Diagnostic ultrasound ( PDFDrive )

CHAPTER 32 Multifetal Pregnancy 1129
A
B
FIG. 32.16 Monochorionic Monoamniotic Twins, 12 Weeks’ Gestational Age. (A) Transverse image of the fetal chests in a single gestational
sac. The umbilical cords are entwined anteriorly. (B) Corresponding color Doppler image demonstrates the cords twisting together and then separating
to insert into the anterior placenta. See also Video 32.9.
output cardiac failure and polyhydramnios, 93 particularly if the
weight of the acardiac twin is more than 70% of the donor twin.
In this situation, intervention to interrupt the abnormal circulation
to the acardiac twin may be beneicial. 94-96
MONOAMNIOTIC TWINS
In a monoamniotic pregnancy, both fetuses are located within
a single amnion and therefore a single chorion. his rare type
of twinning has the highest morbidity and mortality rate of any
type of twin. 33 Monoamniotic twins should be suspected when
no intertwin membrane is identiied by 10 to 12 weeks’ gestational
age and can be conirmed at any age when the umbilical
cords are seen to twist together or become entangled (Figs. 32.16
and 32.17, Videos 32.9 and 32.10). In the late irst trimester,
transvaginal imaging may provide better resolution to conirm
the absence of a membrane. he monoamniotic placenta has
the same three types of vascular anastomoses as found with
diamniotic twins; however, there are more AA anastomoses,
fewer AV anastomoses, and a similar number of VV anastomoses
in monoamniotic twins when compared with diamniotic
monochorionic twins. In addition to all of the potential complications
of a monochorionic gestation, monoamniotic twins
are at risk for cord knotting or entanglement at any gestational
age, although the clinical signiicance of cord entanglement
is unclear, as several studies have shown no improvement in
outcome following ultrasound diagnosis of an entangled cord. 97,98
Discordant fetal anomalies can afect as high as 20% of monoamniotic
pregnancies. 99 Monoamniotic twins are at high risk of
unexpected and usually double intrauterine fetal demise, thought
to be related to fetofetal hemorrhage via large AA anastomoses
following cord compression. 72,100 Survival of monoamniotic twins
may be as high as 80% with intensive surveillance and early
delivery. 101,102
FIG. 32.17 Monochorionic Monoamniotic Twins, 28 Weeks’
Gestational Age. The umbilical cord of one fetus wraps loosely around
a transverse section of the other cord (arrows). See also Video 32.10.
CONJOINED TWINS
Conjoined twins are a rare type of monoamniotic twins that
occur with late division of the embryo. he twins can be conjoined
at any skin site and are diagnosed by demonstrating contiguous
skin covering between two fetuses. he in utero rate of conjoined
twins is estimated at 1 in 50,000 gestations, with an IUFD rate
of 60% and an additional high loss rate ater birth. 103 Conjoined
twins are generally classiied by the site of joining, 104 although
the anatomy of each pair is unique. Careful sonographic evaluation
of each fetus is imperative for a full assessment of the degree of
sharing that exists, which will help with an accurate prognosis