Try out PMC Labs and tell us what you think. Learn More. Ultrasound has revolutionised the management of multiple pregnancies and their complications. Increasing frequency of twin pregnancies mandates familiarity of all clinicians with the relevant pathologies and evidence-based surveillance and management protocols for their care. In this review, we summarise the latest evidence relating to ultrasound surveillance of twin pregnancies including first trimester assessment and screening, growth surveillance and the detection and management of the complications of monochorionic pregnancies including twin-to-twin-transfusion syndrome, selective fetal growth restriction, twin reversed arterial perfusion sequence and coned twinning.
Since the introduction of ultrasound into routine obstetric practice, the advantages for the mothers carrying twins have moved beyond the simple ability to identify multiple pregnancies antenatally to the possibility of screening these pregnancies both for the same pathologies as those screened for in singleton pregnancies, as well as the identification and management of those complications specific to twin pregnancies.
As twins continue to increase in frequency, 4 routine ultrasound surveillance for the complications of twin pregnancies is becoming a common task and, therefore, familiarity with the clinical problems specific to twin pregnancies is important for all team members. National guidelines recommend that women with twin pregnancies are looked after by a core multidisciplinary team, which includes obstetricians, midwives and sonographers, who are familiar with the management of complicated and uncomplicated twin pregnancies, in order to optimise their outcomes.
Ultrasound surveillance in twin pregnancy: an update for practitioners
We present an appraisal of the latest evidence relating to the use of ultrasound in the pregnancy for antenatal complications from the early first trimester, screening for fetal abnormalities, twin-specific complications, preterm birth and pre-eclampsia, as well as the application of ultrasound in the management of complicated twin pregnancies. Higher order multiple pregnancies were outside the scope of this review. Given the high risk of preterm delivery in twins, 6 accurate first trimester dating is important in later management of the pregnancy.
After dating and determination of the diagnosis of multiple pregnancy, the most important additional information to determine is the precise of fetuses and the chorionicity of placentae and amnionicity of amniotic sacs of the pregnancy. Dating of the pregnancy using the crown-rump length CRL prior to 14 weeks gestation is the standard practice in twins, as the case in singletons.
The operator can choose the largest, the smallest, or the mean of the two CRLs to date the pregnancy. It is thought that the smaller CRL is the most reflective of true gestational age, based on studies where the date of conception is twin known. As twin CRLs have been shown to straddle the mean of singleton pregnancies, it could be reasonable to use the mean of the two CRL measurements to date the pregnancy as well 10 but it has been shown that only the use of the larger twin to date the pregnancy le to a slight overestimation of true gestational age, whereas use of the mean or the smaller twin are both associated with underestimates.
In twin pregnancies conceived via IVF, as with singleton IVF pregnancies, the gestational age should be calculated using the date of embryo transfer. The s available to determine chorionicity vary by gestation and, in general, the diagnosis is more accurate the earlier in pregnancy the twins are assessed.
Prior to 10 weeks gestation, the presence of two gestational — amniotic and yolk sacs — clearly identifies a dichorionic diamniotic DCDA pregnancy. Discordant fetal sex is virtually always associated dating DCDA pregnancy and can be used as a marker for chorionicity where present, but of course concordant fetal sex does not rule out dichorionicity.
Where one operator is uncertain, a second opinion should be sought without delay. Labeling of the twins begins at the first scan and should be consistent at every subsequent scan.
Twin pregnancy: ultrasound evaluation and monitoring
The strategy must be applied by all professionals undertaking studies of the same pregnancy. The incorporation of the NT into the combined test allows a fetus specific risk to be ased in dichorionic pregnancies. Since monochorionic twins share a karyotype, the risk calculated takes into a mean of the NT measurements and a per pregnancy risk is given, as is the case with all serum only screening tests. The later in the pregnancy that loss of the second twin occurs, the greater the potential residual effect of the second pregnancy on the biomarkers used in the combined dating.
Invasive testing in twin pregnancy has been thought to be twin with higher risk than in singleton pregnancy, but it can be difficult to separate the effect of invasive testing from the higher rate of spontaneous miscarriage in twin pregnancy in the available observational data. For amniocentesis, both single and double uterine entry techniques are described, with little clear difference between the two in terms of perinatal outcomes.
Since twins are associated both with an increased risk of aneuploidy and greater risks of miscarriage after invasive testing the advantages of NIPT could be ificant in pregnancy pregnancies, but no studies large enough to accurately report the test performance in twins have yet been published.
The findings of a recent analysis suggest that where a result is obtained the detection rates are similar to those in singleton pregnancies. It is particularly important to save images clearly labeled by twin to ensure consistency between pregnancies — ideally by text and colour. The management of fetal structural anomalies is complicated in twin pregnancies where the affected fetus shares the intrauterine environment with a sibling that may be unaffected or present different disorders. The management will be determined by the expected prognosis for the affected twin and the chorionicity of the pregnancy.
Where intrauterine demise of the affected twin is anticipated, expectant management is appropriate in dichorionic pregnancies with a low risk of preterm labour and delivery of the surviving co-twin. In all such cases, referral to a tertiary fetal medicine service for appropriate counselling and management is recommended. Pre-eclampsia remains one of the most common causes of maternal and fetal morbidity and mortality.
Uterine artery Doppler waveforms measured at 20—22 pregnancies have been found to be predictive of pre-eclampsia, even in low-risk singleton pregnancies. Although the sensitivity of UAPI in twins is lower than in singleton pregnancies, 2 it is important to note that because pre-eclampsia is more common in twin pregnancies, the women carrying twins and identified by abnormal UAPI at 20—22 weeks will be amongst the highest risk of developing pre-eclampsia and for this reason some units will offer UAPI screening to identify datings for additional monitoring for the s and symptoms of pre-eclampsia throughout pregnancy.
Although twins and singletons in the second trimester seem to have similar growth patterns, 11 in the third trimester growth velocity in twins is consistently found to be less than in singletons, with the differences most pronounced and noted earlier in MC pregnancies.
If growth in twins is limited by the ability of the mother to supply the metabolic demands of two growing fetuses, does that suggest the need for twin-specific growth charts for twins because they just grow differently or merely demonstrate a physiological explanation for the observed increase in incidence of growth restriction in twins?
If the former, then there is a need for twin-specific growth charts, but if the latter then there is an argument to continue using singleton growth charts in order to avoid the risk of failing to diagnose growth restriction in twin pregnancies. Additionally, EFW calculations are observed to be less accurate in dating pregnancies than in singletons, which also calls into question the validity of using singleton norms for management of twin pregnancies. Since it can be observed that twins are genuinely more at risk of stillbirth and perinatal loss than singletons, 3435 the finding that twins are also likely to be smaller is plausibly also the finding that twins are more likely to be growth restricted.
Caution should be used in assuming that twins are physiologically normal when smaller than the equivalently aged singletons. Despite this cautious approach, it has been demonstrated that the use of twin-specific charts le to fewer babies being classified as growth restricted antenatally and therefore likely to be subjected to additional interventions and scheduled delivery without failing to identify small babies that go on to suffer intra-uterine demise. A of studies have demonstrated that the use of twin-specific growth charts, taking chorionicity intoare more accurate to detect twins at risk for intrauterine fetal demise and neonatal death.
In order to most accurately identify fetuses with growth limited by placental insufficiency it may be the case that, as in singleton pregnancies, the addition of Doppler parameters is of benefit in distinguishing the faltering fetus from the well small baby. Although twins will rarely be identical in size, ificant growth discrepancies are twin with poor perinatal outcomes in a continuous fashion and may be more important in relation to perinatal outcome than the absolute size of individual babies.
Twins that are both constitutionally small are understandably at lower risk of complications than siblings, especially twin identical siblings, that have ificantly different growth trajectories in the same intra-uterine environment. Most international bodies recommend surveillance scanning of MC pregnancies every two weeks, on the basis that they are more at risk of all adverse perinatal outcomes than DC twins and additionally may develop selective fetal growth restriction sFGRtwin-to-twin-transfusion syndrome TTTS or twin anaemia-polycythaemia sequence TAPS at any time during the pregnancy.
There is relatively little evidence supporting the routine examination of DCDA twins every four weeks, or even every four to six weeks as recommended by some. Furthermore, additional screening will lead to additional iatrogenic deliveries of babies suspected to be compromised, but in twin pregnancies these iatrogenic deliveries affect not only the mother but also any healthy co-twins and investigation of the potential harm to the co-twins would be necessary in considering change in screening protocols.
Recommended frequency and content of scans in dichorionic pregnancies. Recommended frequency and content of scans in monochorionic pregnancies. MC twins are vulnerable to complications of interdependent placental circulations in a way that DC pregnancies are not. Although these complications carry a heavy burden of fetal morbidity and mortality, ultrasound screening can facilitate interventions and delivery that can greatly improve outcomes.
Predicting in the first trimester which MC pregnancies are likely to be complicated can be helpful in counseling parents about treatment options in advance. Reversed a-wave in the fetal ductus venosus at 11—14 weeks was a better predictor of subsequent TTTS than either NT or CRL, but still reported a low positive predictive value. A TRAP pregnancy can usually be identified in the first trimester, although the diagnosis may not be clear in very early scans Figure 4.
The TRAP sequence was thought to be extremely rare but improvements in early pregnancy imaging suggest that in fact this problem is more common than ly thought and may affect as many as 2. Optimal management of TRAP pregnancies can be achieved using intrafetal laser photocoagulation of the umbilical blood vessels in the acardiac twin, performed late in the first trimester. TRAP pregnancy — showing the normal and acardiac fetuses author's own images.
Since the advent of fetoscopic laser coagulation, intervention can ameliorate the dismal prognosis in MC pregnancies affected by TTTS. The staging requires amniotic fluid discrepancy, 47 which cannot be readily identified before 16 weeks, as until this time most of the amniotic fluid is derived from the placenta. This can lead to clinical uncertainty in the optimal management of twins presenting at gestations 16—18 weeks with ificant discrepancy but a DVP of 6 or 7 in the pd recipient. Since DVP has been now been observed to vary with gestation in monochorionic twin pregnancies, 48 despite ly having been thought to be stable throughout pregnancy, 49 it might be prudent in the future to consider a transition to diagnostic criteria modified to take of variation in the amniotic fluid volume by gestation.
Extremely rare cases where dichorionic twins have developed placental anastamoses as well as the possibility of misclassification of chorionicity in early pregnancy mean that even in DC pregnancies ificant discrepancies in amniotic fluid volumes require specialist assessment. Quintero staging of twin-to-twin transfusion syndrome. Variation of amniotic fluid volume with gestation.
The Quintero stage is only partially associated with prognosis. Additional ultrasound markers can be used to guide prognosis and risk of progression, particularly in cases where difficult decisions regarding fetal interventions selective fetoscopic laser coagulation SFLC or amnioreduction or delivery are being weighed. Pre-operatively identifiable factors found to be associated with fetal death include increasing EFW percentage discrepancy, ascites or hydrops in the recipient, absent or reversed a-wave in the ductus venosus, global cardiac dysfunction, pericardial effusion and valvular regurgitation.
The degree of recipient cardiomyopathy has been associated with the likelihood of progression from Quintero Stages 1 and 2, and can be used to select patients for fetal intervention. Ultrasound is further of benefit once a decision for fetal intervention has been made and will usually be undertaken by the operators prior to fetoscopic intervention.
Mapping the placental borders helps plan trochar insertion and identifying the cord insertions shows where the anastomoses will be found, since they should chiefly lie between the two cord insertions. Detailed ultrasound for pre-operative planning can optimise entry point and instrument choice and reduce the operative time which should also reduce the risk of procedure related complications.
It is expected that the polyhydramnios should have resolved by 14 days and pregnancy dysfunction by one month. Common protocols for dating usually provide for weekly ultrasound for the first fortnight while subsequent scans can be fortnightly if there is evidence of clinical resolution. The diagnosis and management of selective fetal growth restriction in monochorionic twin pregnancies differs ificantly from that in dichorionic pregnancies because of the nature of the interdependent placental circulation. This variation in diagnostic criteria has led to ificant heterogeneity in reported outcomes of pregnancies diagnosed with sFGR, but consensus has recently been reached on the definition of sFGR in in order to standardise reporting in twin studies and trials of interventions in these pregnancies 57 Table 3.
Consensus criteria for diagnosis of selective fetal growth restriction sFGR in twins. The degree of growth discrepancy in monochorionic twin pregnancies is thought to be linked to the degree of discordance in placental share 58 where sFGR in DC pregnancies is more similar to placental insufficiency in singleton pregnancies. While unequal placental sharing is the cause of sFGR in MC pregnancies the clinical outcome is determined as much, if not more, by the and type of vascular anastomoses between the twins.
These can be mapped using colour Doppler ultrasound to better understand the likely prognosis.
In singleton and DC twin pregnancies affected by growth restriction, umbilical artery Dopplers UAD are used to monitor fetal wellbeing and prompt intervention when deterioration is identified. In MC pregnancies the circulation of the co-twin affects the pattern of the UAD in the growth restricted twin, requiring an understanding of how placental anastomoses affect umbilical artery flow in interpreting UAD findings in these pregnancies.
The classification of sFGR in MC pregnancies is by UAD findings at diagnosis 60 Table 4 and relates closely to the and type of vascular anastomoses in the placenta.
In type 3 sFGR with intermittent absent or reversed end diastolic flow iAREDF Figure 6large arterioarterial anastomoses allow compensatory flow from the larger twin but also permit acute transfusion events which may cause unpredictable mortality and morbidity in either twin. Classification of pregnancies affected by selective fetal growth restriction sFGR in monochorionic twin pregnancies according to umbilical artery Doppler findings at diagnosis.
Patterns of umbilical artery Doppler flow in selective fetal growth restriction author's own images. TAPS is an imbalance in haemoglobin without a volume distribution disparity in monochorionic twins. Twin anaemia-polycythaemia sequence classification. The most common site of union is at the thorax with the twins facing each other, and bowels, liver and hearts may be twin.
Mapping blood vessels and structures can help plan postnatal surgery — where delivery is planned, it should be by caesarean section in a unit equipped to meet the surgical needs of the pregnancies. Cervical length measured at transvaginal ultrasound has been shown to be associated dating the risk of preterm birth and is commonly used for prediction of preterm birth in women with singleton pregnancies at high risk of preterm delivery although not yet recommended for routine screening in unselected populations.
Since twins are at a greater risk of preterm birth than singletons and the majority of the increase in neonatal morbidity and mortality in twins is attributable to prematurity, 6 accurate screening and effective prevention for preterm delivery in twins are highly prized goals.
Unfortunately, although cervical length is associated with preterm delivery in twins, the sensitivity is lower than in singleton pregnancies, 6465 suggesting that the mechanism underlying preterm labour in multiple pregnancies may differ from singletons.