SOGC CLINICAL PRACTICE GUIDELINE
JUNE JOGC JUIN 2011 l 643
Ultrasound in Twin Pregnancies
No. 260, June 2011
This document reflects emerging clinical and scientific advances on the date issued and is subject to change. The information
should not be construed as dictating an exclusive course of treatment or procedure to be followed. Local institutions can dictate
amendments to these opinions. They should be well documented if modified at the local level. None of these contents may be
reproduced in any form without prior written permission of the SOGC.
This Clinical Practice Guideline has been prepared by the
Diagnostic Imaging Committee, reviewed by the Genetics
Committee and the Maternal Fetal Medicine Committee, and
approved by the Executive and Council of the Society of
Obstetricians and Gynaecologists of Canada .
Lucie Morin, MD, Outremont QC
Kenneth Lim, MD, Vancouver BC
DIAGNOSTIC IMAGING COMMITTEE
Lucie Morin, MD (Chair), Outremont QC
Kenneth Lim, MD (Co-Chair), Vancouver BC
Stephen Bly, MD, Ottawa ON
Kimberly Butt, MD, Fredericton NB
Yvonne M . Cargill, MD, Ottawa ON
Gregory Davies, MD, Kingston ON
Nanette Denis, CRGS, Saskatoon SK
Robert Gagnon, MD, Montreal QC
Marja Anne Hietala-Coyle, RN, Halifax NS
Annie Ouellet, MD, Sherbrooke QC
Shia Salem, MD, Toronto ON
Vyta Senikas, MD, Ottawa ON
Jon Barrett, MD, Toronto ON
R . Douglas Wilson, MD (Chair), Calgary AB
François Audibert, MD, Montreal QC
Jo-Ann Brock, MD, Halifax NS
June Carroll, MD, Toronto ON
Lola Cartier, MSc, Montreal QC
Alain Gagnon, MD, Vancouver BC
Jo-Ann Johnson, MD, Calgary AB
Sylvie Langlois, MD, Vancouver BC
Lynn Murphy-Kaulbeck, MD, Moncton NB
Nanette Okun, MD, Toronto ON
Melanie Pastuck, RN, Cochrane AB
MATERNAL FETAL MEDICINE COMMITTEE
Robert Gagnon, MD, Montreal QC
Lynda Hudon, MD (Co-Chair), Montreal QC
Melanie Basso, RN, Vancouver BC
Hayley Bos, MD, London ON
Joan M . Crane, MD, St . John’s NL
Gregory Davies, MD, Kingston ON
Marie-France Delisle, MD, Vancouver BC
Savas Menticoglou, MD, Winnipeg MB
William Mundle, MD, Windsor ON
Annie Ouellet, MD, Sherbrooke QC
Tracy Pressey, MD, Vancouver BC
Christy Pylypjuk, MD, Saskatoon SK
Anne Roggensack, MD, Calgary AB
Frank Sanderson, MD, Saint John NB
Disclosure statements have been received from all members of
the committees .
Key Words: Ultrasound, twins, antenatal, prematurity, cervix,
Objective: To review the literature with respect to the use of
diagnostic ultrasound in the management of twin pregnancies .
To make recommendations for the best use of ultrasound in twin
Outcomes: Reduction in perinatal mortality and morbidity and
short- and long-term neonatal morbidity in twin pregnancies .
Optimization of ultrasound use in twin pregnancies .
J Obstet Gynaecol Can 2011;33(6):643–656
644 l JUNE JOGC JUIN 2011
SOGC CLINICAL PRACTICE GUIDELINE
Evidence: Published literature was retrieved through searches of
PubMed and the Cochrane Library in 2008 and 2009 using
appropriate controlled vocabulary (e .g ., twin, ultrasound, cervix,
prematurity) and key words (e .g ., acardiac, twin, reversed
arterial perfusion, twin-to-twin transfusion syndrome, amniotic
fluid) . Results were restricted to systematic reviews, randomized
control trials/controlled clinical trials, and observational studies .
There were no date restrictions . Studies were restricted to those
with available English or French abstracts or text . Searches were
updated on a regular basis and incorporated into the guideline
to September 2009 . Grey (unpublished) literature was identified
through searching the websites of health technology assessment
and health technology assessment-related agencies, clinical
practice guideline collections, clinical trial registries, and national
and international medical specialty societies .
Values: The evidence collected was reviewed by the Diagnostic
Imaging Committee of the Society of Obstetricians and
Gynaecologists of Canada, with input from members of the
Maternal Fetal Medicine Committee and the Genetics Committee
of the SOGC . The recommendations were made according to the
guidelines developed by The Canadian Task Force on Preventive
Health Care (Table 1) .
Benefits, harms, and costs: The benefit expected from this guideline
is facilitation and optimization of the use of ultrasound in twin
1 . There are insufficient data to make recommendations on repeat
anatomical assessments in twin pregnancies . Therefore, a
complete anatomical survey at each scan may not be needed
following a complete and normal assessment . (III)
2 . There are insufficient data to recommend a routine preterm labour
surveillance protocol in terms of frequency, timing, and optimal
cervical length thresholds . (II-2)
3 . Singleton growth curves currently provide the best predictors of
adverse outcome in twins and may be used for evaluating growth
abnormalities . (III)
4 . It is suggested that growth discordance be defined using either
a difference (20 mm) in absolute measurement in abdominal
circumference or a difference of 20% in ultrasound-derived
estimated fetal weight . (II-2)
5 . Although there is insufficient evidence to recommend a specific
schedule for ultrasound assessment of twin gestation, most
experts recommend serial ultrasound assessment every 2 to
3 weeks, starting at 16 weeks of gestation for monochorionic
pregnancies and every 3 to 4 weeks, starting from the anatomy
scan (18 to 22 weeks) for dichorionic pregnancies . (II-1)
6 . Umbilical artery Doppler may be useful in the surveillance of twin
gestations when there are complications involving the placental
circulation or fetal hemodynamic physiology . (II-2)
7 . Although many methods of evaluating the level of amniotic fluid in
twins (deepest vertical pocket, single pocket, amniotic fluid index)
have been described, there is not enough evidence to suggest
that one method is more predictive than the others of adverse
pregnancy outcome . (II-3)
8 . Referral to an appropriate high-risk pregnancy centre is indicated
when complications unique to twins are suspected on ultrasound .
(II-2) These complications include:
1 . Twin-to-twin transfusion syndrome
2 . Monoamniotic twins gestation
3 . Conjoined twins
4 . Twin reversed arterial perfusion sequence
5 . Single fetal death in the second or third trimester
6 . Growth discordance in monochorionic twins .
1 . All patients who are suspected to have a twin pregnancy on first
trimester physical examination or who are at risk (e .g ., pregnancies
resulting from assisted reproductive technologies) should have first
trimester ultrasound performed . (II-2A)
2 . Every attempt should be made to determine and report amnionicity
and chorionicity when a twin pregnancy is identified . (II-2A)
3 . Although the accuracy in confirmation of gestational age at the first
and second trimester is comparable, dating should be done with
first trimester ultrasound . (II-2A)
4 . Beyond the first trimester, it is suggested that a combination of
parameters rather than a single parameter should be used to
confirm gestational age . (II-2C)
5 . When twin pregnancy is the result of in vitro fertilization, accurate
determination of gestational age should be made from the date of
embryo transfer . (II-1A)
6 . There is insufficient evidence to make a recommendation of which
fetus (when discordant for size) to use to date a twin pregnancy .
However, to avoid missing a situation of early intrauterine growth
restriction in one twin, most experts agree that the clinician may
consider dating pregnancy using the larger fetus . (III-C)
7 . In twin pregnancies, aneuploidy screening using nuchal
transluscency measurements should be offered . (II-2B)
8 . Detailed ultrasound examination to screen for fetal anomalies
should be offered, preferably between 18 and 22 weeks’ gestation,
in all twin pregnancies . (II-2B)
9 . When ultrasound is used to screen for preterm birth in a twin
gestation, endovaginal ultrasound measurement of the cervical
length should be performed . (II-2A)
10 . Increased fetal surveillance should be considered when there is
either growth restriction diagnosed in one twin or significant growth
discordance . (II-2A)
11 . Umbilical artery Doppler should not be routinely offered in
uncomplicated twin pregnancies . (I-E)
12 . For defining oligohydramnios and polyhydramnios, the
ultrasonographer should use the deepest vertical pocket in either
sac: oligohydramnios when < 2 cm and polyhydramnios when
> 8 cm . (II-2B)
AC abdominal circumference
CL cervical length
EFW estimated fetal weight
IUGR intrauterine growth restriction
NPV negative predictive value
NT nuchal translucency
PPV positive predictive value
TTTS twin-to-twin transfusion syndrome
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Ultrasound in Twin Pregnancies
fashion or stratified according to chorionicity. Established
guidelines for the type and frequency of testing are neither
evidence-based nor uniformly followed. Nevertheless,
despite the lack of level I evidence, virtually all twins are
followed routinely with greater fetal surveillance than low-
risk singleton fetuses.2
SONOGRAPHIC DETERMINATION OF
CHORIONICITY AND AMNIONICITY
Early and accurate determination of amnionicity and
chorionicity is critical in the antenatal management of
twins. Ideally, determination of chorionicity should be
done in the first trimester. The management of structural
anomalies, screening for and identification of aneuploidy,
determination of the etiology of fetal growth and/or fluid
discordance, early diagnosis of twin-to-twin transfusion
syndrome, and the management of a surviving twin
following intrauterine demise are examples of clinical
management depending on chorionicity. The high mortality
and morbidity of monoamniotic twins is well-documented,
and early and intensive monitoring and intervention may
Before 10 weeks’ gestation, several sonographic findings
can help determine chorionicity. These are (1) the number
of observable gestational sacs, (2) the number of amniotic
sacs within the chorionic cavity, and (3) the number of yolk
Table 1. Key to evidence statements and grading of recommendations, using the ranking of the Canadian Task Force
on Preventive Health Care
Quality of evidence assessment*Classification of recommendations†
I: Evidence obtained from at least one properly randomized
A . There is good evidence to recommend the clinical preventive action
II-1: Evidence from well-designed controlled trials without
B . There is fair evidence to recommend the clinical preventive action
II-2: Evidence from well–designed cohort (prospective or
retrospective) or case–control studies, preferably from
more than one centre or research group
C . The existing evidence is conflicting and does not allow to make a
recommendation for or against use of the clinical preventive action;
however, other factors may influence decision-making
II-3: Evidence obtained from comparisons between times or
places with or without the intervention . Dramatic results in
uncontrolled experiments (such as the results of treatment with
penicillin in the 1940s) could also be included in this category
D . There is fair evidence to recommend against the clinical preventive action
E . There is good evidence to recommend against the clinical preventive
III: Opinions of respected authorities, based on clinical experience,
descriptive studies, or reports of expert committees
L . There is insufficient evidence (in quantity or quality) to make
a recommendation; however, other factors may influence
* The quality of evidence reported in these guidelines has been adapted from The Evaluation of Evidence criteria described in the Canadian Task Force on
Preventive Health Care .100
† Recommendations included in these guidelines have been adapted from the Classification of Recommendations criteria described in the The Canadian Task
Force on Preventive Health Care .100
since twins make up > 98% of all multiple pregnancies,
and most published studies in the areas covered by this
document are of twins and not higher order multiples,
this guideline discusses only twins. As twins and higher
order multiples were included in some studies, areas of
this document are applicable to higher order multiples (e.g.,
determination of chorionicity and amnionicity), but others
are applicable only to twin pregnancy.
his document was originally to be written for multiple
pregnancy: twins and higher order multiples. However,
From the first trimester until delivery of the second fetus,
the use of ultrasound in the management of twins is both
ubiquitous and indispensable. Some of the most common
clinical uses are determination of chorionicity, confirmation
of gestational age, diagnosis of anomalies and complications,
measurement of cervical length, and assessment of growth
and amniotic fluid, placental localization, and fetal position
for intrapartum management.
Ultrasound is the only safe and reliable method for the
diagnosis and assessment of twins, although improved
detection of twins by routine sonographic examination
has not led to a significant reduction in perinatal mortality.
This may be due to lack of standardized protocols for the
management of twins rather than the technology itself.1
In addition, protocols for increased surveillance in twins
have not been investigated in a prospective, randomized
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SOGC CLINICAL PRACTICE GUIDELINE
1. Number of Gestational Sacs
The relationship between the number of gestational sacs
and the number of embryonic heartbeats gives strong
evidence of chorionicity. Each gestational sac forms
its own placenta and chorion. Thus, the presence of
2 gestational sacs implies a dichorionic pregnancy, while a
single gestational sac with 2 identified heartbeats implies a
monochorionic twin pregnancy.6
2. Number of Amniotic Sacs Within the
When diamniotic twins are identified before 10 weeks’
gestation, separate and distinct amnions may be visible on
ultrasound. The amnion grows outward from the embryonic
disk, and before 10 weeks the separate amnions of a
diamniotic pregnancy will not have enlarged sufficiently to
contact each other and create the inter-twin septum. Each
single amnion is extremely thin and delicate and may be
very difficult to see on transabdominal scanning; however,
endovaginal imaging is often successful in differentiating
3. Number of Yolk Sacs
The number of yolk sacs may help diagnose the amnionicity.7
When 2 yolk sacs are seen in the extra-embryonal coeloma,
the pregnancy will be diamniotic, while a single yolk sac
will in most cases indicate monoamniotic twins. A single
yolk sac seen when there are dual embryos should prompt
a follow-up first trimester scan to definitively assign
After 10 weeks, these sonographic signs are no longer
present: gestational sacs are no longer distinctly separable,
and the inter-twin membrane is formed. At this stage, a
new set of sonographic findings will help determine
amnionicity/chorionicity. These findings are (1) fetal
genitalia, (2) placental number, (3) chorionic peak sign, and
(4) membrane characteristics.
The following order provides a logical sequence to
determine chorionicity after 10 weeks of gestation. Of
note, step 1 is not routinely used at the 10- to 14-week scan.
1. Sex Discordance
Phenotypic discordance identifies dichorionicity in all but
the rarest cases. Concordance of phenotype does not rule
2. Number of Distinct Placentas
A single placental mass likely indicates monochorionicity,
whereas the presence of 2 distinct, separate placentas
identifies dichorionicity. Careful sonographic examination
may help distinguish a single placenta from 2 placentas in
3. Presence or Absence of the Chorionic Peak
(also called the twin peak or lambda sign)
This represents a projecting zone of tissue of similar
echotexture to the placenta, triangular in cross-section
and wider at the chorionic surface of the placenta,
extending into, and tapering to a point within, the inter-
twin membrane. The twin peak sign most often identifies
dichorionicity.8,9 Monochorionicity can be determined by
absence of the twin peak sign.
4. Inter-Twin Membrane Characteristics
The membrane of a dichorionic pregnancy consists of
2 layers of amnion and 2 layers of chorion. It is thicker
and more reflective than the monochorionic diamniotic
membrane. A membrane thickness of > 2 mm identifies
dichorionicity with a positive predictive value of 95%
and monochorionicity with a positive predictive value of
90% for a membrane thickness ≤ 2 mm.10 In the second
trimester, the number of membranes may be counted, and
if there are > 2, then dichorionicity is strongly suggested.11
If a membrane is not detected, careful evaluation to
diagnose or exclude the possibility of monochorionic
monoamniotic twinning is warranted. When an inter-
twin membrane is not visualized, possibilities include
monoamniotic twinning, presence of a twin with
complete oligohydramnios (stuck twin), or a diamniotic
twin pregnancy in which the membrane is present but
not seen owing to its thinness and orientation to the
transducer. The most definitive sonographic finding in the
diagnosis of monoamniotic twins is the demonstration
of cord entanglement from the placental or umbilical
origin. Colour Doppler may facilitate identification of
this finding. Entanglement of limbs or observation
of a limb circumscribing the other is suggestive of
monoamnionicity. Failure to find the membrane between
the 2 cord insertions in the placenta strongly supports
monoamnionicity. The use of transvaginal ultrasound is
often a helpful adjunct to transabdominal scanning in
identifying the membrane.
Accuracy is improved when the assessment of chorionicity
is undertaken before 14 weeks’ gestation rather than
after 14 weeks. Stenhouse et al.8 in a study of 131 twin
pregnancies found the sensitivity after 14 weeks was 77%
for monochorionicity (10/13) and 90% for dichorionicity
(26/29); before 14 weeks, accuracy was 99% for both
groups (98/99 overall, 21 of 22 for monochorionics)
The twin peak sign alone in the second trimester can
accurately identify the chorionicity in many cases, but that
may not be sufficient to guide clinical management in
JUNE JOGC JUIN 2011 l 647
Ultrasound in Twin Pregnancies
all cases.9,12 Scardo et al.12 in their second trimester study
found that the twin peak alone may not be sufficiently
accurate. With a composite of second trimester ultrasound
markers (number of placentas, fetal phenotype, membrane
thickness, and twin peak sign), the sensitivity for correct
identification of monochorionic pregnancies is reported
at 91.7% with 97.3% specificity.12 In the second trimester,
the twin peak sign becomes more difficult to visualize,
and it disappears in about 7% of dichorionic pregnancies
at 16 to 20 weeks. Therefore, the absence of the twin
peak sign in the second or third trimester cannot exclude
1. All patients who are suspected to have a twin
pregnancy on first trimester physical examination
or who are at risk (e.g., pregnancies resulting from
assisted reproductive technologies) should have first
trimester ultrasound performed. (II-2A)
2. Every attempt should be made to determine and
report amnionicity and chorionicity when a twin
pregnancy is identified. (II-2A)
AGE IN TWIN PREGNANCIES
The accurate confirmation of gestational age using
ultrasound is essential to pregnancy management. It
necessitates determining whether there is a high probability
that the measurements of the fetus are appropriate for the
estimated gestational age. Early studies of the reliability
of ultrasound to confirm gestational age used menstrual
dating in women with regular cycles; however, menstrual
dating is fraught with biological variability. More recently,
studies of this nature were done in IVF pregnancies, for
which conception date is known precisely, but it is unclear
whether this will work as well with natural conceptions.
The literature on gestational age confirmation is also not
specific to multiple pregnancies, and in general these studies
were a mixture of singletons, twins, and triplets, with
the vast majority of subjects being singleton.14–17 Studies
assessing the benefits of confirmation of gestational age
by ultrasound have been published,18 but with singleton not
twin pregnancies. A comprehensive and critical review of
this topic is well beyond the scope of this document.
The first trimester is generally considered to be the ideal time
to confirm or establish accurate gestational age dating, and it
is statistically superior to second trimester dating. However,
in 2 dating studies using twins, the difference in accuracy
compared with IVF could be considered clinically insignificant
(underestimate 1 day from IVF dating) with both first (11 to
14 weeks) and second (18 to 22 weeks) trimester ultrasound
estimates being very accurate in relation to conceptual age by
IVF.15,19 Hence, in twins, although there is expert consensus
that first trimester ultrasound dating is preferable, second
trimester dating is also acceptable and accurate.
The best parameter or parameters to use for the most
accurate dating vary according to the gestational age. Many
studies show that singleton dating formulas work equally well
with twins, hence studies in this area are usually a mixture of
singletons and multiples.14–17 In the first trimester, crown–
rump length provides appropriate gestational dating within
5 to 7 days.15–17 First trimester crown–rump length and
second trimester biparietal diameter provide gestational age
with an error of plus or minus 7 days and are very similar in
accuracy.16 In the second trimester, different combinations
of each parameter demonstrate slight differences in
accuracy, with the best estimate using a combination of
head circumference, abdominal circumference, and femur
length.14 Some centres use an average of all parameters,
equally weighted, or use mathematical formulas that give
different significance to each parameter used. There are
more than 30 different formulas in the literature, using
different combinations of parameters.14 In general, about
95% of gestational age estimates in the first and second
trimester will be within 5 to 7 days of the “true” gestational
age, regardless of the parameter or parameters used.14,16,17
In twin pregnancies, modest size discordance is common.
Several studies have cited the need to use the larger twin for
dating purposes to minimize the chance of missing a fetus
that might present with IUGR.16,19 Some studies have based
the estimated gestational age on the mean of the fetuses.14
Salomon et al.20 recently suggested that if the inter-twin
crown–rump length discrepancy was less than the 95th
percentile, according to their charts, the biometry of the
smaller fetus was the more correlative with the conception
date of IVF pregnancies.20 However, the majority of
centres, largely on the basis of expert opinion, use the
larger of the 2 fetuses to date a pregnancy, erring on the
side of overestimation of gestational age and lessening the
chance of missing IUGR in the smaller twin.
Therefore, there is no absolute consensus on the optimal
method to determine gestational age in twin pregnancies.
Most academic centres use the estimated gestational age
based on a known last menstrual period, corrected for a
regular cycle length if the initial ultrasound falls within
an accepted range of days. If the fetal biometry does not
agree, new gestational age estimates can be established with
an anticipated accuracy of 5 to 7 days. Further study in this
area appears to be warranted.
648 l JUNE JOGC JUIN 2011
SOGC CLINICAL PRACTICE GUIDELINE
3. Although the accuracy in confirmation of
gestational age at the first and second trimester
is comparable, dating should be done with first
trimester ultrasound. (II-2A)
4. Beyond the first trimester, it is suggested that a
combination of parameters rather than a single
parameter should be used to confirm gestational
5. When twin pregnancy is the result of in vitro
fertilization, accurate determination of gestational
age should be made from the date of embryo
6. There is insufficient evidence to make
recommendation of which fetus (when discordant
for size) to use to date a twin pregnancy. However,
to avoid missing a situation of early intrauterine
growth restriction in one twin, most experts agree
that the clinician may consider dating pregnancy
using the larger fetus. (III-C)
SCREENING FOR ANOMALIES
IN TWIN PREGNANCIES
Aneuploidy Screening in First Trimester
The literature on aneuploidy screening in twins is relatively
scant, consisting of small studies with < 10 abnormal
fetuses.21–24 Conclusions are inconsistent, and much larger
studies are required to provide definitive answers.
Nuchal transluscency and maternal age in twins
In 1996, Sebire and colleagues22 evaluated NT in 448
twin pregnancies (both dichorionic and monochorionic).
A total of 7.3% of fetuses had an elevated NT above
the 95th percentile. In 88.4% of twin pregnancies, both
fetuses had a normal NT. An elevated NT was seen in
one fetus in 8.7% and in both fetuses in 2.9%. Seven of
8 Down syndrome fetuses were detected with an overall
sensitivity of 88%, which is comparable with the singleton
detection rate. The screen positive rate was higher in
monochorionic twins at 8.4% than in dichorionic twins
at 5.4%.22 In a small study of monochorionic twins,
Vandecruys et al.24 suggested that the best performance
was achieved using the average NT, rather than the larger
or smaller NT measured within a twin pair. Using the
average NT resulted in an estimated 100% sensitivity for
a 4.2% false-positive rate. It would appear that NT in
conjunction with maternal age has the potential to reach
the standard of 75% sensitivity for a 5% screen positive
rate proposed by the SOGC in 200725; however, larger
studies are needed to verify this.
NT may also be useful in the early detection or prediction
of TTTS. One study suggests that an increased NT in
monochorionic twins may be an early manifestation
of TTTS. An NT threshold at the 95th percentile had a
positive and negative predictive value of 43% and 91%,
7. In twin pregnancies, aneuploidy screening using
nuchal transluscency measurements should be
Aneuploidy Screening in the Second Trimester
The use of genetic sonograms to detect Down syndrome
in the second trimester has been well studied in singleton
pregnancies. A detailed scan is performed with a number of
soft markers of Down syndrome, and if there are abnormal
findings, a fetus-specific risk is calculated according to
the soft marker detected. In twins, the singleton genetic
sonogram principles are applied to individual fetuses,
and prenatal diagnosis is offered if sufficient risk exists.27
However, there is very little, if any, information to
estimate the efficacy of this approach in twins. Typically,
in mixed population studies, the data in twin pregnancies
are combined with the singleton data, and abstraction of
efficacy specific to twins is impossible.28 In one study, soft
marker discordance was sought for in twin sets discordant
for Down syndrome. Of the markers studied, nuchal
translucency thickness was found to correctly identify
5 of 9 Down syndrome cases, with the other markers
being significantly less efficacious.29 Therefore, although
there may be some utility of second trimester ultrasound
in screening for Down syndrome in twins, its efficacy is
Congenital anomalies are 1.2 to 2 times more common
in twin gestation.30 In dizygotic twins the rate per fetus
is the same as in singletons, whereas in monozygotic
twins the rate is 2 to 3 times higher.31 The most common
structural abnormalities are cardiac anomalies, neural tube
and brain defects, facial clefts, and gastrointestinal and
anterior abdominal wall defects. Apart from structural
defects, which also occur in singletons, there are 3 types of
congenital anomalies unique to twin pregnancies.30
1. Midline structural defects, believed to be a consequence
of the twinning process, exemplified by conjoined
2. Malformations resulting from vascular events as a
consequence of placental anastomoses, leading to
hypotension and/or ischemia. This can happen to
JUNE JOGC JUIN 2011 l 649
Ultrasound in Twin Pregnancies
a surviving twin after the demise of the other twin.
Anomalies seen as consequence of such events
include microcephaly, periventricular leukomalacia,
hydrocephalus, intestinal atresia, renal dysplasia, and
3. Defects or deformities from intrauterine crowding:
foot deformities, hip dislocation, and skull
Edwards et al.32 evaluated the accuracy of antenatal
ultrasound in the detection of fetal anomalies in 245 twins
managed in a specialized multiples clinic. The prevalence of
anomalies was 4.9%. In this study, antepartum ultrasound
detected 88% of anomalies; ultrasound for the detection
of congenital anomalies in twins therefore appears to be
Twin pregnancies will be scanned multiple times during
pregnancy, predominantly to assess fetal growth. There
are no data to determine whether formal reassessment of
fetal anatomy at each scan is of significant value in anomaly
detection in twins. Only one study of singleton pregnancies33
found that routine repeat anatomy scanning in the early
third trimester resulted in further diagnosis of anomalies.
In the second trimester, a major anomaly was detected in
0.36% of scanned fetuses, and anatomical reassessment in
the third trimester resulted in further diagnosis in 0.22% of
the fetuses previous assessed as “normal.” The anomalies
detected were predominantly lesions that may develop
late in pregnancy and that would not be detectable in the
mid-second trimester. Given the number of ultrasound
examinations per twin pregnancy and the rising rates of
multiple gestations, the resource implications of a policy
of repeated anatomical evaluation are significant. There
are insufficient data to make a recommendation on how
often repeat anatomical survey should be done in twin
Ultrasound scanning for fetal anomalies in twins is clearly
justifiable and best performed between 18 and 22 weeks’
gestation. A management plan necessitates knowledge of
chorionicity and consideration of the risk to the unaffected
1. There are insufficient data to make recommendations
on repeat anatomical assessments in twin pregnancies.
Therefore, a complete anatomical survey at each scan
may not be needed following a complete and normal
8. Detailed ultrasound examination to screen for
fetal anomalies should be offered, preferably
between 18 and 22 weeks’ gestation, in all twin
SCREENING FOR PRETERM BIRTH
Preterm birth is a major cause of mortality and morbidity in
twin pregnancies. Sonographic assessment of the cervical
length can identify twins at significantly elevated risk of preterm
delivery. A number of studies have shown that cervical length
can help identify those twins that may be at either increased or
reduced risk of early delivery. Most of these studies include both
monochorionic and dichorionic pregnancies, and differentiation
on that basis is not known. Studies varied with respect to the
cervical length threshold chosen, the gestational age at which the
cervical length assessment was performed, and the definition of
preterm birth. Tables 2 and 3 show information from studies
that were similar with respect to these 3 variables. None of the
studies included patients who had a cerclage, and all studies
employed transvaginal ultrasound.
The studies listed in Table 2 show that a finding of a
certain cervical length measured between 21 and 24 weeks
correlates highly with preterm birth at < 32 to 33 weeks.
The results are fairly consistent in that the risk of preterm
birth is increased 3- to 5-fold from baseline prevalence.
The absolute PPV ranges from 22% to 38 %. Notably, the
negative predictive values are quite high and consistent
across these studies: 94% to 96%.
Table 3 shows studies that attempted to define a threshold
at which the likelihood of delivery prior to 34 to 35 weeks
is low. The results are more variable than those shown in
Table 2. Risk of spontaneous preterm birth (< 32 to 33 weeks) given various CL thresholds
AuthorPrevalenceN CL (mm)GA (weeks) SensitivitySpecitivity % PPV %NPV %
Goldenberg et al .34
8 .8%147 ≤ 2524 53 .8 85 .826 .995 .0
Skentou et al .35
7 .8% 434≤ 2522 to 2435 .3 91 .8 26 .794 .3
Vayssiere et al .36
5 .4%251 ≤ 25 21 to 23389738 96
Sperling et al .37
6 .0% 383 ≤ 2023 21 .496 .427 .895 .4
Guzman et al .38
9 .2% 131≤ 2021 to 24 42 .0 85 22 .094
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Table 2. Given a CL > 35 mm measured around the mid-
second trimester, the probability of reaching 34 to 35
weeks is quite high (88% to 98%).
Cervical length decreases with increasing gestational age,
and those who deliver preterm have a cervical shortening
rate greater than those who do not. Fujita et al.,41 in a
study of 144 twin pregnancies delivering after 34 weeks,
demonstrated a cervical length decrease of 0.8 mm/week.
Bergelin et al.42 found that the median rate of cervical
shortening in women who delivered at term was 1.8 mm
per week compared with a rate of 2.9 mm/week for those
who delivered preterm. Gibson et al.43 found that a rate
of cervical shortening > 2.5 mm/week predicted preterm
delivery (positive likelihood ratio of 10.8). Thus, it is clear
that progressive shortening greater than expected may
indicate a higher risk of preterm labour.
However, application to clinical practice is less clear.
The 95% confidence interval of inter- and intraobserver
variability (intraobserver repeatability coefficient of
approximately ± 6 mm and the interobserver limits of
agreement was approximately ±10 mm)44 is quite large
relative to the reported rates of cervical change. Observed
changes may simply be observer variability unless the
interobservation interval is quite long. There is also no
proven intervention in this scenario. Thus, the optimal
protocol for serial CL evaluation in twins is unclear.
In women with signs and symptoms of preterm labour
between 23 and 33 weeks, CL was a better predictor of
preterm delivery than funnelling and digital examination.45
Fuchs et al.46 found that among twin pregnancies that
presented in preterm labour, the longer the CL, the less
likely it was that delivery would occur within 1 week. At a
cervical length of > 25 mm, there were no deliveries (0/21)
that occurred within a week, whereas when cervical lengths
were ≤ 15 mm, the rate of delivery was 44% (18 of 32).
2. There are insufficient data to recommend a routine
preterm labour surveillance protocol in terms
of frequency, timing, and optimal cervical length
9. When ultrasound is used to screen for preterm
birth in a twin gestation, endovaginal ultrasound
measurement of the cervical length should be
ASSESSMENT OF FETAL GROWTH
The growth of twins is not significantly different from the
growth of singletons in the first and second trimesters.
However, there is disagreement regarding the rate of
fetal growth in the third trimester in uncomplicated twin
pregnancies. Most studies have described slower fetal growth
after 30 to 32 weeks’ gestation.47–50 The slower growth rate
in twins has been attributed to placental crowding and more
frequent anomalous umbilical cord insertion.
The American Congress of Obstetricians and Gynecologists
technical bulletin on assessment of growth51 suggests
that centres should use growth tables derived from twin
gestations. However, most studies of twin growth curves are
derived from a small sample size that includes pregnancies
with adverse outcomes and do not take into account
chorionicity, race, or gender. The argument in favour of using
twin growth charts is that it likely prevents the over-diagnosis
of IUGR in normally grown twins (which would result in
an increase in iatrogenic preterm delivery). A large cohort
study52 comparing the outcome of twins and singletons,
taking into account chorionicity and fetal growth centiles,
demonstrated that twins with growth restriction (defined
using singleton growth curves) were not protected from
perinatal loss; growth restricted monochorionic twins were,
in fact, at increased risk of perinatal mortality. Therefore,
although it is suggested that the twin growth curve pattern
starts to decelerate from 32 weeks’ gestation, IUGR twins
defined according to the singleton growth curve have worse
outcomes than those defined as appropriately grown using
the same curve. Thus, the literature still suggests that the
use of biometry charts from singletons in the follow-up of
twin pregnancy provide good predictors of adverse perinatal
outcome.52 Further investigation in this area using twin
growth charts is warranted.
Table 3. Probability of exceeding 34 to 35 weeks given CL > 35 mm
threshold %N CL (mm)
GA at scan
% PPV %
Soriano et al .39
> 35 79 .5 44 > 3518 to 24 88 .588 .9 96 .7
Sperling et al .37
> 34 87 383> 3523 62 .8 5490 .1
Yang et al .40
> 3576 .9 65> 35 18 to 2690 93 .397 .6
Vayssiere et al .36
> 35 85 .3225 > 3021 to 23 9027 .3 87 .8
GA: gestational age
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Determination of fetal growth discordance is important,
because studies have shown an association with increased
mortality and morbidity when there are significant
differences in birthweight.53–58 Therefore, detection of
antenatal growth discordance by ultrasound is useful in
identifying twins that may require increased surveillance to
prevent higher fetal/neonatal complications. Confounding
factors in studies of twin growth discordance include
chorionicity, gestational age at delivery, and growth
restriction relative to expected birth weight, as well as
suboptimal sample size. Growth discordance has been
defined in several ways, with the most common being the
difference in estimated fetal weight derived by ultrasound
biometry.59 Another method uses absolute differences
in abdominal circumference.59 Both methods have their
strengths and weaknesses.
Birthweight discordance is defined by the following
formula, using the larger of the twins as the denominator.
There is no single definition of growth discordance in twins.
Clinically significant birthweight threshold definitions
in the literature (based on morbidity and mortality in the
postnatal population) range from 15%54 to 30%.57,60 The
largest (more than 250 000 cases) and most recent postnatal
study55 (which also corrected for IUGR) found statistically
significant odds ratios of neonatal mortality for the smaller
fetus at 25% birthweight discordance and for the larger
fetus at 30% birth weight discordance.
Study findings have not been consistent with respect to
the accuracy of ultrasound to diagnose discordance.61–69
This may be due to the error associated with all of the
ultrasound-derived estimated fetal weight formulas. The
SOGC consensus statement on twin gestations59 suggests
using an EFW discordance of > 20%. Given the relative
imprecision of EFW formulas (none of which were
determined from pure twin populations) and the desire to
have a high index of suspicion, adopting a 20% threshold
is a reasonable option.
Another definition of significant growth discordance
includes abdominal circumference
differences of > 20 mm. A large study by Caravello et al.66
compared the use of AC difference and ultrasound EFW
difference to predict true birthweight discordance. This
study was of twins delivered between the mid-second
trimester and term. Other studies were not all concordant
in term of definitions. The range of sensitivities for IUGR
was 43% to 83% and 33% to 93% for AC and EFW,
respectively. The range of specificities for appropriate for
gestational age was 68% to 91% versus 81% to 98% for AC
and EFW, respectively.66 Studies that directly compared the
2, showed them to be equally efficacious compared with
estimated fetal weight formulas.66,67 Regardless of growth
curves used, increased fetal surveillance is indicated when
abdominal circumference and/or EFW of one or both
twins is < 10th percentile or when growth discordance is
3. Singleton growth curves currently provide the best
predictors of adverse outcome in twins and may be
used for evaluating growth abnormalities. (III)
4. It is suggested that growth discordance be defined
using either a difference (20 mm) in absolute
measurement in abdominal circumference or a
difference of 20% in ultrasound derived estimated
fetal weight. (II-2)
There are few published studies indicating how frequently
routine reassessment should be done in twin pregnancies.
Giles et al.73 in a secondary analysis of their randomized
trial (all twins) reported fewer fetal deaths than expected
in their routine surveillance group: 11.4/1000 live births
(9/1000 live births in the Doppler group) compared with
historical control subjects (85.7/1000). In that study, twins
had repeat biometry scans at 30 and 35 weeks after a normal
25-week scan. Thus routine surveillance of twin pregnancies
every 5 weeks appears to be beneficial. Whether more
frequent surveillance would improve the results further
remains to be seen; however, it is suggested that more
frequent surveillance will result in significantly higher false-
positive rates for IUGR.74
In current practice, the frequency of ultrasound evaluation
in twin pregnancies is determined according to chorionicity
and growth pattern. In general, when monochorionic twin
pregnancies are identified, ultrasound scans are scheduled
every 2 to 3 weeks, starting at 16 to 18 weeks, to better
ascertain early evidence of TTTS.59 For all twin pregnancies,
the anomaly screening scan should be scheduled at 18 to
22 weeks. Most tertiary care centres routinely assess fetal
growth every 2 to 4 weeks, depending on chorionicity, largely
on the basis of expert opinion.59 Monochorionic twins are
scanned more frequently to allow for earlier diagnosis of
TTTS and/or growth restriction or discordance, which have
greater implications for the non-affected twin than they do
in dichorionic pregnancies. Some specialized centres or
clinics perform growth scans more frequently than every
EFW larger twin − EFW smaller twin
EFW largest twin
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SOGC CLINICAL PRACTICE GUIDELINE
2 weeks in uncomplicated monochorionic twin pregnancies,
but there is little evidence beyond expert opinion to support
this practice. Some centres advocate scanning dichorionic
twins every 3 weeks in the third trimester, since the growth
rate slows down after 30 to 32 weeks.
Grobman and Parilla75 found that in twins (of all types)
the positive predictive value of a sonogram for a growth
abnormality at birth significantly decreased if the 20- to
24-week sonogram was normal. Furthermore, in gestations
with normal growth at 20 to 24 weeks a mean of 10.3 (± 3.9)
weeks elapsed before a growth abnormality was subsequently
detected.75 This suggests that some routine growth scans
may be of very limited benefit while increasing the false
positive rate. Increased surveillance is warranted when one
or both fetuses show growth restriction or discordance. In
these circumstances, serial growth scans every 2 to 3 weeks
(or more frequently in monochorionic twins) and fetal
surveillance testing are indicated as for singleton (Doppler,
non-stress test, and/or biophysical profile).
5. Although there is insufficient evidence to recommend
a specific schedule for ultrasound assessment of twin
gestation, most experts recommend serial ultrasound
assessment every 2 to 3 weeks, starting at 16 weeks of
gestation for monochorionic pregnancies and every
3 to 4 weeks, starting from the anatomy scan
(18 to 22 weeks) for dichorionic pregnancies. (II-1)
10. Increased fetal surveillance should be considered
when there is either growth restriction diagnosed in
one twin or significant growth discordance. (II-2A)
USE OF UMBILICAL ARTERY
DOPPLER VELOCIMETRY IN TWINS
Because inequality of the 2 fetal-placental circulations can
cause inter-twin differences in growth, umbilical artery
Doppler velocimetry may improve the detection of IUGR
or fetal growth discordance.65 The largest trial of Doppler
assessment of twin pregnancy (n = 526) compared
routine biometric ultrasound assessment to routine
assessment plus umbilical artery Doppler velocimetry in
a randomized fashion at 25, 30, and 35 weeks’ gestation.73
There were no differences between groups in any
antenatal, intrapartum, or neonatal outcome; there were
fewer unexplained fetal deaths in the Doppler group,
but this was not statistically significant. Unfortunately,
this study was limited by insufficient power and because
monochorionic pregnancies were not analyzed separately.
The available data do not show a clear benefit of Doppler
velocimetry over the use of ultrasound alone; therefore,
routine use of Doppler velocimetry in twin gestations
cannot be recommended at this time.
Of note, in uncomplicated monochorionic twins, uterine
artery waveform abnormalities may be common, and
they reflect retrograde transmission of arterio-arterial
interference patterns in the presence of large arterio-
arterial anastomosis rather than fetal compromise.76,77
6. Umbilical artery Doppler may be useful in the
surveillance of twin gestations when there are
complications involving the placental circulation
or fetal hemodynamic physiology. (II-2)
11. Umbilical artery Doppler should not be routinely
offered in uncomplicated twin pregnancies. (I-E)
ASSESSMENT OF AMNIOTIC FLUID
Currently available evidence78–81 is insufficient to make a
formal recommendation on the best method of amniotic
fluid assessment in twins. Outcome-based studies are
lacking. Identification of the inter-twin membrane is vital
in order to determine the fluid space around each fetus.
Accepted methods for fluid estimation include subjective
assessment, deepest vertical pocket, modified amniotic
fluid index and 2-dimensional pockets. Another method
is to ascertain the presence of fluid, caudal and rostral,
and determine to which fetus it belongs and subjectively
estimate if normal. When amniotic fluid volume appears
reduced or increased, the vertical measurement of the
largest pocket in each sac is taken. The condition is
defined as oligohydramnios when the deepest vertical
pocket < 2 cm and as polyhydramnios when the deepest
vertical pocket is > 8 cm. These definitions correspond
approximately to the 2.5th percentile and 95th percentile
across all gestational ages.82 This is also a common
criterion used in defining TTTS, and for these reasons,
this may be the clinically useful method for assessing
amniotic fluid in twins.83
7. Although many methods of evaluating the level of
amniotic fluid in twins (deepest vertical pocket, single
pocket, amniotic fluid index) have been described,
there is not enough evidence to suggest that one
method is more predictive than the others of adverse
pregnancy outcome. (II-3)
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Ultrasound in Twin Pregnancies
12. For defining oligohydramnios and polyhydramnios,
the ultrasonographer should use the deepest vertical
pocket in either sac: oligohydramnios when < 2 cm
and polyhydramnios when > 8 cm. (II-2B)
Diagnosis of Twin-To-Twin Transfusion Syndrome
Prenatal diagnosis of twin-to-twin transfusion syndrome
is made on the basis of specific ultrasound criteria.
Monochorionic twins with
polyhydramnios sequence and the presence of a large fetal
bladder in the polyhydramnios twin and a small or absent
fetal bladder in the oligohydramnios twin are consistent
with TTTS. Discordance in fetal size with the larger twin
in the polyhydramnios sac is often seen but is not essential
to the diagnosis. A pathognomonic sign for the diagnosis
of TTTS is the appearance of the donor as the stuck twin
contained within the collapsed inter-twin membrane because
of anhydramnios. Doppler studies are also part of the
diagnostic evaluation. Absent or low end diastolic flow in
the umbilical artery of the donor and decreased ventricular
function depicted by tricuspid regurgitation, reversal of A
wave in ductus venosus, and/or cardiac chamber enlargement
in the recipient are seen in more advanced stages of TTTS.
Currently, the Quintero classification method83 is used to
stage and determine the management plan for TTTS.
Stage 1 oligo-polyhydramnios sequence
Stage 2 absent bladder in the donor
Stage 3 abnormal fetal vascular Doppler studies
Stage 4 hydrops of one fetus
Stage 5 death of one fetus
In the absence of oligo-polyhydramnios sequence, the
diagnosis of TTTS should be entertained with caution
when fetal growth discordance is seen in the presence
of velamentous cord insertion, 2 vessel cord, or unequal
DIAGNOSIS OF RARE OBSTETRICAL
COMPLICATIONS UNIQUE TO TWINS
This occurs in approximately 1% of all monozygotic twin
pregnancies. These pregnancies are at elevated risk of fetal death
because of cord entanglement. Early series reported double
survival in only 46% to 65% until 30 to 32 weeks’ gestation.84,85
More recent series reported improved double perinatal survival
of 92% when accurate prenatal diagnosis, serial sonography,
and antenatal testing were done.86 Thus early identification is
important in the management of these pregnancies.
First trimester ultrasound can predict virtually all cases of
monoamniotic twins. Other sonographic indicators include
the presence of a single yolk sac and detection of cord
entanglement.87 In the second trimester, the diagnosis of
monoamnionicity is made on the basis of the following
second trimester ultrasound criteria: (1) single shared
placenta, (2) fetal phenotype concordance, (3) absence
of inter-twin membrane, (4) adequate amniotic fluid
surrounding both fetuses, and (5) free movement of both
twins within the uterine cavity.
Twin Reversed Arterial Perfusion Syndrome
Also known as acardiac twinning, twin reversed arterial
perfusion syndrome occurs in 1 in 35 000 deliveries, 1 in
100 monozyotic twins, and 1 in 30 monozygotic triplets.88
These pregnancies have a 90% risk of preterm birth and
a 30% risk of congestive heart failure in the normal twin
(also called pump twin).89 Diagnosis of acardiac twins is
made when one monochorionic twin has the absence of
cardiac pulsation along with poor definition of fetal parts.
Definitive diagnosis is established with colour Doppler
demonstrating reversal of blood flow within the abnormal
fetus. Blood-flow pattern reveals a paradoxical direction of
arterial flow towards rather than away from the acardiac twin
and retrograde flow in the acardiac twin’s abdominal aorta.
Differential diagnosis includes intrauterine fetal demise or an
abnormal monochorionic twin, or placental tumours.
After the diagnosis of twin reversed arterial perfusion
syndrome sequence is made, fetal hemodynamic function
should be assessed by fetal echocardiography; hydrops in
the pump twin being a poor prognostic feature. In addition,
estimation of the weight ratio of the acardiac to the pump
twin should be established. In a 1990 study, Moore et al.89
found that when the weight of the acardiac twin was ≥ 70%
of the weight of the normal pump twin, the incidence of
preterm birth, polyhydramnios, and fetal hydrops was 90%,
40%, and 30%, respectively. When the ratio was < 70%, the
rates were 70%, 30%, and 10%.89 When the weight ratio
was < 50% the complication rates were 18%, 0%, and 35%
compared with 44%, 25%, and 94% when > 50%.92 The
overall perinatal mortality was 55% in this untreated cohort.89
A multitude of treatment options have been described in the
literature, and the optimal method depends on gestational
age and centre experience. In a review of all reported cases of
minimally invasive therapies, Tan et al.88 reported an overall
pump twin survival of 74%. Two recent series reviewed the
use of radio frequency ablation and reported pump twin
survival rate of around 90%.90,91 However, in one small case
series of untreated, antenatally diagnosed acardiac twins,
the perinatal survival of the pump twin was 90%, with 40%
demonstrating spontaneous cessation of flow in the acardiac
654 l JUNE JOGC JUIN 2011
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twin over time.92 Because of the complexity of these cases
and the possible management options, including expectant
management,92,93 referral to a tertiary care unit is indicated.
The incidence of conjoined twins varies between 1 in 50 000
and 1 in 100 000 births.94,95 The diagnosis can be made by
ultrasound examination in the first trimester. If the embryo
appears bifid, follow-up imaging should be performed to
confirm the diagnosis. Other clues to the diagnosis include
the inability to separate the fetal bodies and skin contours, lack
of a separating membrane between the twins, the presence
of more than 3 vessels in the umbilical cord, heads remaining
at the same level and body plane, extremities in unusual
proximity, and failure of the fetuses to change their relative
positions over time. Of all conjoined twins, only those who
are omphalopagus have a reasonable chance of survival.96
Single Fetal Death
It is estimated that only 50% of twin pregnancies identified
in the first trimester will result in 2 live born infants.97 When
the demise occurs early in pregnancy, the prognosis for the
surviving fetus is excellent.98,99
Demise of one fetus occurs in 2% to 5% of twin
pregnancies during the second and third trimesters. The
occurrence of single fetal death is 3-fold to 4-fold higher
in monochorionic twins than in dichorionic twins. It is
also more common in high-order multiples, complicating
14% to 17% of triplet pregnancies. The loss of a fetus in a
twin gestation has been associated with adverse outcomes
for the surviving fetus. The greatest risk to the surviving
fetus, regardless of chorionicity, is preterm delivery and
the associated complications of prematurity. Overall,
50% to 80% of surviving twins are born preterm, most
often because of preterm labour. In monochorionic twins,
multi-organ damage in the surviving twin can occur.
Ischemic injury, which is thought to occur at the time of
the demise, has been documented in the spleen, kidney,
gastrointestinal tract, skin, and brain of the surviving
twin. Up to 20% of surviving fetuses in monochorionic
twin pregnancies may experience neurologic injury, such as
multicystic encephalomalacia. These abnormalities may not
be diagnosed by ultrasound until much later in pregnancy,
far removed from the ischemic event. Immediate delivery
may not prevent the development of such complications.
In dichorionic twin pregnancies, the risk of major perinatal
morbidity or mortality to the surviving twin appears to be
negligible, apart from the risk related to preterm delivery.
8. Referral to an appropriate high-risk pregnancy centre
is indicated when complications unique to twins are
suspected on ultrasound. (II-2) These complications
1. Twin-to-twin transfusion syndrome
2. Monoamniotic twins gestations
3. Conjoined twins
4. Twin reversed arterial perfusion sequence
5. Single fetal death in the second or third trimester
6. Growth discordance in monochorionic twins.
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