Early ultrasound assessment and accurate determination of chorionicity is crucial so that appropriate care of multiple pregnancy can be provided. It is best achieved in the first trimester of pregnancy using the Lambda ‘λ’ and ‘T’ signs. Accurate labelling of the twins is needed to ensure that the same individual fetus is measured through the pregnancy so that the longitudinal growth pattern can be correctly assessed. Discrepancy in crown–rump length indicates a possibility for future development of selective intrauterine growth restriction. Careful early ultrasound assessment is needed to identify structural and chromosomal anomalies, as twin pregnancies are at increased risk. Twin-to-twin transfusion syndrome, selective intrauterine growth restriction and congenital abnormalities represent the major determinants of perinatal loss in monochorionic pregnancies, and diagnosis and prognosis are discussed in detail. Treatment of twin reverse arterial perfusion sequence is more effective in early pregnancy, so early identification is needed. Outcome of conjoined twins is guarded, and is dependent on the extent of fusion, degree of sharing of organs, associated anomalies, and presence of cardiac failure in utero .
Introduction
Multiple pregnancies are at increased risk of neonatal mortality and morbidity compared with singleton pregnancies, This may result from preterm birth, congenital abnormalities, growth problems, and unique complications related to monochorionicity, such as twin-to-twin transfusion syndrome (TTTS) and twin reversed arterial perfusion (TRAP) sequence. Early ultrasound assessment is crucial, so that appropriate care of these pregnancies can be provided. Determination of chorionicity in the first trimester allows stratification of the obstetric risk and the planning of subsequent scans, whereas accurate labelling of twins is fundamental for the evaluation of the longitudinal growth pattern of twins, especially when discordance is observed.
Early ultrasound assessment of congenital anomalies is important because congenital anomalies are more frequent in multiple gestations, and the presence of a structural or chromosomal abnormality leading to intrauterine demise may profoundly affect survival and neurological outcome of the surviving co-twin, particularly in monochorionic pregnancies.
In this manuscript, we provide an up-to-date review of the role of early ultrasound assessment in twin pregnancies, with particular focus on determination of chorionicity, twin labelling, early diagnosis of complication related to monochorionicity, and prediction of adverse outcomes.
Assessment of chorionicity
The pregnancy loss rate in monochorionic twins is significantly higher than that of dichorionic pregnancies, especially because of complications related to monochorionic placentation, such as TTTS . Previous studies estimating pregnancy loss in twin pregnancies have found that most of these deaths take place before 24 weeks of gestation, when the likelihood of developing TTTS is significantly higher than in the third trimester of pregnancy .
Accurate determination of chorionicity is, therefore, mandatory in the routine care of twin pregnancies to distinguish which pregnancies are at risk and to allow the early detection of these specific complications. Several methods of chorionicity determination have been reported, such as the number of placentas, fetal sex, the presence and thickness of the inter-twin membrane, and the presence of ‘lambda’ or ‘twin peak’ sign in case of dichorionic and ‘T’ sign in case of monochorionic twins . Ultrasound determination of chorionicity is best achieved in the first trimester of pregnancy using the Lambda (λ) and T signs. ‘λ’ or ‘twin peak’ sign refers to the classical appearance of the inter-twin membrane at its placental attachment. In the case of dichorionic placentation, the presence of a triangular tissue projection extending from the base of the inter-twine membrane gives the latter a characteristic appearance of the Greek letter ‘λ’. This projection is produced by extension of chorionic villi into the inter-chorionic space of the twin membrane at the point where it encounters the chorion and placenta of the co-twin ( Fig. 1 a). In the case of monochorionic placentation, the lack of tissue projection into the inter-twin membrane prevents the formation of ‘λ’ sign, and the inter-twin membrane inserts perpendicularly in the placental plate ( Fig. 1 b) . These two signs are mutually exclusive in twins with a single placental mass .
The largest study on the accuracy of the λ and T signs, and the number of placental in determining chorionicity in first-trimester diagnosis of chorionicity reported a sensitivity and specificity of 100% and 99.8%, respectively, when these ultrasound markers are used to diagnose chorionicity between 11 and 14 weeks of gestation . Recently published papers are shown in Table 1 .
| Reference | Number of pregnancies | Gestational age (weeks) | Ultrasound marker | Confirmatory test | Sensitivity (%) | Specificity (%) |
|---|---|---|---|---|---|---|
| Dias et al., 2011 | 613 | 11–14 | λ and T signs, placental masses. | Placental histology or discordant sex at birth. | 100 | 99.8 |
| Lee (2006) | 410 | <14 | λ and T signs, placental masses, fetal sex | Placental histology | 88.9 | 99.5 |
| Menon, 2005 | 463 | 3–14 | λ and T signs, placental masses, fetal sex, inter-twin membrane thickness. | Placental histology | 100 | 97.9 |
| Carrol et al., 2002 | 150 | 10–14 | λ and T signs, placental masses. | Placental histology or discordant sex at birth | 98.0 | 97.4 |
| Stenhouse et al., 2002 | 138 | 7–14 | λ and T signs, placental masses, fetal sex | Placental histology or discordant sex at birth | 100 | 98.7 |
Although highly predictive of dichorionic placentation in the first trimester of pregnancy, the λ-sign gradually disappears during the second trimester of pregnancy, limiting its predictive accuracy . In the absence of a clear assignment, the pregnancy should be considered monochorionic and be monitored as such. It is important to look for the lambda sign at the insertion of the inter-twin membrane on the placental surface, and not the uterine wall ( Fig. 2 ). Membrane insertion on the uterine wall gives a false impression of a ‘T’ sign when the pregnancy, in fact, is dichorionic.
The thickness of the inter-twin membrane has been reported to predict chorionicity reliably. Carrol et al. found that the median thickness of the inter-twin membrane in dichorionic twins was significantly higher than that of monochorionic twins (2.2 v 0.9 mm), and reported similar sensitivities of 98% and 93% for λ-sign and membrane thickness, respectively. The investigators also found that a combination of these two ultrasound markers improved sensitivity by 1.3% compared with the use of T and λ-sign or membrane thickness alone. The assessment of the inter-twin membrane, however, is technically difficult to obtain, and is affected by high inter- and intra-observer variability .
Other investigators have used ultrasound determination of fetal sex as a marker of chorionicity; the main limitation of this assessment is that fetal sex cannot be reliably determined by early ultrasound scans, and that all monochorionic twins and one-third of dichorionic twins are of the same sex .
Assessment of chorionicity
The pregnancy loss rate in monochorionic twins is significantly higher than that of dichorionic pregnancies, especially because of complications related to monochorionic placentation, such as TTTS . Previous studies estimating pregnancy loss in twin pregnancies have found that most of these deaths take place before 24 weeks of gestation, when the likelihood of developing TTTS is significantly higher than in the third trimester of pregnancy .
Accurate determination of chorionicity is, therefore, mandatory in the routine care of twin pregnancies to distinguish which pregnancies are at risk and to allow the early detection of these specific complications. Several methods of chorionicity determination have been reported, such as the number of placentas, fetal sex, the presence and thickness of the inter-twin membrane, and the presence of ‘lambda’ or ‘twin peak’ sign in case of dichorionic and ‘T’ sign in case of monochorionic twins . Ultrasound determination of chorionicity is best achieved in the first trimester of pregnancy using the Lambda (λ) and T signs. ‘λ’ or ‘twin peak’ sign refers to the classical appearance of the inter-twin membrane at its placental attachment. In the case of dichorionic placentation, the presence of a triangular tissue projection extending from the base of the inter-twine membrane gives the latter a characteristic appearance of the Greek letter ‘λ’. This projection is produced by extension of chorionic villi into the inter-chorionic space of the twin membrane at the point where it encounters the chorion and placenta of the co-twin ( Fig. 1 a). In the case of monochorionic placentation, the lack of tissue projection into the inter-twin membrane prevents the formation of ‘λ’ sign, and the inter-twin membrane inserts perpendicularly in the placental plate ( Fig. 1 b) . These two signs are mutually exclusive in twins with a single placental mass .
The largest study on the accuracy of the λ and T signs, and the number of placental in determining chorionicity in first-trimester diagnosis of chorionicity reported a sensitivity and specificity of 100% and 99.8%, respectively, when these ultrasound markers are used to diagnose chorionicity between 11 and 14 weeks of gestation . Recently published papers are shown in Table 1 .
| Reference | Number of pregnancies | Gestational age (weeks) | Ultrasound marker | Confirmatory test | Sensitivity (%) | Specificity (%) |
|---|---|---|---|---|---|---|
| Dias et al., 2011 | 613 | 11–14 | λ and T signs, placental masses. | Placental histology or discordant sex at birth. | 100 | 99.8 |
| Lee (2006) | 410 | <14 | λ and T signs, placental masses, fetal sex | Placental histology | 88.9 | 99.5 |
| Menon, 2005 | 463 | 3–14 | λ and T signs, placental masses, fetal sex, inter-twin membrane thickness. | Placental histology | 100 | 97.9 |
| Carrol et al., 2002 | 150 | 10–14 | λ and T signs, placental masses. | Placental histology or discordant sex at birth | 98.0 | 97.4 |
| Stenhouse et al., 2002 | 138 | 7–14 | λ and T signs, placental masses, fetal sex | Placental histology or discordant sex at birth | 100 | 98.7 |
Although highly predictive of dichorionic placentation in the first trimester of pregnancy, the λ-sign gradually disappears during the second trimester of pregnancy, limiting its predictive accuracy . In the absence of a clear assignment, the pregnancy should be considered monochorionic and be monitored as such. It is important to look for the lambda sign at the insertion of the inter-twin membrane on the placental surface, and not the uterine wall ( Fig. 2 ). Membrane insertion on the uterine wall gives a false impression of a ‘T’ sign when the pregnancy, in fact, is dichorionic.
The thickness of the inter-twin membrane has been reported to predict chorionicity reliably. Carrol et al. found that the median thickness of the inter-twin membrane in dichorionic twins was significantly higher than that of monochorionic twins (2.2 v 0.9 mm), and reported similar sensitivities of 98% and 93% for λ-sign and membrane thickness, respectively. The investigators also found that a combination of these two ultrasound markers improved sensitivity by 1.3% compared with the use of T and λ-sign or membrane thickness alone. The assessment of the inter-twin membrane, however, is technically difficult to obtain, and is affected by high inter- and intra-observer variability .
Other investigators have used ultrasound determination of fetal sex as a marker of chorionicity; the main limitation of this assessment is that fetal sex cannot be reliably determined by early ultrasound scans, and that all monochorionic twins and one-third of dichorionic twins are of the same sex .
Pregnancy dating
Uncertainty about gestational age has been shown to be associated with a significant increase in perinatal mortality and morbidity, and accurate pregnancy dating is needed to provide optimal antenatal care of pregnancy . In singleton gestations, evidence shows that accurate pregnancy dating reduces the number of unnecessary induction of labour, and that the use of CRL is superior to that of menstrual date .
The potential concerns associated with pregnancy dating in twin pregnancies are whether CRL charts derived from singletons pregnancies can be use to assess gestational age reliably, and whether the pregnancy should be dated according to the larger twin, smaller twin, or to an average of the two.
Several formulae to date singleton pregnancies from CRL have been reported .
Because no clinically validated twin formulae for fetal growth in the first trimester are available, it has been questioned whether singleton CRL charts should be used to date twin pregnancies. Martins et al. showed no significant difference between CRLs of twins and singletons between 7 and 10 weeks of gestation, whereas Wisser et al. showed a maximum discrepancy of 1.6 days between twins and singletons. When comparing the CRL of the smaller and larger twin with that of singleton pregnancies, Salomon et al. and Chaudhuri et al. reported that the CRL of the smaller twin was closest to the actual gestational age, whereas Dias et al. found no significant difference between both smaller and mean CRL in twins with that of singleton gestations.
A difference in twin CRLs is a common finding in twin gestations. It is not related to chorionicity, and seems to reflect a physiological variation rather than a pathological condition . A policy of dating twin pregnancy according to the smaller CRL would minimise parental anxiety about apparent reduced growth in the first trimester. Reduced fetal growth, however, might be linked with chromosomal or structural abnormalities, and it is difficult to be confident whether the small twin is normally or pathologically small during the early stages of pregnancy.
A policy of dating by using the larger CRL may be more effective because it is relatively infrequent for a twin to be pathologically large. This policy, however, may exaggerate the relative smallness of the other twin, and may increase parental anxiety. Dating by mean CRL may be a reasonable alternative, limiting parental anxiety without compromising accuracy.
Twin labelling
Multiple pregnancies are scanned frequently because of the increased risk for congenital anomalies, aneuploidy, and growth restriction compared with singleton pregnancies . Accurate labelling of the twins is needed to ensure that the same individual fetus is measured through the pregnancy so that the longitudinal growth pattern can be assessed.
Standardised and reliable labelling is also mandatory at first-trimester Down’s syndrome risk assessment in case invasive prenatal diagnosis is subsequently required. Twin order and presentation are relevant information for obstetricians planning timing and mode of delivery. Birth order is also especially important in twin pregnancies discordant for fetal abnormalities that are not immediately evident on external examination at birth (e.g. if one twin is affected by a duct-dependent cardiac lesion).
A variety of protocols for labelling twins have been used by different units and even individual operators. These include fetal presentation, sac position, and placental site. The lack of consistency in labelling predisposes to errors in twin identification.
Identifying each fetus by the position of its placenta is of limited value, as the placental position change with advancing gestation, and technique cannot be used with twin pregnancies where the placenta is either monochorionic or fused dichorionic. Antenatal determination of fetal sex is difficult in early pregnancy, and is of no value in same-sex twin pregnancies.
The position of each twin relative to maternal cervix is often used to label twin pregnancies; however, the position of either fetus relative to the cervix can change considerably, especially in early pregnancy, thus limiting the clinical applicability of this method.
A recent study proposed a consistent method of twin identification throughout pregnancy based on the relation between the gestational sac and maternal cervix . In this study at the 11–14-week ultrasound assessment, the fetus contained in the gestational sac closest to the internal os was designated as Twin 1. The relative orientation of the fetuses to each other ( Fig. 3 ) was then defined as either lateral (left/right) or vertical (top/bottom). Lateral fetal orientation was associated with an inter-twin membrane running vertically along the longitudinal axis of the uterus, whereas vertical fetal orientation was associated with an inter-twin membrane running horizontally across the longitudinal axis of the uterus ( Fig. 3 ).
The reproducibility of this method is likely to be good because the position of the gestational sac relative to the cervix remains constant throughout the pregnancy, allowing objective differentiation between the two fetuses. This assignment should be clearly documented so that it is consistently followed. Furthermore, using multiple criteria (e.g. left and right, upper and lower, gender, placental location, presence of discordant anomaly) is likely to reduce the error.
Interestingly, 16% of the twins changed presentation when the scan orientation was compared with the birth order, and this change in twin order was significantly higher for twins delivered by caesarean section compared with those delivered vaginally ( Fig. 4 ) . This finding was subsequently confirmed in a large cohort of twin pregnancies . The finding that antenatal ultrasound labelling does not predict twin birth order reliably in a significant proportion of twin deliveries, has huge clinical implication. It is often assumed by obstetricians, neonatologists, and parents alike, that the prenatal nomenclature used to identify twins will also apply to the birth order and postnatal nomenclature. Obstetricians need to be aware of the poor association between antenatal ultrasound labelling and twin birth order when planning delivery of twins discordant for fetal growth or for non-cephalic presentation of the leading twin. Obstetricians and neonatologists should also be aware of the likelihood of a discrepancy between pre- and post-natal twin order when delivering babies with discordant anomalies where immediate postnatal intervention is required at birth. For example, when delivering twins discordant for cardiac abnormalities or diaphragmatic hernia, dedicated perinatal assessment is required in order to quickly identify the affected twin after delivery. Correct and consistent identification of antenatal and postnatal twins is also important in studies on perinatal outcomes.
Discrepancy in biometry
Size discordance represents one of the major determinants of adverse perinatal outcomes in twin pregnancies . Although a degree of discordance in fetal size is invariably present in all twin pregnancies, inter-twin discordance in size has been associated with a multitude of adverse outcomes including stillbirth, neonatal death, preterm birth, respiratory distress and admission to neonatal intensive care unit . On the assumption that discordant growth in twins begins as early as the first trimester of pregnancy, it has consequently been suggested that discordance in CRL may have a role in predicting birth-weight discordance. Discordance is most often expressed as a difference in measurements (biometry or estimated weight) expressed as a percentage of the larger fetus. A total of 12–15% discordance is typically the 95th centile.
Discordance in early fetal growth has been associated with other adverse outcomes, such as pregnancy loss, chromosomal abnormalities, and structural malformations, leading to the widely held belief that discordance in size in the first trimester may have a role in predicting fetal growth discordance and adverse perinatal outcomes .
Discordance in CRL is a factor commonly taken into consideration in counselling parents about the outcome of the pregnancy, and different thresholds have been investigated. The role of CRL discordance in predicting the outcome in twin pregnancies is still conflicting, with different studies reporting contrasting results ( Table 2 ).
| Structural or chromosomal abnormalities included | ||||||||
|---|---|---|---|---|---|---|---|---|
| References | Chorionicity | Number of pregnancies | Crown–rump length discordance cut-off (% or mm) | Number of pregnancy losses (%) | Early mortality | Perinatal mortality | Birth weight discordance and sIUGR | Preterm birth |
| Sebire et al., 1998 | Monochorionic and dichorionic | 539 | Not stated | 36 (6.7) | Associated | – | Not associated | – |
| Kalish et al., 2004 | Dichorionic only | 159 | 10% | 9 (5.6) | Associated | – | Associated with birth weight discordance >20% | – |
| Bartha et al., 2005 | Monochorionic and dichorionic | 57 | Not reported | 5 (8.5) | – | – | Associated with birth weight discordance ≥ 25% and sIUGR | Associated |
| Salomon et al., 2005 | Monochorionic and dichorionic | 182 | 11.4–14.3% | 14 (7.7) | Not associated | Not associated | Not associated | Not associated |
| Fajardo-Exposito et al., 2011 | Monochorionic and dichorionic | 46 | – | – | – | – | Not predictive | – |
| Bhide et al., 2011 | Monochorionic and dichorionic | 507 | 12.2–19.3% | 39 (7.7) | Associated (monochorionic only) | – | Associated (dichorionic only) | – |
| Harper et al., 2012 | Dichorionic only | 610 | 11% | 28 (4.6) | Associated (<20 wks) | Not associate | Not associated | Not associated |
| Kalish et al., 2003 | Dichorionic only | 130 | 10% | – | Associated | – | Associated (sensitivity 18.8% specificity 92.1%) | – |
| Banks et al., 2007 | Dichorionic | 108 | 5% | – | – | – | Not associated | – |
| Tai and Grobman, 2007 | Monochorionic and dichorionic | 178 | 11.1% | 19 (10.7) | – | Not associated | Predictive of birth weight discordance > 20% (area under curve: 0.72) | Not associated |
| El Kateb, et al., 2007 | Monochorionic only | 239 | 10% | 16 (15.5) | Associated | Not associated | – | |
| Kagan et al., 2007 | Monochorionic only | 512 | Not reported | 42 (8.2) | Associated with fetal loss ≤18 wks) | Not associated | Not associated | |
| Fareeduddin et al., 2010 | Dichorionic only | 78 | 9% | – | – | – | Not associated | Associated |
| Fratelli et al., 2011 | Monochorionic only | 135 | 5–20% | 4 (3.0) | – | – | Predictive (area under curve: 0.77) | – |
| Matias et al., 2011 | Monochorionic only | 237 | – | – | – | – | Not associated | – |
| Shahshahan et al., 2011 | Dichorionic only | 118 | 11% | 10 (8.5) | – | Associated | Associated with birth weight discordance >20% and sIUGR | Not associated |
| Memmo et al., 2011 | Monochorionic only | 242 | 7.1% | – | – | – | Predictive of sIUGR <26 weeks (area under curve: 0.89) | – |
| D’Antonio, et al., 2013 | Monochorionic and dichorionic | 2155 | Not found | 65 (3.0) | Not associated | Not associated | Not associated | Not associated |
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
