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316 E.P. Gaziano, U. F. Harkness
severe discordance [19±22]. When the weight difference
between twins is >25%, perinatal death rate is
increased by a factor of 2.5 and risk of fetal death is
increased by a factor of 6.5 [23].
Differences in ultrasound measurement parameters
or Doppler indices in twins are commonly reported
as the percent difference and are expressed as delta
change or intrapair difference. A number of studies
[24±35] rely upon the discordance concept and have
reported Doppler results accordingly.
The advent of high-resolution real-time ultrasonography
and duplex Doppler systems diminishes the
need to view multiple gestations only in terms of discordance
because the individual fetus in multiple gestations
can be independently and accurately assessed.
Expressing either Doppler or estimated fetal weight
(EFW) differences between twins is less useful, as
neonatal morbidity appears to be best predicted by
individual fetal assessment. Using 30% disparity in
birth weight, O'Brien et al. [36] were able to determine
only one in five growth-restricted twins. Bronsteen
and colleagues [37] evaluated 131 consecutive
sets of surviving infants and showed that individual
evaluation for intrauterine growth was more effective
than discordance or other classifications for predicting
adverse neonatal outcomes. Regarding neonatal
morbidity among twins, neither birth-weight discordance
of > 20% or > 25% was a factor for predicting
adverse events compared with individual birth-weight
percentiles of < 15th and < 10th, respectively. The
most clinically relevant outcomes, such as neonatal
death, congenital anomalies, small for gestational age
(SGA) and periventricular leukomalacia, are defined
by a birth-weight difference of 30% [38]. In a study
of 192 twin pairs who had ultrasound within 16 days
of delivery, intertwin estimated fetal weight discordance
of 25% or more had a sensitivity of 55%, specificity
of 97%, positive predictive value of 82%, and
negative predictive value of 91% for predicting actual
birth-weight discordance [39]. The antepartum diagnosis
of discordance was correct in 4 of 5 cases; however,
discordance may be overdiagnosed in almost
20% of cases and almost half of the significantly discordant
twin pairs were missed.
Gaziano et al. [33] focus on the individual fetus
and classify the Doppler value, EFW, abdominal circumference,
or other measurement parameter for
each fetus as a percentile for the gestational age at
which it is obtained. Deter et al. [40, 41] recommended
multiple individual parameter assessment
(individualized growth assessment) for the twin fetus
suspected of growth restriction. This approach simplifies
interpretation of clinical data, allowing the fetus
to be followed over time and permitting focused surveillance.
Doppler Sonographic Prediction
of Fetal Growth Restriction
A number of difficulties are inherent in assessing the
fetal Doppler results and their relationship to growth
restriction. Firstly, the heterogeneous cause of FGR in
singletons also apply to multiple gestations and can
be broadly categorized as follows: (a) malformations,
intrauterine infections, or chromosomal abnormalities;
(b) maternal vascular and nutritional deficits;
and (c) constitutionally small fetuses. In addition,
causes specifically related to twins must be considered,
such as twin transfusion, primary uterine or
placental pathology, and anomalies due to the twinning
process. Secondly, although the umbilical arteries
are relatively accessible compared with the
internal fetal vessels, careful establishment of individual
fetal circulations by real-time ultrasonography is
necessary to prevent sampling the same umbilical
circulation twice. This point is particularly important
for suspected TTS, as the smaller ªstuckº twin
with oligohydramnios may be relatively difficult to
image. Thirdly, studies differ in instrumentation,
pulsed-wave vs continuous-wave (CW) technology,
and the resistance index selected: systolic/diastolic
ratio (S/D); pulsatility index (PI); or resistance index
(RI). A variety of end points for Doppler-diagnosed abnormalities
are reported. Outcomes may be reported as
birth-weight differences of > 20% or > 25% and the cutoff
for defining growth restriction is reported at various
percentile ranges (< 10th, < 5th, or < 2.5th). Finally,
the latter definitions for FGR, as for singletons, are
not always satisfactory, as some constitutionally small
twins are normal in all other respects, while some
ªnormalº-weight fetuses may be growth restricted.
Pathologic studies suggest an anatomic lesion in
some cases of FGR. For example, histologic examination
of placentas shows fewer placental tertiary stem
villi in twin pregnancies complicated by ªplacental insufficiencyº
and abnormal S/D ratios than in fetuses
with normal S/D ratios [42]. These findings are similar
to those observed in some singleton pregnancies
with FGR and suggest a placental vascular pathology
rather than a uteroplacental one [1, 42]. Furthermore,
in twin pregnancies the PO 2 levels in the individual
fetus appear unaffected among a wide range of umbilical
RI values [43]. Differences in the RIs for twin
fetuses were of little value for detecting PO 2 differences
until the differences in the RI between the
fetuses was 76% [43].
Doppler prediction for fetal growth restriction has
been evaluated by a number of investigators [24±35].
Table 20.2 summarizes the experience of these investigators,
indicating the method of assessment, outcome
variable measured, and the sensitivity for SGA
prediction.