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Dizygotic twins
2 zygotes
Monozygotic
twins
Chapter 32 ■ Multifetal Pregnancy 1147
100%
Dichorionic diamniotic
33%
66%
1 zygote
1%
Monochorionic diamniotic
Monochorionic monoamniotic
SONOGRAPHIC DETERMINATION
OF CHORIONICITY AND
AMNIONICITY
Chorionicity can be determined with high reliability in
the first trimester with accuracy of 98% to 100%.
Very early in gestation, between 6 and 9 weeks, the
membrane thickness can be used to determine chorion-
icity and the number of yolk sacs used to determine
amnionicity. If a thick membrane (>
2 mm) is seen, it
19-21
FIGURE 32-1. Diagram illustrat-
ing zygosity and placentation in
twins. Of all twins, 70% are dizygotic
(from 2 sperm and 2 eggs) and 30% of
twins are monozygotic (from 1 sperm and
1 egg). From all dizygotic twins, 100%
form dichorionic diamniotic placenta.
From all monozygotic twins, 33% are
dichorionic diamniotic, 66% are monochorionic diamniotic, and 1% are monochorionic monoamniotic.
is a dichorionic gestation. If there is a thin or no perceptible membrane seen this early in gestation, it is a monochorionic gestation. In a monochorionic diamniotic
gestation, a thin membrane is usually seen surrounding
the individual embryos if the gestational age is sufficient
for a visible amnion to be present (about 8 weeks). If a
membrane is not seen, assessing the number of yolk sacs
is helpful. A monoamniotic gestation is present when
there is one yolk sac with two embryos.
22-25
two yolk sacs with two embryos, even if an intervening
membrane is not seen, it still has to be a diamniotic
If there are
1148 PART IV ■ Obstetric Sonography
gestation (Fig. 32-2; Videos 32-1, 32-2, and 32-3).
Follow-up later in gestation will demonstrate the two
amniotic sacs in these cases.
As the gestation progresses, other factors can be used
to determine chorionicity and amnionicity.
Membrane Thickness
Membrane thickness, in experienced hands, has 100%
intraobserver concordance and 91% interobserver concordance,
(Fig. 32-3). Problems can arise late in gestation when a
“thick” membrane becomes perceptibly “thin.”
26
making it a reliable tool to assess chorionicity
27
Technical factors such as assessing the membrane close to the
placental attachment site, or having the membrane
perpendicular to the ultrasound beam, can have the
opposite effect and cause a “thin” membrane to appear
“thick.”
16
In a study of twins between 10 and 14 weeks’
gestation, a membrane thickness cutoff of 1.5 mm or
greater was 100% specific and 92.6% sensitive for
dichorionicity, with a positive predictive value (PPV) of
100% and negative predictive value (NPV) of 80%.
20
Another study examined twins later in gestation, at 20
to 35 weeks, and assessed membrane thickness using
two-dimensional (2-D) and three-dimensional (3-D)
ultrasound. Using a cutoff of 1.8 mm, the sensitivity
and specificity for dichorionicity were 97% and 94%,
respectively, for 3-D and 83% and 83%, respectively,
for 2-D sonography.
28
The “twin peak” sign can be used to distinguish between
these two entities.
30
This finding, also called the lambda
sign, is produced by proliferating chorionic villi growing
into the potential space between the two layers of chorion
in the intertwin membrane
31,32
(Figs. 32-4, B and C).
The presence of twin-peak sign indicates dichorionicity,
but its absence does not indicate monochorionicity.
This is especially true in the second and third trimesters,
as the sign regresses in conspicuity with progression of
gestation.
33
In a monochorionic gestation, there is a single placenta and a single chorion, and thus the potential space
for the twin-peak sign does not exist. The two layers of
amnion extend to the placenta, referred to as the “T”
sign (Fig. 32-4, D; Video 32-5). This sign predicts
monochorionicity with sensitivity of 100% and specificity of 98.2%.
20
Umbilical Cord
Sometimes it can be difficult to identify an intervening
membrane to distinguish diamnionicity from monoamnionicity. In general, the insertion sites of the umbilical
cords into the placenta are closer to each other in monoamniotic twins than in diamniotic twins (Fig. 32-5, A).
However, the only way to diagnose monoamnionicity
with certainty is to identify one twin’s cord either around
the other twin or entangled with the cord of the other
35
(Fig. 32-5, B).
twin
20,33,34
Fetal Gender
In the second and third trimesters, if different genders
are seen, by definition the twins have to be dizygotic and
thus dichorionic. The only exception to this rule is a rare
case of postzygotic nondysjunction, in which one twin
appears male, with an XY karyotype. However, the Y
chromosome becomes lost in the second twin, resulting
in a 45 XO female fetus with Turner syndrome. If the
fetuses are of the same gender, zygosity cannot be determined using this criterion, and thus chorionicity cannot
be based on gender alone.
Placenta
If two clearly separate placentas are present, one for each
twin, dichorionicity can be stated with certainty
29
(Fig.
32-4, A; Video 32-4). The only exception to this rule is
a succenturiate lobe in a monochorionic twin. Therefore, attention must be paid to the size of the placentas
being about equal, and each fetus having a separate placenta. In some cases, because of the sites of implantation,
two placentas are located adjacent to each other, making
it difficult to determine if there are indeed two “fused”
placentas or only a single placenta. The former would
imply dichorionicity and the later monochorionicity.
Accuracy
Using the tools just described, chorionicity, even later in
gestation, can be determined quite reliably. One study
of 410 twins (≤24 weeks’ gestation) accurately identified
chorionicity in 392 (95.6%).
36
Another study of 150
twins earlier in gestation (10-14 weeks) used a combination of membrane thickness, number of placental sites,
and lambda and T signs and accurately predicted chorionicity in all but one.
20
Another study of 100 twins with
scans in the second trimester used fetal gender, placental
number, twin-peak sign, and dividing-membrane thickness and predicted chorionicity, amnionicity, and zygosity with 91% or higher sensitivity and specificity.
37
MORBIDITY AND MORTALITY
Infant mortality of twins is five times that for singletons
(37 vs. 7 per 1000 live births).
to 15% of neonatal deaths.
and mortality, gestational age at delivery, chorionicity,
and sonographic findings are each independent, statistically significant, prognostic factors.
delivery is higher for multiples than singletons and
increases with increasing number of gestations. However,
when controlling for gestational age, outcomes of
38
Twins account for 12%
39
When assessing morbidity
19
The risk of preterm
A B
Chapter 32 ■ Multifetal Pregnancy 1149
B
A
C
E
D
FIGURE 32-2. First-trimester transvaginal scans assess-
ing chorionicity and amnionicity. A, Dichorionic diamni-
otic gestation. Sagittal view of twins at 8 weeks shows two intrauterine
gestational sacs with two embryos, separated by a thick intervening
membrane. B, Monochorionic diamniotic gestation. Transverse view
of twins at 9 weeks shows a thin membrane (amnion) surrounding each
embryo. C, Monochorionic diamniotic gestation. Transverse view of
twins at 6 weeks shows no perceptible membrane. However, two yolk
sacs and two embryos are seen. D, Dichorionic triamniotic gestation.
Transverse view of triplets at 8 weeks shows three embryos. One embryo
has its own amnion and chorion, with a thick dividing membrane with
the other sac, which contains two embryos and two yolk sacs. It is difficult on this image to appreciate the amnions in the second chorionic
sac, but they must be present, given two yolk sacs. E, Sextuplets at 9
weeks. Transverse view shows five of the six sacs, all separated by the
thick membrane, as each fetus has its own chorion and amnion.
1150 PART IV ■ Obstetric Sonography
FIGURE 32-3. Three-dimensional membrane.
Dichorionic diamniotic gestation. 3-D image of a 17-week gestation shows a thick, intervening membrane (arrow) separating
twins.
SONOGRAPHIC FINDINGS FOR
DETERMINING CHORIONICITY
AND AMNIONICITY
Different fetal genders = dizygotic and thus
dichorionic
Two separate placentas = dichorionic but can be
monozygotic or dizygotic
Lambda or twin-peak sign = dichorionic, but if
absent does not mean monochorionic
T sign with 1 placenta = monochorionic diamniotic
1 yolk sac with 2 embryos very early in gestation =
monoamniotic
Entangled cords or cord of one fetus around other
fetus = monoamniotic
otherwise normal multiples are similar to singletons.40
This suggests that prematurity and intrauterine
growth restriction (IUGR) are the main issues that
increase neonatal morbidity and mortality in multiple
gestations.
Although most twin gestations are dichorionic, the
monochorionic pregnancies account for up to 50% of
the mortalities of twin pregnancies.
41
Monochorionic
gestations have a higher rates of fetal loss before 24
weeks, stillbirth after 28 weeks, preterm delivery, IUGR
and discordant growth, and neurologic morbidity compared to dichorionic twins.
twins, the monoamniotic gestations have the highest
mortality, up to 50%, primarily from cord accidents.
42-46
Of the monochorionic
47
Intrauterine Fetal Demise
The incidence of twins is higher than actually documented because early pregnancies with subsequent loss
of a fetus are not diagnosed as multiple gestations. In
one study of 1000 first-trimester scans, twins were seen
in 3.3%. Of these 21.2% “vanished” later in the first
trimester.
48
Thus, depending on if and when a firsttrimester scan is performed, a twin gestation with early
demise may be classified as a singleton pregnancy.
Higher rates of at least one embryonic loss have been
reported in multiple gestations conceived with ART,
with rates of 35% to 36% in twins, 53% to 59% in
triplets, and 47% to 65% in quadruplets.
49,50
Some of
this increase could be caused by an earlier onset and
increased frequency of performing first-trimester scans.
In general, single fetal death occurs in 2.6% to 9% of
19,51,52
twins.
twins than dichorionic twins.
of the other fetus depends on the chorionicity and is
inversely related to the time of death of the twin fetus.
This is twice as common in monochorionic
53
The chance of survival
52
When loss of a twin occurs before 16 weeks, outcome is
much better than when loss of a twin occurs after 16
weeks, when there is up to 50% chance of prematurity,
22% risk of IUGR, and 13% perinatal mortality
54
(Fig.
32-6). In monochorionic twins with loss of a twin after
the first trimester, the “twin embolization” syndrome
can occur, as discussed later.
Structural Anomalies
Major malformations are twice as common and minor
malformations 1.5 times more common in twins than
singletons.
major malformations compared with twins, and higherorder gestations have a more than sixfold increase.
55
Triplets have up to a threefold increase in
56
The
later in gestation the cleavage into twins occurs, the higher
the incidence of structural abnormalities. Monozygotic
twins have more structural defects compared with dizygotic twins and singletons,
versus 1.9% to 2.5%, respectively.
57
with rates of 3.1% to 3.7%
58
Of the monozygotic
twins, the monochorionic gestations have increased risk
of malformations compared to the dizygotic gestations.
Even though monozygotic twins have the same chromosomal makeup, they can have discordant anomalies. The
extreme example of late cleavage is conjoined twins,
which all have structural abnormalities.
Of all anomalies, those involving the central nervous
system (CNS) are most common (Fig. 32-7). Cerebral
palsy is more likely to occur in twins than singleton
pregnancies.
59
The prevalence of cerebral palsy in twins
is 7.3 to 12.6 per 1000 infants, increasing in triplets up
to 28 to 44.8 per 1000, compared to singletons with
rates of 1.6 to 2.3 per 1000.
60,61
Antenatal necrosis of the
cerebral white matter is more common in monochorionic twins, likely from the presence of vascular connections in the placenta.
44
A B
TW B
MAT LT
Chapter 32 ■ Multifetal Pregnancy 1151
*
C D
FIGURE 32-4. Placental findings to determine chorionicity. A, Dichorionic gestation. Transverse view of twins at 12
weeks’ gestation shows two separate placentas, one anterior and one posterior. B, Dichorionic gestation. Sagittal view of twins at 14
weeks’ gestation shows two “fused” posterior placentas, with a “twin peak” sign (arrow). C, Dichorionic gestation. 3-D image shows the
“twin peak” sign with placental tissue extending into the potential space between the two layers of chorion (*), and a thick membrane
(arrow). D, Monochorionic diamniotic gestation. Transverse view of twins at 22 weeks shows a thin membrane perpendicular to a single
posterior placenta, forming the “T” sign (arrow).
Other Screening Tests
for Anomalies
There are multiple tests used to screen for anomalies
with twins. Maternal serum alpha fetoprotein (MSAFP) is increased in all twins (because more than one
fetus is present). However, when the MS-AFP in twins
is greater than 4.5 multiples of the median (MOM),
there is an increase in perinatal mortality, especially
when it is a monochorionic diamniotic gestation.
47
Screening for trisomy by nuchal translucency (NT)
is performed in twins in a similar manner as performed
in singletons. The detection rates are similar, but the
false-positive rates in normal monochorionic twins are
higher, at 8%.
62
NT, combined with serum human
chorionic gonadotropin and pregnancy-associated
plasma protein A, identifies 90% of trisomy 21 in singletons, and 80% in twins.
63
Chorionic villus sampling (CVS) and amniocentesis can safely be performed in twins, without associated
increase in fetal loss rate.
rate is 1.2% to 1.5%.
64-67
For CVS, the sample error
64,68
In some cases, CVS of one
twin is performed at the same time as fetal reduction
of the other twin. This same-day dual procedure
does not increase the rate of fetal loss.
68
Some have
proposed delaying the reduction to slightly later in
gestation (13-14 weeks), after CVS results are confirmed. This allows for increase in detection rate of
abnormal findings, including abnormal NT, growth
discordance, cystic hygroma, and CNS anomalies, but
1152 PART IV ■ Obstetric Sonography
2
1
A
B
A B
FIGURE 32-5. Monoamniotic twins at 16 weeks’ gestation. A, Transverse view shows a single anterior placenta with the
cord insertion sites (1, 2) of the twins in proximity to each other. B, Color Doppler image shows the cord of the fetus on the right (A)
wrapped around the fetus on the left (B), confirming that the twins are in the same sac and are monoamniotic.
A B
FIGURE 32-6. Vanishing twin. A, Dichorionic diamniotic gestation with first-trimester demise of one twin. Scan at 17 weeks’
gestation shows the live fetus and an empty sac anterior to the fetal head. There has been resorption of the embryonic tissue, although a
small sac from demised twin remains. B, In a different gestation after demise of co-twin, note the small compressed twin (arrow) against
the wall of the uterus.
does not increase the risk of pregnancy loss caused by
reduction.
69
When performing amniocentesis, indigo carmine or
Evan’s blue dye is often instilled into one sac after
centesis of both sacs is also often performed in monozygotic twins because even though they are “identical,”
they can have varied karyotypes, including mosaicism
and small-scale mutations.
73
removal of fluid, to ensure that the second sample is
obtained from the correct sac. Methylene blue is not
used in pregnancy due to risks of fetal hemolytic anemia,
intestinal atresia, and fetal demise.
70-72
Amniocentesis of
both sacs is performed in dichorionic twins because the
majority of these are chromosomally different. Amnio-
Growth Restriction and
Discordant Growth
Abnormal fetal growth is defined in two ways: (1) the
estimated fetal weight is below the 10th centile on a
Chapter 32 ■ Multifetal Pregnancy 1153
A B
FIGURE 32-7. Anencephaly. One fetus of a twin gestation at 19 weeks. A, Sonogram. B, T2-weighted MR image. Arrowhead shows
the dividing membrane. Arrow indicates anencephalic twin. Structural anomalies are more common in twins than in singletons, especially
anomalies of the central nervous system.
singleton curve; or (2) there is a 20% or more discordance in estimated weight between twins.
Birth weight discordance complicates more than
15% of twin pregnancies.
74
There is a higher rate of
IUGR in twins compared to singletons, probably caused
by uteroplacental insufficiency resulting from increased
metabolic demand or an abnormality involving placental
implantation. Triplets are even more likely to have
discordant growth compared to twins.
75
The neonatal
mortality rate increases with increasing growth discordancy. In a large twin study, with no discordance the
mortality rate was 3.8:1000 live births; with 15% to
19% discordance, 5.6:1000 live births; with 20%
to 24% discordance, 8.4:1000 live births; with 25% to
30% discordance, 18.4:1000 live births; and with 30%
or more discordance, 43.4:1000 live births. In this series,
fetal weight of less than 10th percentile was more
common in discordant twins (60%) compared to nondiscordant twins (5%).
76
Growth discordance can be detected as early as the
first trimester. A difference of 3 mm or less in mean sac
diameter or crown-rump length (CRL) between twin
gestations has up to a 50% embryo loss rate, much
greater than when the difference is 1 mm or less. A
recent study found 45% discordance at birth when firsttrimester CRL was greater than 3 days discrepant, versus
9% discordance at birth with CRL 3 days discrepant or
earlier. Discordance seen in the first trimester increases
the risk of congenital anomalies
gestation.
78
77
and IUGR later in
Doppler waveforms, fetal biometry (especially abdominal circumference), and estimated fetal weight are all
helpful in assessing for discordance in the second and
third trimesters.
Premature Delivery
The risk of preterm delivery is higher for multiple pregnancies compared to singletons.
relationship between the number of fetuses and the gestational age at delivery.
3
The rate of premature delivery
is increased up to five times in twins and nine times
in triplets compared to singleton gestations.
delivery accounts for much of the increase in morbidity
and mortality in multiple gestations.
79
There is an inverse
80
Preterm
40
Cervical Incompetence
Cervical length on transvaginal scan in the middle of
the second trimester is inversely related to risk of
preterm delivery
81
(Fig. 32-8). Women with twins or
higher-order multiple gestations are more likely to have
a shorter cervix than women with singleton preg-
82
nancy.
One study of twins found that a cervical
length of 2.5 cm or less at 24 weeks was the most
powerful predictor of preterm delivery.83 Another study
using 2.0 cm as the cutoff for cervical length found a
NPV of 99%, 98%, 95%, and 93% for delivery at
<28, <30, <32, and <34 weeks’ gestation, respectively.
84
Prophylactic cerclage in patients with twins has not
1154 PART IV ■ Obstetric Sonography
1.85cm 1.10cm
A
FIGURE 32-8. Transvaginal imaging of the cervix. A, Short cervix (calipers) measuring 1.8 cm. Transvaginal scan of cervix
in a twin pregnancy at 19 weeks. B, Funneling with closed cervical length of 1.0 cm (calipers). Transvaginal scan of the cervix in a different patient with twins at 30 weeks’ gestation.
B
been shown to be effective in preventing preterm
delivery.
85-87
Placental Abnormalities: Marginal
and Velamentous Insertion
When the umbilical cord, instead of inserting directly
into the placenta, inserts into the membrane with blood
vessels traversing to the placenta, it is termed velamen-
tous insertion.
88
An umbilical cord that inserts at the
periphery of the placenta, rather than centrally, is referred
to as marginal insertion. Abnormal insertion sites of the
cord into the placenta are associated with increased perinatal morbidity and mortality
in multiple gestations than singletons.
third of monoamniotic pregnancies have either a marginal or a velamentous insertion.
89,90
and are more common
47,91
Up to one
92
Velamentous and
marginal cord insertion sites are associated with IUGR
and growth discordance, twin-twin transfusion syndrome (TTTS), and preterm labor.
93-96
Umbilical Cord Doppler Ultrasound
Doppler sonography is helpful in assessing the well being
of a fetus, and can help in the assessment of intrauterine
IUGR, TTTS, and discordant growth of twins. The
systolic-to-diastolic (S/D) ratio is often used to assess
level of resistance of blood flow in the umbilical artery.
As the pregnancy progresses, there is a decrease in the
peripheral resistance of blood flow because of an increase
in arterioles in the placenta.
97
Thus, normally the S/D
ratio decreases after 20 weeks’ gestation. The low-resistive system results from the presence of end diastolic
flow. The normal S/D ratio in the third trimester ranges
from 1.7 to 2.4. When obtaining Doppler waveforms, it
is important to sample at a free-floating loop of cord;
falsely elevated ratios can be obtained if sampling is from
the fetal end at the cord insertion site.
An elevated S/D ratio is associated with increase
in morbidity and mortality in twins.
S/D ratios of twin pairs are predictive of discordant
100,101
growth
(Fig. 32-9). An elevated S/D ratio occurs
98
99
Differences in
when approximately 30% of the fetal villous vasculature
is abnormal. When this increases to 60% to 70% abnormal vessels, there is absent or reversed flow.
102,103
COMPLICATIONS
Monochorionic Twins
Complications unique to monochorionic twins are
caused by the single shared placenta. Vascular communications are almost always found in monochorionic
gestations. The communication can be arterial-arterial,
venous-venous, or arterial-venous. The arterial-arterial
and venous-venous connections are end-to-end anastomoses that occur at the placental surface. Arterial-arterial
communications are much more common than venousvenous communications.
venous anastomosis, which has a feeding artery, enters
the placenta to the capillary bed of a cotyledon, where
it drains into the venous system of the other twin.
Depending on the imbalances in the types of communications, various complications can arise.
104
In contrast, the arterial-
Chapter 32 ■ Multifetal Pregnancy 1155
0
1
2
3
4
5
Umb–PS
Umb–ED
TWIN A
A B C
FIGURE 32-9. Umbilical artery Doppler in twins at 28 weeks’ gestation. A, Normal arterial waveforms. Doppler
image of one twin shows normal arterial waveforms with normal S/D ratio and presence of diastolic flow. B, Elevated S/D ratio with
absent end diastolic flow. Doppler image of the other twin. Biometry (not shown) revealed this fetus to be in the 3rd percentile for weight.
Note that the spectral Doppler scale is too low, cutting off peak systole of the waveform. C, Absent diastolic flow on Doppler ultrasound
of a twin at 23 weeks’ gestation by dates and 19 weeks’ gestation by ultrasound.
Twin-Twin Transfusion Syndrome
Twin-twin transfusion syndrome occurs in up to 15%
of monochorionic twins.
105
One hypothesis on the etiology of TTTS is an imbalance in the formation of the
arterial and venous connections of both fetuses with a
single placenta.
106
The presence of one-way arterialvenous connections, without the presence of other connections that can compensate for this unidirectional
flow, can cause TTTS.
107-110
One study examining placentas found that those from pregnancies with TTTS
had significantly fewer anastomoses than those without
TTTS, although in those with TTTS the anastomoses
present were more likely to be deep rather than superficial (80% vs. 36% in control).
107
The superficial anastomoses, especially arterial-arterial, are believed to be
protective against TTTS, decreasing its incidence by
ninefold.
109
Another study of monochorionic twins
found TTTS in 58% pregnancies with no arterialarterial anastomoses, compared to 5% when arterialarterial anastomoses were present.
where TTTS develops despite the presence of arterialarterial anastomoses, there is improved outcome.
111
In pregnancies
109
In TTTS, one twin is referred to as the “donor” and
the other the “recipient.” The donor has blood shunted
away from it, thus causing anemia, hypovolemia, IUGR,
and oligohydramnios. The recipient of this excess blood
is the larger twin, with hypervolemia, cardiac overload,
polyhydramnios and possibly hydrops.
The criteria to diagnose TTTS include documentation of a monochorionic gestation, with significant
weight discrepancy of the fetuses (difference >20%) and
polyhydramnios in the larger twin and oligohydramnios
in the smaller twin. When TTTS is severe, it may be
difficult to see the membrane around the donor twin due
to severe oligohydramnios. The membrane appears
“stuck” to the fetus, and the fetus has limited motion,
being adherent to the uterine wall. In severe cases the
donor twin may not have fluid visible in the stomach or
bladder. In contrast, the recipient, who has polyhydramnios, may have a large bladder (Fig. 32-10; Videos 32-6
and 32-7).
Other disorders that should be considered when there
is discordance in the amount of amniotic fluid around
twins, or discrepancy in twin size, are placental insuf-
ficiency, umbilical cord abnormalities, intrauterine
infection, congenital anomaly (e.g., renal agenesis in
one twin), and premature rupture of membranes (Fig.
32-11).
Quintero et al.
112
proposed a staging system to help
determine severity and predict outcome. The overall
perinatal survival rate for TTTS is 65%; however, it is
higher for stage I-II at 76% and lower for higher stages
at 52% (Table 32-2).
Options for treatment of TTTS include serial amnioreduction, laser photocoagulation of communicating
vessels, septostomy, and termination.
113-116
One study of
173 cases of TTTS, in which 78 were treated with
amnio reduction and 95 with laser therapy, reported at
least one surviving infant in 67% of cases with amnioreduction and in 83% with laser therapy. The neurologic
morbidity was lower with laser, 4% versus 24% with
1156 PART IV ■ Obstetric Sonography
A
B
A B
C D
FIGURE 32-10. Twin-twin transfusion syndrome. A, Twins at 18 weeks’ gestation. Note the discrepancy in abdominal size
of the fetuses, with fetus B being smaller than fetus A. A separating membrane is seen (arrow), with the membrane close to fetus B due
to oligohydramnios. A stomach bubble was not seen in this fetus. B and C, Different twin gestation at 17 weeks. B, “Recipient” twin
shows polyhydramnios and a distended bladder. C, “Donor” twin had oligohydramnios and no visible fluid in the bladder. D, Severe
polyhydramnios of a recipient twin with the donor twin “stuck” anteriorly in a different case of twins at 21 weeks’ gestation.
amnioreduction.
with amnioreduction of 60% to 65%,
therapy, 70%, with at least one fetus surviving in 81%.
Another study performing septostomy in a small group
of patients with TTTS reported 83% survival.
112
Other studies report survival rates
117
and for laser
115
118
Twin Embolization Syndrome
Demise of one twin in a monochorionic pregnancy
occurs in up to 20% of cases, the majority occurring in
the first trimester.
119
A major complication of TTTS
with the death of one twin is twin embolization syndrome. One theory for this occurrence is that thromboplastin-rich blood from the dead twin travels via placental
anastomoses to the live twin.
current theory proposes that the injuries are not caused
by emboli, but by changes in perfusion with blood loss
from the survivor to the more relaxed circulation of the
dead twin. This hypoperfusion can affect many organs,
especially those that typically are well perfused.
result is structural defects resulting from ischemia.
The brain is extremely susceptible to the effects of
twin embolization syndrome
intestinal tract can also be affected, with splenic and
hepatic infarcts, as well as atresias of the small bowel,
colon, and appendix. Other anomalies include renal cortical necrosis, pulmonary infarctions, limb anomalies,
and aplasia cutis.
123
120
The more accepted
121
122
(Fig. 32-12). The gastro-
The
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