference (AC),
thigh circumference (ThC), femur diaphysis length
(FDL), and EFW. The authors recommended using a
multiple-parameter technique rather than standard
growth population curves for the detection of FGR in
twins. According to their data, single anatomic parameters
are inadequate for distinguishing the normal
twin from the growth-restricted twin. By assessing
the number and magnitude of deviations of the parameter
changes, all growth-restricted infants in 34
twin pregnancies were identified; none in the sample
were misclassified [41].
Cerebral Vessels
In one study of 17 consecutive twin pregnancies, the
sensitivity for FGR prediction was 83% for PI of internal
carotid artery (ICA) compared with 33% for
the PI of the umbilical artery [48]. Multiple vessel interrogation
may increase diagnostic accuracy, and in
singletons the ratio between the PI of the umbilical
artery and the PI of the internal carotid artery show
sensitivities of 84.2% for FGR prediction at a PPV of
97% [49]. The cerebral/umbilical artery ratios may
have a greater sensitivity for FGR prediction than
does a single-vessel evaluation [50].
Summary
Correlations between ultrasound biometry, Doppler,
and FGR identification in multiple gestations are
summarized as follows:
1. An abnormal umbilical artery Doppler result is
significantly associated with the birth of a growthrestricted
infant.
2. A wide range of accuracy for FGR detection is reported,
varying according to the population characteristics
and the technique employed.
3. Ultrasound biometry is superior to fetal Doppler
examination for purposes of diagnosis.
4. Multiple parameter measurements and serial measurements
are superior to final individual values
for either Doppler studies or biometry.
5. Combinations of biometric with Doppler measurements
and cerebral/umbilical artery ratios probably
increase the likelihood of FGR detection.
Other than methodologic or technical parameters,
a number of factors account for the differences
among the studies regarding FGR prediction. Not all