CHAPTER 7 • Multiple Pregnancies in the First Trimester. First Trimester Ultr

 CHAPTER 7 • Multiple Pregnancies in the First Trimester

INTRODUCTION

The widespread use of assisted reproductive technologies along with increasing

maternal age has resulted in a steady rise in the frequency of multiple pregnancies over

the past two decades.1 Pregnancies with twins and higher order multiples are at an

increased risk for many maternal and fetal/child complications to include miscarriage,

gestational diabetes, hypertensive disorders, preterm birth, fetal genetic and congenital

malformation, fetal growth restriction, perinatal death, and cerebral palsy.2,3

Ultrasound is an integral part of the clinical care of multiple pregnancies, from the

initial diagnosis to guiding the delivery of the neonates. In this chapter, we review the

utility of ultrasound in the diagnosis and management of multiple pregnancies in the first

trimester with a focus on twin pregnancies. Detailed evaluation of fetal congenital

abnormalities is covered in subsequent chapters of this book. Table 7.1 lists the

benefits of first trimester ultrasound in multiple pregnancies.

PREGNANCY DATING IN TWINS

There are no significant differences in the first trimester fetal biometric measurements

between fetuses of multiple pregnancies and singleton fetuses and thus singleton-derived

reference measurements of crown-rump length (CRL), biparietal diameter (BPD), head

circumference (HC), abdominal circumference (AC), and femur length can be used in

multiple pregnancies.4 Criteria for first trimester assignment of gestational age on

ultrasound are also similar in twins and singletons and are discussed in detail in

Chapter 4 of this book. Twin pregnancies are best dated when the CRL measurement is

performed between 11+0 and 13+6 weeks of gestation.5 In twin pregnancies conceived

by in vitro fertilization, the oocyte retrieval date or the embryonic age should date the

pregnancy.5 In early gestation, the growth rate of twins is no different than that of

singletons. On occasions, differential growth in twin fetuses can be detected by

differences in biometric measurements such as CRL, BPD, and HC. When this situation••••••

is noted, it is recommended to date the twin pregnancy based on the biometric

measurements (CRL) of the larger twin.5 For twin pregnancies presenting after 14

weeks of gestation, the larger HC should be used for pregnancy dating when biometric

discrepancy is noted.6

Table 7.1 • Benefits of First Trimester Ultrasound in Multiple Pregnancies

Diagnosis of multiple pregnancies

Pregnancy dating

Determining chorionicity

Evaluation of fetal anatomy

Assessment for the presence of multiple pregnancy complications

Guiding chorionic villous sampling and other interventions

ETIOLOGY AND PLACENTATION IN TWINS

Twin pregnancies are classified into two main categories—dizygotic and monozygotic

—based upon the number of eggs fertilized at conception.

Dizygotic Twins

Dizygotic twins, also called fraternal, occur when two eggs are fertilized with two

separate sperms resulting in two fetuses that are distinct genetically but share the same

uterus. Dizygotic twins are always dichorionic/diamniotic, as each fetus has its own set

of placenta and membranes. Several factors affect the rate of dizygotic twinning

including maternal age, race, increasing parity, geographic area and presence of

assisted reproduction.7 The rate of dizygotic twinning varies significantly around the

world with high rates reported in parts of Nigeria and low rates reported in Southeast

Asia and Latin America.7,8

Monozygotic Twins

Monozygotic twins (also referred to as identical) occur when one egg is fertilized by

one sperm followed by division of the embryo into two. These twins are therefore

typically identical genetically. Unlike dizygotic twins, the rate of monozygotic twins is

fairly constant throughout the world at 1/250 pregnancies9 excluding pregnancies of

assisted reproduction. Monozygotic twins are associated with higher pregnancy

complications and perinatal morbidity and mortality than dizygotic twins. Monozygotic

twins can have various types of placentation based upon the timing of the division of the

fertilized egg. Table 7.2 shows types of placentation in monozygotic twins in relation to

the timing of embryo cleavage. Although conceptually monozygotic twins are identical,

post-fertilization genetic events result in genetic heterogeneity between the twin

pairs.10,11 Furthermore, discordance in fetal malformations, which poses significant

challenges in clinical management, is not uncommon in monozygotic twins.Zygosity and Chorionicity in Twins

Zygosity refers to whether the twins are genetically identical or not, whereas

chorionicity refers to the type of placentation in twins. As shown in Table 7.2,

monozygotic twins (identical) can have different types of placentation based upon the

time of division of the fertilized egg, whereas dizygotic twins always have two separate

placentas that on occasions can appear fused on ultrasound. Parents commonly ask at the

time of the ultrasound examination whether their unborn twins are identical or not. It is

important to note that the sonographic diagnosis of identical twins can only be made

when the criteria for a monochorionic pregnancy (discussed later in this chapter) are

met. When a dichorionic spontaneous twin pregnancy is diagnosed by ultrasound, the

chance of identical twins in this setting is about 10%. From the point of view of

pregnancy care chorionicity is therefore more important than zygosity.

Table 7.2 • Types of Placentation in Monozygotic Twins in Relation to

Timing of Embryo Cleavage

Embryo

Cleavage

(d)

Placentation Type Frequency

0–3 Dichorionic/diamniotic ~25%

4–8 Monochorionic/diamniotic ~75%

9–12 Monochorionic/monoamniotic ~1%

13–15 Conjoined Rare

DETERMINING CHORIONICITY/AMNIONICITY IN

TWINS

First trimester ultrasound can determine the type of placentation in twins with high

accuracy. The diagnosis of dichorionic/diamniotic twin pregnancy can be made

accurately when two separate and distinct chorionic sacs are seen in the endometrial

cavity as early as the fifth week of gestation (Fig. 7.1). Indeed, until about 8 weeks of

gestation, the presence of two distinct gestational sacs on ultrasound with

embryos/cardiac activities confirms a dichorionic/diamniotic twin gestation (Fig. 7.2).

Later on in early gestation, when two adjoining gestational sacs or fetuses are seen

within the endometrial cavity, the characteristic of the dividing membrane(s), when

present, is the most accurate way for determining chorionicity. Indeed, chorionicity

should be ideally determined between 11+0 and 13+6 weeks of gestation if feasible.6 If

the placenta appears to fill the junction of the dividing membrane(s) at its insertion into

the placenta, resulting in a thick wedge-shaped configuration (lambda or twin-peaksign), this is diagnostic of dichorionic/diamniotic placentation (Figs. 7.2 to 7.4).

Figure 7.1: Sagittal plane of the uterus at 5 weeks of gestation with two

distinct chorionic sacs A and B. The thick separation of the chorionic sacs

(arrows) suggests a dichorionic twin gestation.

Figure 7.2: Dichorionic-diamniotic twins (A and B) at 9 week of gestation. Note

the thick dividing membrane with a twin-peak sign (asterisk) at the placental

insertion of the membranes.Figure 7.3: Dichorionic-diamniotic twins (A and B) at 11 weeks of gestation.

Note the thick dividing membrane with a twin-peak sign (asterisk) at the

placental insertion of the membranes.

In monochorionic pregnancies, the dividing membrane attach to the uterine wall in a

thin T-shaped configuration without any placental tissue at its insertion site (Figs. 7.5

and 7.6). The shape of the placental attachment of the dividing membranes (T-shaped)

has a very high sensitivity and specificity for the diagnosis of monochorionicity between

11 and 14 weeks of gestation. Commonly, the presence of communicating fetal vessels

on the surface of the twin placenta can be documented by ultrasound in color Doppler

and this finding confirms the presence of monochorionic pregnancy (Fig. 7.6). The

demonstration of such vessels however has no clinical relevance to twin pregnancy

management. Although in general the number of yolk sacs correlates with the number of

amnions (Fig. 7.7), this rule has many exceptions, as monoamniotic twins can be

associated with a single yolk sac, a partially divided yolk sac, or two yolk sacs. For

pregnancies beyond 8 weeks of gestation, the number of placental masses can be

assessed as the presence of two distinct placental masses signifies a dichorionic

gestation. The reliability of the number of placental masses is questionable, however, as

in about 3% of monochorionic twin pregnancies two placental masses can be seen on

ultrasound.12 The thickness of the twin separating membrane can also be used for

determining chorionicity, and similar to findings in the second trimester, this technique

is not reliable to be solely used for chorionicity diagnosis. Occasionally the use of

three-dimensional ultrasound can help in assessing membrane thickness in the first

trimester of pregnancy (Fig. 7.8). Discordance in fetal gender at 13 weeks of gestation

and beyond implies the presence of dichorionic gestation.Figure 7.4: Dichorionic-diamniotic twins (A and B) at 13 weeks of gestation.

The separating membrane (asterisk) is thick with a twin-peak or lambda sign (l)

at the placental insertion of the membranes.

Figure 7.5: Monochorionic-diamniotic twins (A and B) at 13 weeks of

gestation. The dividing membrane (asterisk) is thin with a T-shape configuration

at placental insertion (T). See Figure 7.6.Figure 7.6: Monochorionic-diamniotic twin pregnancy at 13 weeks of gestation.

A thin separating membrane is visible with a T-shape configuration at placental

insertion separating twin A from twin B. The use of color Doppler shows in this

case an artery with a course from twin A to B (red arrow). Such connections

are present in almost all monochorionic placentas and can occasionally be

demonstrated on ultrasound by color Doppler as shown here.Figure 7.7: Monochorionic-diamniotic twins (A and B) at 8 weeks of gestation.

Note the presence of two yolk sacs. A thin separating membrane is not visible

in this image. The presence of two yolk sacs at this gestation suggests

monochorionic-diamniotic pregnancy but does not confirm it. The presence of a

dividing membrane on follow-up ultrasound examinations with high-resolution

transducers, confirmed this diagnosis.

When no dividing membrane is noted on ultrasound, especially with high-frequency

transvaginal or transabdominal transducer, the diagnosis of monoamniotic twins can be

performed (Fi g . 7.9). Color and pulsed Doppler confirms the diagnosis of

monoamniotic twins by demonstrating the presence of cord entanglement (Fig. 7.10),

which is almost universally seen in these pregnancies (discussed later in this chapter).

Conjoined twins are diagnosed by ultrasound in the first trimester when shared tissue is

noted between twins and confirmed on color Doppler evaluation demonstrating shared

vasculature (discussed later in this chapter).Figure 7.8: Three-dimensional (3D) ultrasound in surface mode in a

monochorionic-diamniotic twin at 11 weeks of gestation with a thin membrane

(asterisk) (A) and in a dichorionic-diamniotic twin at 10 weeks of gestation with

a thick separating membrane (asterisk) (B). 3D can support but not replace 2D

gray scale ultrasound in the diagnosis of chorionicity.Figure 7.9: Monochorionic-monoamniotic twins (A and B) at 10 weeks of

gestation. Note the presence of a single amniotic sac (labeled) with no dividing

membrane.Figure 7.10: Monochorionic-monoamniotic twins (A and B) at 13 weeks of

gestation with cord entanglement seen on color and pulsed Doppler modes.

Note the presence of a mass of cord (arrows) on color Doppler. Pulsed

Doppler with a wide sample gate confirms cord entanglement by demonstrating

two distinct Doppler waveforms (A and B) within the same Doppler spectrum.

The first trimester ultrasound is thus very accurate in determining chorionicity in

twin pregnancies with rates approaching 100% when correlated with delivery.

Chorionicity should be determined before 14 weeks of gestation if feasible as the

accuracy of ultrasound in determining chorionicity decreases with advancing gestation.

It is therefore imperative that an early gestation ultrasound, preferably in the first

trimester, be part of the management of twin gestation and that chorionicity is

determined and reported at that time when feasible. As pregnancy advances, the

accuracy of determining chorionicity and amnionicity decreases. The accuracy ofdetermining chorionicity and amnionicity is estimated around 90% in the second and

third trimester of pregnancy with the twin-peak or lambda sign being the most accurate

and reliable method.13,14

ULTRASOUND LABELING OF TWIN FETUSES

Accurate labeling of twin pregnancy by ultrasound is important and should be clearly

reflected in the report. Traditionally, twins have been labeled as twin A and twin B

based upon fetal presentations in relationship to the cervix. This is confusing as fetal

presentations may change during pregnancy and it is not uncommon for twin B to be

born first at cesarean section, which presents confusion for parents. It is recommended

to follow a descriptive process for twin labeling that takes into account the location of

each gestational sac in relationship to maternal right or left side and the position of the

sac in the uterus as upper or lower.5 For instance, twin A can be referred to as “on

maternal left with posterior placenta and lower gestational sac.” This can be performed

in the first trimester by the 13th week of gestation.

MONITORING OF TWIN PREGNANCY

One of the most important benefits of first trimester ultrasound is the diagnosis of twins

and the designation of chorionicity. When dichorionic twins are diagnosed in the first

trimester, follow-up ultrasound is recommended at 18 to 20 weeks of gestation and if

uncomplicated every 4 weeks thereafter.5 When monochorionic twins are diagnosed in

the first trimester, follow-up ultrasound is recommended at 16 weeks and every 2 weeks

thereafter in order to detect monochorionic complications such as twin-twin transfusion

syndrome (TTTS) and twin anemia polycythemia syndrome (TAPS; both discussed later

in this chapter).5 With each ultrasound examination in the second trimester and beyond,

fetal biometry, amniotic fluid volume, and umbilical artery Doppler should be obtained

to screen for twin discordance and in monochorionic pregnancies, middle cerebral

artery Doppler is also recommended to screen for fetal anemia.5 In the presence of twin

complications, more rigorous follow-up is recommended.

In one study, a combined risk assessment approach in the first and second trimester

(16 weeks) ultrasound identified a subgroup of monochorionic twin pregnancies with a

risk of complicated fetal outcome, reported as greater than 70% with a survival rate of

only 69%.15 The presence of intertwin CRL difference or discordant amniotic fluid

volumes in the first trimester along with 16 weeks of gestation differences in AC,

discordance in amniotic fluid volumes, or site of cord insertions predicted poor

outcome.15

SCREENING AND TESTING FOR CHROMOSOMAL

ABNORMALITIES IN TWINS

First trimester screening for chromosomal abnormalities in twins can be performed withmaternal age, nuchal translucency (NT), and biochemical markers such as free betahuman chorionic gonadotropin (β-HCG) and pregnancy-associated plasma protein A

(PAPP-A), with maternal age and NT alone or with cell-free DNA (cfDNA). In

monochorionic twins, Down syndrome risk is calculated as the average risk of both

fetuses, whereas in dichorionic twins, the risk is calculated per fetus A and B. It is

unclear whether the detection rate of Down syndrome is lower in twins than in

singletons, as studies have shown conflicting results.6,16 In the presence of a vanishing

twin with a visible fetal pole, screening for chromosomal abnormalities is best

performed with maternal age and NT alone, as the demised twin will alter biochemical

markers. cfDNA is a Down syndrome screening modality associated with very high

sensitivity and low false-positive rate. More data are accumulating on the role of

cfDNA as a robust screening test in twin gestation with Down syndrome–reported

detection rates of 94.4% with a near 0% false-positive rate.17

Invasive testing with chorionic villous sampling and amniocentesis for diagnostic

purposes appears to carry a higher loss rate in twins than in singletons irrespective of

the modality and approach used.18 It is important therefore to council pregnant women

with twins appropriately and discusses the complexity inherent in genetic screening and

diagnostic testing and the clinical implication of an abnormal diagnostic test. The option

for selective feticide should also be discussed with the patient during genetic

counseling.

CONGENITAL ANOMALIES IN TWINS

The risk of fetal anomalies is greater in twins when compared with singletons and this

risk is especially increased for monochorionic and monoamniotic twin

pregnancies.15,19,20 In a retrospective analysis of prospectively collected data on 1,064

twin pregnancies, detection of structural abnormalities in the first trimester occurred in

27.3% of cases.21 This is compared to approximately half of fetal malformations

detected in the first trimester in singleton pregnancies.22 Similar to the pattern seen in

singleton pregnancies, the likelihood for first trimester detection of structural

abnormalities in twin pregnancies was best for cranial vault (Fig. 7.11), midline brain,

and abdominal wall defects.21,22 Monochorionicity and discordance on CRL and NT

were associated with an increased risk of fetal anomalies with a moderate predictive

accuracy.21

It is important to note that a smaller than expected CRL in a twin member is not only

associated with an increased risk for fetal malformations, but also with an increased

risk for chromosomal abnormalities, fetal loss, preterm delivery, and birth weight

discordance.4,5,23 The critical threshold for CRL-twin discordance in the first trimester

has not been clearly defined. In general, studies addressing this issue defined twin

discordance as a CRL difference of at least 10% or 7 days.5,23–25 Pregnancy counseling,consideration for genetic testing, and detailed first trimester ultrasound examination is

therefore appropriate when early gestation biometric discordance is noted in multiple

pregnancies.

The presence of twin discordance in fetal anomalies presents a challenging clinical

scenario. In such cases, management at a center with expertise in fetal medicine is

recommended. When one fetus of a dichorionic twin pregnancy presents with a lethal

anomaly that carries a high risk for in utero demise, conservative management is

generally recommended (Fig. 7.11),5 whereas in monochorionic twin pregnancy, this

situation may warrant an intervention with cord occlusion, laser, or radiofrequency cord

ablation of the anomalous twin in order to protect the health of the normal twin should a

demise occur.5

Figure 7.11: Dichorionic twins at 13 weeks of gestation with a thick dividing

membrane (asterisks). These twins are discordant for anomaly as seen on

three-dimensional ultrasound in surface mode. Note the presence of

anencephaly in one twin and a normal head in the other twin.

COMPLICATIONS OF TWIN GESTATION

Vanishing Twin and Twin Demise

Vanishing twin describes the clinical situation where following documentation of a twin

gestation by ultrasound, subsequent follow-up ultrasounds demonstrate thedisappearance of one of the gestational sacs or the demise of one of the twins (Fig.

7.12). Vanishing twin is not an uncommon phenomenon. When ultrasound examinations

are performed in the first trimester, about a third of twin pregnancies will ultimately

result in singletons.26 This is even more common in higher order multiples, occurring in

about 50% of triplets.27 In general, patients with vanishing twins are asymptomatic and

the pregnancy outcome does not appear to be affected. As stated previously,

biochemical markers for genetic screening are typically affected, especially when the

vanishing twin occurs later in the first trimester. In case of a vanishing twin with a still

measurable fetal pole, NT alone or in combination with maternal age and other

sonographic markers should be used for aneuploidy risk assessment.28

The presence of a single demise in a twin pregnancy complicates the clinical

management, especially in the presence of monochorionic placentation. In this setting,

careful attention should be given to ultrasound imaging with the application of color

Doppler to rule out the presence of an acardiac twin with twin-reversed arterial

perfusion (discussed later in this chapter). Follow-up ultrasound examinations in the

second trimester are also important to rule out the presence of malformations in the

surviving twin, especially involving the central nervous system. Of note, the earlier in

gestation that the demise of a co-twin occurs in a monochorionic twin pregnancy, the

lower is the risk of neurologic complication in the surviving twin member. In general,

demise of a co-twin embryo/fetus in the first trimester in a dichorionic pregnancy

typically results in a favorable outcome for the surviving twin member.

Twin-Twin Transfusion Syndrome

TTTS, which complicates 10% to 20% of monochorionic twins,29 is an abnormality that

is seen predominantly in the second and third trimesters of pregnancy. TTTS is believed

to occur when vascular anastomoses exist in a monochorionic placenta with net blood

flow going to one fetus at the expense of the other. The recipient twin fetus is typically

plethoric, larger in size, and has polyhydramnios due to excess urination. The donor

twin fetus is anemic, smaller in size, and has a “stuck” appearance due to

oligohydramnios with restricted movements. TTTS can progress quickly and lead to

preterm labor and delivery if untreated. Risk factors for TTTS in monochorionicdiamniotic pregnancies include the presence of velamentous cord insertion and/or

placental arteriovenous (AV) anastomosis without compensating arterioarterial

anastomosis.30 Given that placental vascular anastomoses cannot be accurately

diagnosed prenatally on ultrasound, close ultrasound follow-up (every other week) of

monochorionic-diamniotic twins is recommended starting at 16 weeks of gestation.5Figure 7.12: Dichorionic twins at 13 weeks of gestation with a normal fetus (1),

shown in A on midsagittal plane and a small demised embryo (2), shown on

gray scale ultrasound in B. In three-dimensional ultrasound in surface mode

(C), fetuses (1) and (2) are seen, separated by a thick membrane (asterisk).

Second and third trimester ultrasound is essential for the diagnosis and management

of TTTS. Criteria for establishing the diagnosis of TTTS by ultrasound include a

monochorionic placenta, polyhydramnios in one sac with a maximum vertical pocket of

equal to or greater than 8 cm and oligohydramnios in the other sac with a maximum

vertical pocket of less than 2 cm, in the absence of congenital abnormalities that may

explain fluid and growth discrepancies. In Europe, the diagnosis of polyhydramnios is

made when the maximum vertical pocket is greater to or equal to 8 cm by 20 weeks of

gestation and 10 cm after 20 weeks.5 Concurrent confirmatory features include a small

or non-visible bladder in the donor twin and an enlarged bladder in the recipient twin.

On rare occasions, TTTS can be suspected in the first trimester by the presence of

CRL and NT discordance in the setting of a monochorionic-diamniotic twin

pregnancy.31,32 Other first trimester warning signs include twin discordance on Doppler

findings, especially of the ductus venosus (DV).33 It is important to note however that

the predictive value of CRL, NT, and DV for TTTS in the first trimester is relatively

poor with significant false positives and negatives.5 Close monitoring of themonochorionic pregnancy by ultrasound is still the optimal approach to the early

diagnosis of TTTS.

Twin Reversed Arterial Perfusion

Twin reversed arterial perfusion (TRAP), known as acardiac twinning, is a very rare

condition characterized by monochorionic placentation and absence of a functioning

heart in one fetus of a twin pregnancy (Figs 7.13 to 7.15). Color Doppler and threedimensional ultrasound in the first trimester is helpful in confirming the presence of

TRAP and in assessing the size of the acardiac twin in relationship to the normal twin

(Figs. 7.13 to 7.15). TRAP can be considered as a severe form of TTTS. The normal

fetus perfuses the acardiac mass by an arterial-to-arterial anastomosis on the placental

surface. Typically in normal conditions, the umbilical arteries carry blood from the

fetus to the placenta. In TRAP, the anastomosis allows for reverse perfusion to the

acardiac mass (Fig. 7.15), thus the acronym TRAP. The acardiac fetus commonly has

multiple anatomic and growth abnormalities. Given that the normal fetus has to perfuse

his/her body and that of the acardiac mass, there is a significant increase in cardiac

workload and a risk for cardiac failure and hydrops. The overall perinatal mortality of

the normal fetus in TRAP syndrome is in the range of 30% to 50%.34,35 Beyond the first

trimester, frequent echocardiographic evaluation of the normal twin in TRAP syndrome

may help recognize cardiovascular stress and help guide management. The ratio of the

estimated weight of the acardiac twin to that of the normal twin has been used to assess

mortality risk.34 Treatment options include expectant management with close

observation, or cord coagulation of the acardiac twin. Bipolar cord coagulation of the

acardiac twin appears to be the most feasible option for cord occlusion and is best

performed before 24 weeks of gestation. Treatment intervention before 16 weeks of

gestation is preferable when technically feasible.36Figure 7.13: A: Gray scale ultrasound of an acardiac twin reversed arterial

perfusion (TRAP) in a monochorionic twin pregnancy at 9 weeks of gestation.

Note the presence of an amorphous mass of tissue with an amniotic membrane

covering (small arrows) and a yolk sac, representing the acardiac twin. The

normal twin is seen with its own yolk sac. B: Three-dimensional ultrasound in

surface mode 2 weeks later showing the amorphous acardiac twin with the

TRAP and the adjoining normal fetus.Figure 7.14: Two acardiac twin fetuses (A and B) in monochorionic-diamniotic

pregnancies at 14 and 13 weeks of gestation, respectively, complicated by twin

reversal arterial perfusion (TRAP) sequence. Acardiac twins may have various

appearances but typically body edema is present. Often, a part of a spine (A)

and some bones (A and B) are found and occasionally some parts of the lower

body may be present along with lower extremities. In general, the mass

appears amorphous without any typical anatomic features. See Figure 7.15.

Twin Anemia Polycythemia Syndrome

TAPS is another form of TTTS characterized by significant discrepancy in fetal

hemoglobin in monochorionic twins with normal fluids in both sacs. Suggested

pathophysiology includes small AV placental anastomosis with slow blood transfusion

from one twin member to the other. Incomplete laser treatment of TTTS may also lead to

TAPS.37 The diagnosis is typically performed in the late second or third trimester of

pregnancy. Intertwin discordance in peak systolic velocities of the middle cerebral

arteries (anemia in one twin member) suggests the diagnosis. Pregnancy outcome of

TAPS is generally more favorable than the classic forms of TTTS and TRAP.37Figure 7.15: Two-dimensional ultrasound in gray scale (A) and color Doppler

(B), along with three-dimensional ultrasound in surface mode (C) of a

monochorionic twin pregnancy at 12 weeks of gestation with twin reversal

arterial perfusion (TRAP) sequence. Note in A the presence of edema

(asterisk) and a lower extremity with a femur bone (arrow). Color Doppler of

the umbilical artery in B shows opposite flow (blue color), with flow directed

from the placenta to the acardiac fetus (blue arrow), typical of the TRAP

sequence. Three-dimensional ultrasound shows the acardiac twin with both legs

(arrow) and lower body formed with edema (asterisk).

Cord Entanglement in Monoamniotic Twins

Monochorionic/monoamniotic twins (monoamniotic twins) account for about 1% of all

monochorionic twins. The diagnosis is established when a monochorionic placenta is

noted in a twin pregnancy in the absence of a dividing membrane. It is important to

confirm this diagnosis after multiple sonographic evaluations. The transvaginal

approach is recommended in the first trimester given the high resolution of the

transducer and its proximity to the pregnancy. Monoamniotic twins tend to have

placental cord insertions that are in close proximity and are at significant risk of cord

entanglement. Cord entanglement can be suspected in the first trimester by gray scale

and confirmed by color and pulsed Doppler evaluation. In our experience, cord

entanglement is an almost universal finding in monoamniotic pregnancies and can often

be diagnosed in the first trimester.In the first trimester, cord entanglement appears as a mass of cord between the two

fetuses. Color Doppler will confirm that this mass is indeed entanglement of umbilical

cords (Fig 7.10) and pulsed Doppler can confirm the diagnosis by documenting two

distinct Doppler waveforms, with different fetal heart rate patterns (twin A and twin B)

on one Doppler spectrum (Fig. 7.10). In order to obtain these waveforms, a wide

Doppler gate should be applied to the suspected cord entanglement region. The authors

have correlated the presence of umbilical artery waveform notching on pulsed Doppler

evaluation in monoamniotic twins with cord entanglement in the second and third

trimesters of pregnancy.38 In the absence of signs of fetal compromise, the presence of

cord entanglement with or without umbilical artery waveform notching in monoamniotic

twins does not appear to contribute significantly to perinatal morbidity and mortality

however.39,40

Conjoined Twins

Conjoined twins are very rare complications of monochorionic twinning, which results

from incomplete division of the fertilized egg between days 13 and 15 from conception.

The incidence varies and is reported between 1 in 50,000 and 1 in 250,000 births.41

The anatomic site of conjunction describes conjoined twins. Complex types are

described by a combination of forms. The five common types of conjoined twins and

their frequencies are listed in Table 7.3.

Table 7.3 • Types and Frequency of Conjoined Twins

Type Frequency

Craniopagus (head) 1%–2%

Thoracopagus (chest) 75%

Omphalopagus (abdomen) Rare

Pygopagus (rump) 20%

Ischiopagus (pelvis) 5%

The diagnosis of conjoined twins can be easily made in the first trimester by gray

scale and color Doppler ultrasound by demonstrating shared tissue on gray scale (Figs.

7.16 to 7.19) and vasculature on color Doppler between twins (Figs. 7.16B, 7.17C, and

7.18A). Three-dimensional ultrasound in surface mode in the first trimester can also

confirm the presence of conjoined twins by demonstrating the anatomic site of shared

tissue (Figs. 7.17 to 7.19). The prognosis is generally poor and is dependent on the

degree and site of fusion and the extent of joined organs. Sharing of major organs

complicates postnatal management and worsens the prognosis. Extensive

multidisciplinary counseling should be part of the prenatal management of conjoinedtwins.

Figure 7.16: Conjoined twins noted on two-dimensional gray scale ultrasound

at 9 weeks of gestation (A). Note the fusion of twins in the pelvic area

(asterisk). Cephalic regions of twins are labeled. B: The conjoined twins with

color Doppler ultrasound confirming vascular connectivity between the two

embryos (asterisk). Color Doppler can be used to confirm the diagnosis of

conjoined twins, and differentiate it from monoamniotic non-fused embryos that

are closely positioned in the amniotic cavity. Cephalic region of twins is labeled.Figure 7.17: Thoracopagus conjoined twins have typically the heads close to

each other (A). At the level of the chest an abnormal heart is shared by both as

shown in B and in color Doppler in C. In another fetus with

thoracoomphalopagus at 12 weeks of gestation, three-dimensional ultrasound

in surface mode (D) shows that the twins are joined at the chest and abdomen.Figure 7.18: Conjoined twin gestation with thoracopagus at 10 weeks of

gestation with joined hearts seen in color Doppler in a longitudinal view (A) and

displayed in three-dimensional ultrasound in surface mode (B). Thickened

nuchal translucency (asterisk) is present in both fetuses.1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

Figure 7.19: Conjoined twins at 13 weeks of gestation with

thoracoomphalopagus. Note the presence of closed spina bifida, shown as

cystic meningocele (asterisks), seen in an axial view at the level of the

abdomen in A and in three-dimensional ultrasound in surface mode in B

(asterisk).

CONCLUSION

The first trimester ultrasound plays an important role in the management of multiple

pregnancies. As discussed in this chapter, first trimester ultrasound allows for

pregnancy dating and for determination of chorionicity with high accuracy. With recent

improvements in transducer technology, ultrasound is currently able to diagnose a

substantial number of major fetal malformations in the first trimester. This is of

particular relevance to multiple pregnancies given an overall increased rate of fetal

malformations as compared to singletons, especially for monochorionic pregnancies.

Following chapters in this book present a systematic approach to the diagnosis of fetal

malformations in the first trimester of pregnancy.

R E F E R E N C E S