o_1977r8vv9vk1ts2ms0kd8pksa.pdf

criterion of a hemoglobin difference of > 5 g/dl. [59].
Classic TTS was confirmed in only 44% of fetuses by
direct injection of transfused blood to the donor
while simultaneously assessing evidence for transfused
cells in the recipient fetus by direct fetal blood
sampling [59].
Ultrasound evaluation of TTS includes a general
fetal anatomic survey to rule out gross anomalies and
an assessment of the fetuses for signs of fetal hydrops,
including scalp edema, ascites, hydrothorax,
and pericardial effusion. The sex of the fetuses should
be determined and membrane thickness assessed for
evidence of zygosity. To infer zygosity, placental localization
in multiple pregnancies is best assessed during
the early part of the second trimester (12±14
weeks) when fused or separate placentas can be distinguished.
The amniotic fluid volume in each sac should be
assessed, and detection of hydramnios or oligohydramnios
by a modified four-quadrant technique may
be helpful. A cutoff of 8 cm vertical depth for amniotic
volume is suggested [60]. In addition, the degree
of hydramnios is correlated with the probability of
fetal abnormality [60]. For twin pregnancies, abnormal
monochorionic placentation (TTS) is the most
frequently cited association with hydramnios [60].
The fetal weight should be estimated ultrasonographically
for each fetus using the formulas of Hadlock
et al. [61], Shepard et al. [62], or others and then
the EFW classified into a percentile ranking for the
gestational age at which it was determined. The individual
fetal growth can then be classified as appropriate
for gestational age (AGA), SGA, or LGA. On the
basis of Rossavik growth models and using detailed
measurements in normal twins, individual assessment
for the growth of the twins can be performed with
the same methods used for singletons and the results
are similar [63]. A multiple parameter individualized
growth assessment technique as described by Deter et
al. [40, 41] may be the most precise biometric method,
but its clinical application remains to be evaluated.
Standards of birth weight in twin gestation stratified
by placental chorionicity are a recent contribution.
Singleton charts tend to underestimate twin
growth at earlier gestational ages and overestimate
twin growth at later gestational ages [64].
Irrespective of the neonatal findings regarding hemoglobin
differences, the ultrasonographic findings
described previously portend a poor prognosis. Based
on ultrasound findings, Pretorius et al. [12] noted
possible TTS in nine twin pairs. Patients in their series
with TTS had a relatively high death rate. The
overall mortality rate for TTS depends on birth
weight and gestational age at delivery and is reported
to be as high as 70% [6]. Among pregnancies delivera
Chapter 20 Doppler Velocimetry and Multiple Gestation 319
FGR fetuses exhibit abnormal Doppler velocimetry. In
singleton fetuses, for example, the birth of an SGA infant
was associated with a normal umbilical artery
Doppler waveform in 44%±51% of such births [51,
52]. It is unknown what proportion of these infants
were constitutionally small instead of growth restricted.
Twin Transfusion Syndrome
Twin transfusion syndrome (TTS) and its manifestations
result in a high frequency of fetal loss and significant
neonatal morbidity. Becoming clinically evident
at the lower limits of fetal viability, intensive
maternal and fetal therapy may result in the birth of
viable but profoundly ill neonates. The complex vascular
arrangements possible within the placental circulations
of these monochorionic pregnancies suggest
an area of promise for fetal Doppler investigation.
The TTS is characterized by clinical findings
which include a recipient fetus who is usually appropriate
for gestational age (AGA) or large for gestational
age (LGA), and a donor fetus who is small or
growth restricted. The recipient's sac demonstrates
hydramnios that is sometimes massive. Oligohydramnios
is often present in the donor's sac. The marked
oligohydramnios results in the ªstuckº twin due to
compression from the recipient sac [53]. Ultrasonographically,
the separating membrane may be difficult
to visualize because of its close adherence to the donor
and its thin monochorial origin. Signs of fetal
hydrops or cardiac failure may develop in either twin,
usually the larger.
A scoring system has been proposed for TTS based
on sonography, Doppler findings, and cordocentesis
[54]. Sonographic and Doppler findings (minor)
include an abdominal circumference difference
>18 mm, poly- or oligohydramnios, signs of monozygosity,
and an intrapair S/D ratio of > 0.4. Major
criteria include evidence of transplacental shunt, a
birth-weight difference of >15%, and a hemoglobin
difference of >5 g/dl. Antenatally or postnatally two
criteria are needed (two major or one major and one
minor). Other authors have suggested variations on
these criteria for defining TTS [55, 56].
Although discordant fetal size, amniotic fluid differences,
concordant gender, and evidence of monochorial
placentation suggest TTS, neonatal criteria for
TTS, which include a hemoglobin differential of
> 5 g/dl, is not uniformly found. In four cases of TTS,
cordocentesis was performed and no intrapair hemoglobin
differences of > 2.7 g/dl were observed [57]. In
another study using cordocentesis, others have noted
significant hemoglobin differences in only one of nine
fetuses with TTS [58]. Ultrasonographic findings in
TTS do not always correlate with the classic neonatal