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Chapter 20
Doppler Velocimetry and Multiple Gestation
Emanuel P. Gaziano, Ursula F. Harkness
The role for fetal Doppler sonography includes detecting
fetal growth restriction (FGR), serving as a
modality for fetal surveillance, predicting adverse
neonatal events, and determining the optimal time
for delivery. Multiple-gestation pregnancies afford a
unique opportunity for fetal Doppler application, as
abnormalities in growth and potential disturbances of
the fetal circulation are relatively common. Doppler
data add a physiologic parameter not previously
available to the clinician. By reflecting downstream
resistance to flow in the umbilical artery, it serves as
an accurate proxy for placental insufficiency [1]. By
including the cerebral circulation, inferences may be
made as to the redistribution of fetal blood flow under
conditions of chronic asphyxial stress, while multiple
vessel interrogation is useful in studying the extremes
of circulatory changes seen in some monochorionic
twins with twin transfusion syndrome
(TTS). In multiple as in singleton pregnancies, application
of fetal Doppler velocimetry permits the
assessment of the sequence of fetal cardiovascular
adjustment to asphyxial stress in the arterial and
venous circulations yielding useful insights into the
timing of delivery.
Among the risks for multiple gestations listed in
Table 20.1, Doppler velocimetry helps define the status
of the fetus with restricted growth, TTS, and congenital
anomalies. In these instances Doppler information
facilitates the appropriate and selective application
of perinatal resources. For decisions regarding
patient management, Doppler data become a part of
Table 20.1. Major complications: multiple gestation pregnancy
Preterm labor
Fetal malformations
Intrauterine growth restriction
Fetal loss (vanishing twin)
Twin-transfusion syndrome
Amniotic fluid volume changes: polyhydramnios, oligohydramnios
Preeclampsia
Cord prolapse or entanglement
Birth trauma
the clinical mosaic to be used in tandem with other
historical, examination, sonographic, laboratory, and
biophysical data.
Twin Prevalence
The prevalence of twin gestation varies from 1 to 5%
depending on the gestational age at assessment, as a
significant number of twins suffer intrauterine fetal
demise of one of the pair [2]. The overall incidence
of spontaneous twin gestations has declined, but reproductive
technologies, including ovulation induction
and surgical transfer of gametes or ova, have resulted
in an increasing number of high-order multiple
gestations [2].
Twin Placentation
Twins are derived from either a single ovum (monozygotic)
or two ova (dizygotic). Dizygotic twinning is
influenced by maternal central gonadotropin levels [3,
4] and varies among ethnic groups. For example,
Nigerian women have a high level and frequency of
twinning and Japanese women have a low level and
frequency. Occurring at a fixed rate of 1 per 250
births, proportionately more complications occur in
twin monozygotic pregnancies probably owing to
both placental vascular anastomosis and placental
asymmetry [5, 6].
Because dizygotic twin pregnancies arise from two
entirely separate placental disks, each gestational sac
has one amnion, one chorion, and rare vascular anastomosis
[5]. Diamniotic dichorionic gestations account
for about 80% of all twin pregnancies, and
their placentas may be fused or separate [7]. This
type of placentation also may include monozygotic
twins if the zygote divides early after fertilization.
The type of placentation seen in a given monozygotic
twin pregnancy depends on when in development
splitting occurs [8]. If division of the ovum occurs
before the chorion develops in the first 2 or 3 days, a
diamniotic dichorionic gestation will result [8]. If the
split occurs between 3 and 8 days post-fertilization, a