Diagnostic ultrasound ( PDFDrive )

CHAPTER
32
Multifetal Pregnancy
Mary C. Frates
SUMMARY OF KEY POINTS
• Multifetal pregnancies have higher rates of perinatal
morbidity and mortality than singletons.
• Chorionicity is the major determining feature for the
inherent unique complications faced by multiple
gestations.
• Fetal growth differences and congenital malformations are
increased in all types of multiple gestations.
• In monochorionic twins with a single demise, there is a
high risk of severe cerebral and other injuries in the
survivor.
• Sonography permits the diagnosis of syndromes unique to
monochorionic twins, including twin-twin transfusion
syndrome, twin anemia polycythemia sequence, twin
reversed arterial perfusion sequence, and conjoined twins.
CHAPTER OUTLINE
ZYGOSITY/CHORIONICITY
DETERMINATION OF CHORIONICITY
GENERAL ISSUES
LOSS OF A TWIN
COMPLICATIONS OF
MONOCHORIONICITY
Twin-Twin Transfusion Syndrome
Twin Anemia Polycythemia Sequence
Twin Reversed Arterial Perfusion
Sequence
MONOAMNIOTIC TWINS
CONJOINED TWINS
Pregnancies with more than one fetus have become an increasingly
common obstetric occurrence, primarily as a result of
assisted reproductive technologies. Most multifetal pregnancies
are twins, with higher-order multiples also possible. he rate of
twins occurring naturally is 1 in 80 births, 1 yet the rate of twin
births continues to rise. According to the Center for Disease
Control Vital Statistics, 2 whereas the number of overall births
in the United States was stable to slightly decreased in 2013
compared with 2012, the rate of twins reached a new high of
33.7 per 1000 births. he rate of triplet or higher births has been
dropping since 1998, with a 2013 rate of 119.5 per 100,000 births.
Modiications of assisted reproductive techniques are in part
responsible for the declining rate of triplet and higher-order
multiple gestations since the peak in 1998. 3 Multifetal pregnancies
are subject to all the same considerations of singleton pregnancies
but are also at particular risk for premature delivery and growth
restriction. In 2013, more than one of every two twins and more
than 9 of every 10 triplets was born preterm or with low birth
weight. 2 In addition, there are unique issues that arise in multiple
gestations, and it is critical for imagers to have a full understanding
of these possible complications.
ZYGOSITY/CHORIONICITY
Twins are either dizygotic (fraternal) or monozygotic (identical).
Among spontaneous twins, approximately two-thirds are dizygotic
and one-third are monozygotic. Dizygotic twins occur when
two separate ova are fertilized by two separate sperm. he rate
of dizygotic twinning varies with maternal age (increased with
older mothers), race, and family history. Monozygotic twins occur
when a single ovum is fertilized by a single sperm and the embryo
then divides from 2 to 14 days ater fertilization. he timing of
the division will dictate the type of twin. Spontaneous monozygotic
twinning is uniform across the world (3.5/1000), and historically
is not afected by maternal age, race, or other known factor. 1
However, there is currently an increased incidence of monozygotic
twins from 2 to 12 times the expected rate following in vitro
fertilization. 4-6 It has been reported that in patients undergoing
assisted reproduction, the timing of single embryo transfer can
afect the rate of monozygotic twins, with a higher rate seen
following a 5- to 6-day transfer as compared to a 2- to 3-day
transfer and a higher rate in patients who have undergone assisted
hatching. 7,8 Higher-order multiples may have any combination
of zygosity.
For management of a twin pregnancy, the type of twin is
characterized by chorionicity rather than zygosity. Chorionicity
is the major determining feature for the inherent unique complications
faced by multiple gestations, with monochorionic pregnancies
at the highest risk. 9 herefore accurate determination of
chorionicity is critical to ensure appropriate obstetric management
of a twin pregnancy. 10
Dizygotic twins are always dichorionic diamniotic, meaning
that each twin has its own placenta (chorion), amnion, and
amniotic luid. Each twin exists in a separate intrauterine
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