ultrasound diagnosis of fatal anomalies

15 Multiple Pregnancy
Determination of Zygosity
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Perinatal mortality is still three times higher in
twin gestations than in singleton pregnancies.
The reason for this appears to be a considerably
higher incidence of premature birth, placental
insufficiency, and congenital malformations in
twins than in singleton pregnancies. According
to the Hellin rule, the frequency of twin pregnancies
is one in 85 pregnancies, that of triplets one
in 85 2 , and that of quadruplets one in 85 3 . These
data apply to Caucasian populations. The
frequency of twin pregnancies varies considerably
in relation to geography and ethnic origin. In
Japan, there is one twin pregnancy to 155 singletons,
about half of that expected in Caucasians.
By contrast, there are regions of Nigeria in which
one twin pregnancy occurs for 25 singletons—
about three times higher than the rate predicted
by Hellin.
It is important to differentiate between dizygotic
and monozygotic twins, as risks in pregnancy
differ accordingly. Two-thirds of multiple
pregnancies are dizygotic and one-third are
monozygotic. The geographical variation in the
rate of twins is due only to the differing
frequency of dizygotic twins; the incidence of
monozygotic twins remains constant throughout
the world, at one in 250 pregnancies.
This natural incidence of twin pregnancies
has dramatically changed in recent years due to
advances in fertility therapies. Twin pregnancies
and multiple gestations are seen more
frequently today due to follicle stimulation, ovulation
treatments, and artificial insemination
therapies. This primarily affects dizygotic twins.
Monozygotic pregnancies arise from a single fertilized
ovum that splits at various times during
the first 2 weeks of embryogenesis. Depending
on the stage of embryogenesis at which the zygote
splits, four types of identical twins can result:
1. Dichorionic, diamniotic twins result from
the division of the zygote within the first 3 days
after fertilization. Each fetus has its own
placenta and is surrounded by two layers—the
chorion and amnion—and is thus separated from
the other fetus by four layers. In the prenatal
stage, these twins can not be differentiated from
dizygotic twins if they are of the same sex. This
applies to about 30% of monozygotic twin pregnancies.
2. Cellular division occurring between 3 and
8 days after fertilization results in monochorionic,
diamniotic twins. Although there are
two amniotic cavities, the placenta is shared by
the two fetuses. The twins are separated by only
two layers—the amnions of each twin. This constellation
is present in about 70% of monozygotic
twin pregnancies.
3. Monochorionic, monoamniotic twins occur
with division between the 8th and 13th day after
fertilization. The twins are not separated by
membranes. This pattern constitutes about 1% of
monozygotic pregnancies. It is associated with a
high rate of intrauterine mortality of almost 50%,
resulting from umbilical cord complications.
4. Conjoined twins are caused by incomplete
division of the embryonic disk after day 13. The
incidence is one in 70 000 completed pregnancies.
In monochorionic diamniotic twins (which always
arise from a single ovum), there is a special
risk that has to be considered, that of twin-totwin
transfusion syndrome (TTTS). Blood is transfused
from one twin (donor) to the other (recipient),
due to vascular shunts within the
joined placenta. Three forms of vascular shunt
have been described: connections between two
arteries (arterioarterial), between two veins
(venovenous), and between one artery and one
vein (arteriovenous). Although these vessel
anastomoses are present in 85% of monochorionic
pregnancies, clinical manifestation of
twin–twin transfusion syndrome occurs only in
10–15% of cases. This means that blood transfusion
between twins is a common phenomenon
and not an exception. Chronic twin-to-twin
transfusion syndrome is found only when there is
an imbalance of transfusion from one twin to the
other; arteriovenous shunts are responsible for
this. In addition to this chronic TTTS, there is also
an acute twin–twin transfusion syndrome arising
during birth in monochorionic twins. This may
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