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Since antiquity, multiple births have fascinated mankind. Twins,
triplets, and higher order births attract public attention and bemusement that
often belies appreciation of specific management challenges not
associated with singleton pregnancies. In addition, certain complications
of pregnancy, such as preterm birth, occur more frequently with
higher order gestations. Optimal management requires knowledge of
issues associated with multifetal gestation during pregnancy, at
delivery, and into the neonatal period.
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Multifetal gestations are classified on the basis of zygosity
and chorionicity.1Dizygotic twins arise
from independent fertilization of separate ova. Typically, two placentas
form with distinct chorionic and amniotic membranes separating each
fetus. Monozygotic twins develop from a single
fertilized ovum that subsequently divides at or before the blastocyst
stage of embryogenesis. The factors promoting fission of the conceptus
are poorly understood. The timing of embryonic division determines
whether the monozygotic twins will be separated by amnion and chorion
(dichorionic, diamniotic), amnion only (monochorionic, diamniotic),
or not separated (monoamniotic, monochorionic). Dizygotic twins
account for about two thirds of spontaneous twin births and are
almost always dichorionic, diamniotic. Monochorionic twins share
a single placenta with a shared circulation. This developmental
phenomenon carries a significant mortality risk and may lead to complications
such as twin-twin transfusion syndrome (discussed under “Complications” in this
chapter).2 Higher order multiples can be a combination
of monozygotic and dizygotic gestations. Monozygotic triplets arising
from a single fertilized ovum are extremely rare.
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The twin birth rate in 2005 was 32.2 per 1000 births, a 42% increase
from 1990, and up 70% since 1980.3 The
incidence in 2005 was unchanged from 2004. It is not clear whether
this represents a true stabilization of a persistent upward trend
over the previous 25 years. The triplet+ birth rate (the
number of triplet, quadruplet, and higher order multiple births)
increased dramatically during the 1990s, peaking at 193.5 per 100,000
births in 1998. Since then, the rate has slowly declined to 161.8
per 100,000 births in 2005. However, this rate is still more than
fourfold higher than in 1981.
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The increase in twin and higher order multiple births has occurred
in conjunction with increases in maternal age and the use of assisted
reproductive technologies, ovulation-inducing agents, and artificial
insemination. It is noteworthy to consider that nearly half of very-low-birth-weight
multiples hospitalized in newborn intensive care units may involve pregnancies
assisted by infertility treatments.4 In the late
1990s, the American Society of Reproductive Medicine recommended
reducing the number of embryos transferred per cycle. This may explain
the decline in triplet+ births since that time.5 The
remarkable increase in multiple births is recognized as a significant public
health concern because of the associated mortality and morbidity
risks discussed next.
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Multifetal gestations exhibit certain complications also encountered
during a singleton gestation. Of these, the single most significant
is preterm delivery (see Table 46-1). Once
delivered, the preterm product of a multifetal gestation is at risk
of experiencing any ...