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a Chapter 20 Doppler Velocimetry and Multiple Gestation 315
Fig. 20.3. Deep arteriovenous
anastomosis in monochorionic
diamniotic twin
placentation. Twin-to-twin
transfer. IUGR intrauterine
growth restriction, AGA appropriate-for-gestational
age
weeks occur at gestational ages of 26 weeks or less
[15]. Whereas multiple gestations resulted in 3% of
live births in 2000, 14% of infant deaths in the same
year were multiples [16].
Studies on multiple gestations have focused on
using fetal Doppler data in three clinical areas: (a)
identification or prediction of growth restriction; (b)
the TTS; and (c) prediction of perinatal morbidity
and mortality. We address the use of Doppler velocimetry
for each of these separately.
Fig. 20.4. Monochorionic monoamniotic twin placentation
with no dividing membrane. Umbilical cords from the
twins are entangled
nic placentation, and perinatal mortality (25%±50%)
occurs principally with this type of placentation compared
with the 10% mortality in the diamniotic dichorionic
group [2, 11].
Figure 20.4 demonstrates a monoamniotic monochorionic
twin gestation and no dividing membrane.
These fetuses are at risk for cord entanglement and
death. Recently, 50% mortality was reported for pregnancies
with monoamniotic monochorionic placentation
[12].
Fetal Doppler and Complications
of Multiple Gestation
In the United States, the infant mortality rate among
multiple births was more than five times higher than
among singleton births in both 1989 and 1999 [13]
and the perinatal mortality was similarly increased
[14]. Two-thirds of the perinatal losses before 30
Growth Restriction and Discordance
Prior to 30 weeks' gestation, the major causes of neonatal
death relate to immaturity while stillbirths due
to growth restriction contribute a significant number
of perinatal losses after 32 weeks' gestation [15].
Growth restriction occurs in one-fourth of multiple
gestations [17]. Representing 1% of pregnancies,
twins account for 12% of early neonatal deaths and
17% of all growth-restricted infants [17]. Triplets and
other higher-order multiple gestations also lend a disproportionate
contribution to the number of growthrestricted
fetuses.
Evaluation of Fetal Growth
Two methods are used to evaluate fetal growth in
multiple pregnancies: (a) growth discordance (differences
in fetal weight frequently expressed as a percent);
and (b) longitudinal assessment of the weight
or growth for the individual fetus.
Since the introduction of Babson et al.'s correlation
between severe growth discordance and neonatal developmental
delay [18], the concept of differences in
weight (discordance) has gained popularity. No uniform
cutoff value is reported for growth discordance,
although 15% or more birth-weight difference between
twins is considered mild discordance, and
more than 25% birth-weight difference is classified as