This chapter should be cited as follows:
Chaveeva P, Molina FS, et al, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.419333
The Continuous Textbook of Women’s Medicine Series – Obstetrics Module
Volume 18
Ultrasound in obstetrics
Volume Editors:
Professor Caterina M (Katia) Bilardo, Amsterdam UMC, Amsterdam and University of Groningen, Groningen, The Netherlands
Dr Valentina Tsibizova, PREIS International School, Florence, Italy
Chapter
Ultrasound Evaluation of Multiple Gestation
First published: October 2024
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INTRODUCTION
The incidence of twin and higher-order multiple pregnancies has increased worldwide, accounting for about 2–3% of all other pregnancies, and this is mainly because of the use of assisted reproductive technology (ART) for conception. Multiple embryo transfers are recognized as the main factor contributing to the rise in higher-order multiple gestations. Across different countries, the rate of twin births resulting from ART procedures varies from 2% to 27%, and for triplet pregnancies, it ranges from 0.1% to 4%.1
Ultrasound is a non-invasive method of assessment of pregnant women adopted globally to increase the quality of prenatal care. Thus, it plays a major role in the diagnosis and management of multiple pregnancies, which are at significantly increased risk of maternal, perinatal and neonatal morbidity and mortality compared to singleton gestations. Several studies and guidelines have proposed specific antenatal care based on ultrasound determination of chorionicity in multiple gestations. The unique complications related to monochorionic multiple gestations, such as twin-to-twin transfusion syndrome (TTTS), twin anemia–polycythemia sequence (TAPS), selective fetal growth restriction, and twin reversed arterial perfusion (TRAP) sequence can only be differentiated by ultrasound examination.
It is essential for clinicians providing antenatal care to conduct guided and standardized assessments in multiple pregnancies. This allows for the identification and differentiation of normal and abnormal pregnancy findings, leading to better pregnancy care.
ULTRASOUND IN THE FIRST TRIMESTER
Chorionicity and amnionicity
The diagnosis of multiple gestations has shifted from the second to the first trimester of pregnancy since the introduction of nuchal translucency thickness as a marker for chromosomal abnormalities at 11–13 weeks. As a result, first-trimester ultrasound examination has become a globally recognized routine scan during pregnancy.2,3 The implications of first-trimester ultrasound in multiple pregnancies are threefold: firstly, it is used to determine chorionicity and amnionicity, as well as to accurately date the pregnancy and label the fetuses; secondly, it serves as a means for assessing and screening for chromosomal abnormalities and structural defects; lastly, first-trimester ultrasound helps in identifying a subgroup of pregnancies that would benefit from invasive prenatal management.4,5,6
Twins
The prevalence of dichorionic diamniotic (DC) twins in the first trimester is about 80%, monochorionic diamniotic (MCDA) twins about 20%, and monochorionic monoamniotic (MCMA) twins less than 1%. Among these three types of twins, there is a substantial difference in pregnancy outcome. The risk of miscarriage before 24 weeks of gestation is 2% in DC twins, compared with 8% in MCDA and 22% in MCMA twins. Similar trends apply to other adverse pregnancy outcomes such as preterm birth before 34 weeks of gestation, which is 7% in DC twins, twice as high in MCDA twins and three times higher in MCMA twins. Likewise, fetal demise after 24 weeks is 1% in DC twins, 2.5% in MCDA twins and 9% in MCMA twins.7
The importance of determining chorionicity in the first trimester lies in its correlation with proper management planning, the frequency of ultrasound scans and screening for specific complications related to the different types of twins.
During the 11–14-week scan, the amnion and chorion have not yet fused, and the amniotic membranes and layers of the intertwin membrane are visible by ultrasound.
In DC twins, ultrasound shows a lambda sign, visualized as three layers: a thick chorion in the middle and two amniotic membranes on each side (Figure 1). In MCDA twins, only a thin layer of the two amniotic membranes separates the twins, visualized by ultrasound as a T-sign (Figure 2).5 To facilitate visualization of this thin membrane, the ultrasound beam should not be parallel to the membrane as it could easily be overlooked. Therefore, changing the angle of insonation during the scan of suspected MC twins to a more oblique view will help identify the presence of a T-sign.
1
The ‘lambda’ sign in dichorionic diamniotic twins.
2
The ‘T’-sign in monochorionic diamniotic twins.
In cases in which the lambda sign is absent, the intertwin membrane should be thoroughly examined to identify MCMA twins, who share a single amniotic cavity. In such situations, additional findings such as the close insertion of the umbilical cords into the placenta and close proximity of the two fetuses could serve as diagnostic sonographic markers. Another ultrasound finding is the presence of cord entanglement as early as the 11–13-week scan, which could also enhance differentiation of the rarest type of MC twins (Figure 3).8
3
Cord entanglement in monoamniotic twins at 11–13 weeks, with (a) and without (b) color Doppler.
Triplets
Triplet pregnancies are less prevalent than twins and singletons, and diagnosis and assessment of chorionicity is critical to managing these pregnancies as early as the first trimester. Trichorionic triamniotic triplet pregnancy, characterized by the presence of three gestational sacs and therefore three chorions and three amnions, is the most common type of triplet pregnancy. This is followed by dichorionic triamniotic triplet pregnancy, which has one gestational sac with its own chorion and amnion and another gestational sac with one chorion and two amniotic sacs. The rarest type of triplet pregnancy involves one gestational sac and one chorion, but three amniotic sacs, resulting in a monochorionic triamniotic triplet pregnancy (Figure 4).
4
Triplet pregnancy: (a) trichorionic triamniotic; (b) dichorionic triamniotic; (c) monochorionic triamniotic.
Different scenarios with amnionicity within a single gestational sac are also possible in triplet pregnancies. For instance, dichorionic monoamniotic triplet pregnancy features two gestational sacs with two amniotic sacs, one separate fetus and one monoamniotic pair. Monochorionic monoamniotic triplet pregnancy is when the three fetuses share not only one chorion but also one amniotic cavity. The prevalence of trichorionic triplets is approximately 80% among all triplet pregnancies, with those having monochorionic or dichorionic placentation accounting for about 20%.9 Determining chorionicity and amnionicity in triplet pregnancy, and using the ultrasound lambda sign by assessing the appearance of the membranes and thickness at the insertion site, has proven to be an effective ultrasound marker.10
The accuracy of first-trimester determination of chorionicity is reported to be over 95%.11 In the second trimester, the ultrasonographic lambda and T-signs have disappeared, making the diagnosis of chorionicity challenging. However, ultrasound can still distinguish DC twins based on differences in placental location and, to a lesser extent, on fetal gender. When assessment of chorionicity is difficult or the gestational age is advanced, it is best to manage such pregnancies as MC.
Crown–rump length
The criteria for measuring fetal crown–rump length (CRL) in multiple pregnancies do not differ from those in singleton pregnancies. The CRL is used as a sonographic marker to determine gestational age.12 Notably, a discrepancy in CRL measurements in twins can serve as an ultrasound marker for potential pregnancy complications in both DC and MC twins.
A large study evaluating the relationship between ultrasound findings in twin pregnancies at 11+0 to 13+6 weeks and pregnancy outcomes reported that the prevalence of structural defects is higher in those twins with a CRL discrepancy of more than 10%, and this risk doubles in those with a discrepancy of more than 15%, accounting for 9% and 20% respectively.13 In MC twins, a discrepancy in CRL is also a marker for an increased risk of fetal loss or the need for intrauterine laser therapy.14,15
Although the predictive performance of screening by CRL discordance ≥10% or ≥15% is relatively low, these findings nonetheless provide clinicians with a tool to identify the subgroup of twin pregnancies that will require further counseling and follow-up to either exclude or confirm the presence of congenital structural defects or assess the need for therapeutic intervention.
Nuchal translucency thickness
Measurement of NT in combination with maternal age provides effective screening for aneuploidy in twin and higher-order multiple pregnancies. However, for a detection rate of 90%, similar to singleton pregnancies, the false-positive rate is slightly higher in DC twins at 6% rather than 5%, and in MC twins, it is about 8–9%. This is primarily due to the fact that an NT discrepancy between the two fetuses of a MC pair also serves as an ultrasound marker for early TTTS.16
In DC twins, the risk calculation is based on the specific risk for each fetus, whereas in MC twins, the risk calculation is the average risk of the two fetuses. Similarly, in higher-order multiple pregnancies, NT measurements should be individually assessed for each fetus.
The risk assessment for aneuploidy in twins could be improved by the use of maternal serum biochemical markers, such as pregnancy-associated plasma protein-A (PAPP-A) and beta-human chorionic gonadotropin (β-hCG). However, these markers require specific adjustments for chorionicity and have been reported to be useful only in twin pregnancies, not in triplet and higher-order multiple pregnancies.17
Assessment of increased fetal NT in MC twins (≥95th or ≥99th percentile) has been reported to be a marker for adverse pregnancy outcomes such as fetal demise or the development of TTTS and the need for laser treatment before 20 weeks of gestation.
A large study involving 12,732 fetuses reported the use of NT as a marker for structural defects in twin pregnancies. The incidence of any defect was reported to be higher in those fetuses with NT ≥95th percentile compared to those without: 16.5% vs 4.5% in DC twins and 19.2% vs 5.9% in MC twins. The overall incidence of structural defects and NT ≥95th percentile was about 17%, and it was 4.6% with structural defects and NT <95th percentile.13
Ductus venosus
Measurement of blood flow in the ductus venosus (DV) at 11–13 weeks of gestation has been shown to improve the effectiveness of screening for chromosomal abnormalities in singleton pregnancies. This ultrasound marker has also been reported to be beneficial in first-trimester screening for cardiac defects. Furthermore, an abnormal 'a-wave' flow in the DV is associated with an increased risk of fetal loss.18,19
The use of DV in multiple pregnancy has been evaluated in MC twin pregnancies. An abnormal 'a-wave' (absent or reversed) has been described as being associated with the development of severe TTTS. The prediction of TTTS by abnormal flow in the DV in at least one twin of the monochorionic pair is approximately 30%, which underscores the importance of close monitoring and timely diagnosis of this condition.20 As such, detection of this abnormal finding can guide clinical management, including potential interventions such as fetoscopic laser coagulation, and also help to inform parents about potential risks and outcomes.
Labeling
Labeling of twin and triplet pregnancies is an important part of both antenatal and postnatal follow-up. Several strategies have been proposed for labeling, including the fetuses' location within the uterus, with respect to maternal left and right, or proximity to the internal cervical os, designating them as the upper or lower fetus.
Ideally, clinicians should follow consistently the same labeling strategy throughout the pregnancy and clearly document this. This approach minimizes the chance of confusion (Figure 5). Correct labeling allows clinicians to accurately track and compare growth patterns among the fetuses, as well as effectively communicate with the neonatology team after delivery.
5
Labeling of twins.
Structural defects
Ultrasound plays a major role in first-trimester screening for structural defects in not only singleton pregnancies but also twin and higher-order multiple pregnancies. The overall incidence of structural defects is higher in twins and even higher in multiple gestations. These defects can be classified into two categories: concordant, in which both fetuses are affected, and discordant, in which only one fetus is affected by a given defect. Although, in dizygotic twins, the prevalence of structural defects per fetus is the same as in singletons, the rate in monozygotic twins is increased 2–3 times. A large study reported that the incidence of structural defects in the first trimester in euploid MC vs DC twins (as a proxy for chorionicity) is 2.8% compared to 1.3%.13
Standardized ultrasound protocols performed in the first trimester can detect 35.6% of the defects, including 27.1% of those in fetuses of DC twin pregnancies and 52.6% of those in fetuses of MC twin pregnancies. As in singleton pregnancies, major fetal defects such as acrania, alobar holoprosencephaly, encephalocele, pentalogy of Cantrell, exomphalos and body-stalk anomaly can always be detected, as can TRAP sequence and conjoined twinning.
The same trend of detectability as in singletons has been described for those defects in which first-trimester ultrasound evaluation may not always be possible, such as cleft lip, diaphragmatic hernia, structural heart defect and renal agenesis, and for never-detected defects, for which visualization in the first trimester is not possible. These include, amongst other anomalies, severe ventriculomegaly, hypoplastic cerebellum or vermis, congenital pulmonary airway malformation, arrhythmia, rhabdomyoma, hemivertebra and duodenal atresia.13
ULTRASOUND DIAGNOSIS OF COMPLICATIONS SPECIFIC TO MONOCHORIONIC PREGNANCY
Twin reversed arterial perfusion sequence
TRAP sequence is a rare complication diagnosed in the first trimester with a prevalence of approximately 1 in 15,000 pregnancies. This condition is usually found in MCDA or MCMA twin pregnancies.21 Although the incidence of this condition is more common in twin pregnancies than in triplet pregnancies, the two most commonly diagnosed triplet pregnancies complicated with TRAP sequence are dichorionic triamniotic and monochorionic triamniotic.
TRAP sequence should be suspected on ultrasound when a malformed fetus is present in a MC pregnancy with the absence of cardiac activity in one twin (Figure 6). The possibility of TRAP sequence should always be considered when there is a diagnosis of intrauterine demise of one of the twins, and color Doppler should be used to identify the pathognomonic feature of the reversed blood flow in the umbilical artery.22
6
Twin reversed arterial perfusion sequence (TRAP): (a) acardiac twin, viewed with and without color Doppler, in MCDA twins complicated by TRAP sequence; (b) 2D (left) and 3D (right) representation of the ‘pump’ twin connected to the acardiac twin.
A MC pregnancy complicated by TRAP sequence may result in intrauterine death, neonatal death, or preterm delivery due to polyhydramnios and high-output cardiac failure of the pump twin.23 Several management options have been reported in relation to the treatment of TRAP sequence, with the survival rate of the pump twin estimated at about 78–80%.24,25 These options can include conservative management, fetoscopic laser coagulation, bipolar cord coagulation and radiofrequency ablation, depending on the specific circumstances and prognosis.
Conjoined twins
Conjoined twinning is one of the rarest complications, occurring in approximately 1 in 50,000 gestations or less than 1% of monochorionic twin pregnancies.26 By definition, conjoined twins are monoamniotic twins, and the different types are based on the site of fusion: thoracopagus (fusion at the thoraces) (Figure 7), omphalopagus (fusion at the abdomen), pygopagus (fusion at the pelvis/lower body), ischiopagus (fusion at the perineum) and craniopagus (fusion at the head).
Conjoined twins are associated with increased intrauterine mortality, at about 60%, which could be due to the organs being conjoined or the associated abnormal hemodynamics. The diagnosis of conjoined twins is usually made in the first trimester of pregnancy. While many surgical separation procedures have been introduced over the years to improve pregnancy outcomes, the survival rate remains at around 28%.27
7
Thoracopagus conjoined twins.
Twin-to-twin transfusion syndrome
TTTS is a serious complication that occurs in about 10–15% of all monochorionic twins or higher-order multiple pregnancies. The imbalanced chronic blood transfusion between the two fetal umbilical circulations via vascular anastomoses on the shared placenta is the necessary factor for the development of this condition.
The diagnosis of TTTS is ultrasound-based following fortnightly monitoring starting at 16 weeks of gestation. The fetus seen on ultrasound to have a distended bladder and high cardiac output, polyuria and polyhydramnios, is called the recipient fetus and the fetus with a small or non-visible bladder with hypovolemia, oliguria and oligohydramnios is called the donor fetus. In the setting of monochorionic triplets or higher-order multiple pregnancies the presence of two donors or two recipients is common. TTTS is recognized as a fluid discrepancy condition in which the striking finding is the presence of increased amniotic fluid in the sac of the recipient twin and reduced or absent amniotic fluid in the sac of the donor twin. The definition of polyhydramnios varies throughout gestation, the measurements used in daily clinical practice being the deepest vertical pocket of >6 cm at 15–17 weeks; >8 cm before 20 weeks, and >10 cm after 20 weeks.28,29 The donor twin displays oligo- or anhydramnios, with a deepest vertical pocket of <2 cm and an ultrasound finding of “stuck” twin, either to the placenta or to the uterine wall due to the polyhydramnios of the recipient twin (Figure 8). Visualization of the donor twin wrapped in its membranes as a free-floating fetus within the polyhydramnios of the recipient twin represents the so-called “cocoon sign”.30
8
Twin-to-twin transfusion syndrome. (a) Polyhydramnios in recipient (left) and oligohydramnios in donor (right); (b) color Doppler at the level of the bladder: bladder not visible in donor twin (left), bladder distended in recipient twin (right); (c) reversed a-wave flow in ductus venosus in recipient (left) and reversed umbilical artery flow in donor (right).
The severity of TTTS is accepted universally to be assessed according to the Quintero staging system,31 which uses ultrasound features such as the difference in amniotic fluid, bladder filling and Doppler patterns between the two fetuses, and deterioration to fetal hydrops or intrauterine death of one or both twins, these representing five stages (Table 1).
1
Staging description of twin-to-twin transfusion syndrome.
I | Discordant amniotic fluid volume between twins – deepest vertical pocket <2 cm in one and >8 cm in the other (before 20 weeks) or >10 cm after 20 weeks |
II | Bladder of donor twin not visible |
III | Critically abnormal Doppler findings in either twin (umbilical artery Doppler in the donor and/or venous Doppler in the recipient) |
IV | Ascites, pericardial or pleural effusion or overt hydrops |
V | Single or double intrauterine death |
The disease occurs between 16 and 26 weeks of gestation in about 75% of cases and less so, at about 15%, before 16 or after 26 weeks, making the prognosis for both babies unfavorable if an intrauterine intervention does not take place. The risk of perinatal mortality due to composite complications such as premature delivery, fetal demise or miscarriage of the pregnancy accounts for about 80–85% of the cases complicated with TTTS without treatment.32 Fetoscopic laser coagulation of all vascular anastomoses is recognized as the gold-standard treatment.33
Ultrasound assessment following laser treatment plays a major role in documenting the successful interruption of the connecting fetal circulations that leads to normalization of amniotic fluid, bladder filling in both fetuses and cardiac function in the recipient fetus. While there are no universally specified protocols for ultrasound follow-up scans after intrauterine laser therapy, it is generally accepted that weekly scans for the first 2 or 3 weeks can help screen for potential complications. These complications include recurrent TTTS, occurring in about 1% of cases, or the development of TAPS in about 2–15% of cases.34 TAPS, often labeled as iatrogenic, typically results from small-caliber anastomoses missed during laser surgery. Ultrasound features of TAPS include a discrepancy in Doppler measurements in the middle cerebral artery peak systolic velocity (MCA-PSV) between fetuses. This discrepancy indicates fetal anemia in the former TTTS recipient (now TAPS donor) and polycythemia in the former TTTS donor (now TAPS recipient). In 55% of cases this condition occurs post-laser surgery.35
In TTTS cases, the recipient twin often shows cardiac compromise related to a decreased diastolic function due to a thickened myocardium, predominantly at the right ventricle level. This finding can be explained by the volume overload theory and an increase in cardiac output in these fetuses. Cardiovascular abnormalities in the recipient twin can include hypertension, ventricular hypertrophic cardiomyopathy, tricuspid regurgitation and, most commonly, right ventricular outflow tract obstruction. About one-third of the recipient twins have abnormal DV Doppler assessment at stage III in the Quintero staging system and about 30–50% of recipient twins have tricuspid regurgitation. By contrast, the donor twin generally has normal cardiac function, with only 5–10% presenting with abnormal Doppler in the DV and about 3% with tricuspid regurgitation.36 In cases in which laser treatment successfully interrupts the intertwin transfusion, the short and long-term outcomes for cardiac function are favorable, often leading to rapid normalization of cardiac function in both recipient and donor twins. However, pulmonary stenosis can be observed post-surgery, even in the donor twin, with an overall prevalence of about 8–10%. This finding necessitates prenatal and postnatal cardiac surveillance.37
Twin-to-twin transfusion syndrome in monoamniotic twins
The prevalence of MCMA twins is less than 1%, and the rate of all specific complications related to shared placentation like TTTS, TAPS or selective fetal growth restriction (sFGR), is less prevalent in monoamniotic twins. This is due to the unique nature of this rare twin pregnancy type, characterized by very close cord insertions and a particular placental vascular architecture. The incidence of TTTS in MCMA twins is less than 5%, and the diagnosis is made through ultrasound examination.38 The main difference between MCMA and MCDA twins is that MCMA twins share a single amniotic sac and therefore, the diagnosis of TTTS is made in the presence of polyhydramnios, defined as measurement of the deepest vertical pocket >8 cm before 20 weeks or >10 cm after 20 weeks, accompanied by a distended bladder in one twin and a small or invisible bladder in the other twin. Doppler assessment in MCMA twins should be carefully interpreted due to diastolic notching in the umbilical artery flow pattern, which results from cord compression. Although this can persist for an extended period, it is generally a transient finding.39
To assess umbilical artery blood flow, the pulsed-wave Doppler gate is best placed at the level of the fetal bladder to minimize the risk of assessing the umbilical cord of the same twin twice.
Twin-to-twin transfusion syndrome in triplet pregnancies
MC triplet pregnancies, like MCDA twins, carry a high risk of developing TTTS. The most commonly observed types of MC triplets are dichorionic triamniotic (DCTA) and monochorionic triamniotic (MCTA). In these cases, the TTTS typically involves either the two fetuses from the MC pair in DCTA triplets or the three fetuses in the MCTA triplets, where depending on the placental anastomoses, there can be two donors or two recipients.
The management strategy for these conditions is similar to that for MCDA twins, which includes using fetoscopic laser coagulation of the vascular anastomosis to treat the disease by disconnecting the two or three fetal circulations. The survival rate of at least one fetus in the case of DCTA triplets is reported to be about 94% and in MCTA triplets, about 81%.40
Twin anemia–polycythemia sequence
TAPS is a complication found in MC pregnancies, characterized by a large discrepancy in hemoglobin levels between the two fetuses, without amniotic fluid discordance.35,41
Spontaneous TAPS typically occurs during the viable period after 26 weeks of gestation, with an incidence of about 5%. In cases in which laser surgery has been performed, TAPS arises in 2–13% of cases, predominantly within the first 3 to 4 weeks following the procedure but it could also be diagnosed several weeks later. This is typically due to the presence of small-caliber anastomoses.42
The antenatal diagnosis of TAPS relies on ultrasound assessment of MCA-PSV. This value is elevated (>1.5 MoM) in the donor twin and decreased (<1 MoM) in the recipient twin;34,43 or, as stated in the new emerging classification, an intertwin difference in MCA-PSV >0.5 MoM (Figure 9).44 The severity of the disease is determined based on Doppler assessment with a five-stage classification system (Table 2).
9
Twin anemia–polycythemia sequence (TAPS). (a) Placental dichotomy: hypoechogenic placental share for polycythemic twin (left) and enlarged hyperechogenic placental share for the anemic twin (right); (b) ‘starry sky’ liver of the polycythemic twin; (c) discordant middle cerebral artery peak systolic velocity (MCA-PSV) Doppler measurements: decreased in the polycythemic twin (left) and increased in the anemic twin (right).
2
Twin anemia–polycythemia sequence: staging classification according to Slaghekke et al. (2010)43 and Tollenaar et al. (2019).44
Stage | Slaghekke et al. | Tollenaar et al. |
1 | MCA-PSV donor >1.5 MoM and MCA-PSV recipient <1.0 MoM, without other signs of fetal compromise | >0.5 MoM difference between fetuses in delta MCA-PSV |
2 | MCA-PSV donor >1.7 MoM and MCA-PSV recipient <0.8 MoM, without other signs of fetal compromise | >0.7 MoM difference between fetuses in delta MCA-PSV |
3 | Stage 1 or 2 changes in MCA-PSV, with cardiac compromise of the donor | Stage 1 or 2 changes in MCA-PSV, with cardiac compromise of the donor |
4 | Hydrops in donor | Hydrops in donor |
5 | IUFD of either fetus in a pregnancy known to be affected by TAPS | IUFD of either fetus in a pregnancy known to be affected by TAPS |
DV, ductus venosus; IUFD, intrauterine fetal demise; MCA, middle cerebral artery; MoM, multiples of the median; PSV, peak systolic velocity.
Additional ultrasound findings related to the diagnosis of TAPS include the appearance of a dichotomous placenta, which is enlarged and hyperechogenic on the side of the anemic donor twin, and hypoechogenic on the side of the recipient polycythemic twin (Figure 9). The recipient twin has also been observed to have "starry sky" appearance of its liver due to congestion, and the donor twin with cardiomegaly as a result of fetal anemia.45
Ultrasound screening for complications associated with monochorionic multiple pregnancies, like TTTS or TAPS, should ideally begin as early as 16 weeks of gestation. This should continue at 2-week intervals until delivery, provided there are no complications.
ULTRASOUND IN THE SECOND AND THIRD TRIMESTERS
Fetal growth
Multiple pregnancies are at higher risk for poor intrauterine growth. The pattern of growth velocity differs from that of singletons starting from the mid-second trimester, and it varies in MC and DC gestations.46,47 The growth discrepancy between the fetuses is also an important parameter for appropriate management of these pregnancies. Discrepancy percentages are calculated using the formula (Fetus A – Fetus B) × 100/Fetus A, where Fetus A is the heavier fetus and Fetus B is the lighter one. The same rule applies to triplet pregnancies: (Fetus A – Fetus C) × 100/Fetus A, where Fetus C is the smallest triplet and (Fetus A – Fetus B) × 100/Fetus A, where Fetus B is the medium-sized fetus. A fetal weight discrepancy of 25% is a significant ultrasound finding that requires closer monitoring and, if necessary, referral to a specialist center.
sFGR is defined in DC and MC pregnancies when one fetus is growing appropriately for gestational age and the other fetus' estimated weight (EFW) falls below the 10th percentile, with a discrepancy between the two fetuses of 25%.
For twins without complications, ultrasound assessment should be performed every 4 weeks after 20 weeks in DC twins and every 2 weeks after 16 weeks in MC twins to monitor growth and Doppler indices of the umbilical artery, middle cerebral artery and DV. However, closer surveillance should be offered in cases with abnormal Doppler findings.48
Dichorionic twin pregnancy
The Doppler patterns in DC twin pregnancies with sFGR are similar to those in singleton pregnancies and weekly wellbeing assessment including EFW, amniotic fluid and Doppler indices would be recommended in DC twin cases. If the condition of the smaller fetus deteriorates, appropriate counseling should be provided, depending on the gestational age, considering options such as corticosteroid prophylaxis and preterm delivery or expectant management, and bearing in mind the possibility of spontaneous fetal demise of the smaller baby. Since the two fetuses in a DC pair have separate circulations, in cases of intrauterine demise of the smaller fetus, the outcome is usually favorable for the surviving twin. However, the main risks include miscarriage and severe preterm birth.49
Monochorionic twin pregnancy
sFGR in MC twins occurs as commonly as TTTS and can affect 1 in 10 pregnancies. The cause of sFGR can be attributed to an unequally shared placental mass, the vascular architecture, and the pattern of the placental anastomoses. This condition can occur in isolation but very often (up to 70%) is associated with TTTS or TAPS.50,51
The Gratacós classification system categorizes sFGR into three types based on the umbilical artery Doppler pattern in the smallest twin. Type I is characterized by normal umbilical artery blood flow, type II with persistent absent or reversed end-diastolic flow and type III with intermittent absent or reversed end-diastolic flow (Figure 10).52 Type I usually requires expectant management with ultrasound follow-up before delivery, and has a favorable outcome for both fetuses. In type-II and type-III cases the management is not well established and depends on the gestational age at which the condition occurs and, therefore, the possibility of either intrauterine fetal therapy (cord occlusion or laser coagulation of the placental anastomoses) or preterm delivery.
10
Selective fetal growth restriction in monochorionic twin pregnancy. In type I (a), the umbilical artery Doppler waveform has positive end-diastolic flow, while in type II (b) there is persistent absent or reversed end-diastolic flow (AREDF) and in type III (c) there is a cyclical/intermittent pattern of AREDF.
Ultrasound assessment for sFGR type I, type II and type III should be performed on a weekly basis. In type II sFGR, the prognosis is the worst with 90% eventual deterioration and a high risk of perinatal morbidity and mortality. The Doppler pattern in these cases is similar to that in a singleton pregnancy and the assessment of the blood flow in the DV can help to predict intrauterine demise, therefore it should be part of the ultrasound surveillance. In contrast, type III has an unpredictable outcome with the possibility of sudden intrauterine demise (up to 20%) and this may be explained by the presence of a large intertwin artery-to-artery anastomosis or a close insertion of the two umbilical cords. Ultrasound has a poor predictive value for fetal demise in these cases.52 Moreover, in about 14% of sFGR type III twins, the Doppler assessment of the umbilical artery of the small fetus could change and normalize. This is observed in more advanced gestation and can be explained by the fact that the placental share between the two fetuses is not as imbalanced as in type II.53,54
ULTRASOUND IN CASES OF SINGLE INTRAUTERINE DEMISE
The long- and short-term outcomes in cases of single intrauterine demise are influenced by factors such as chorionicity, amnionicity and gestational age. Although more common in twin pregnancies compared to singletons, spontaneous fetal demise in non-complicated MC twins is relatively rare. The main complications of intrauterine single demise in cases of MC gestations are related to potential ischemic brain lesions in the surviving twin, caused by blood exsanguination via the vascular anastomoses to the placenta of the demised cotwin.55 Such brain injury is a primary risk factor for neurological impairment in the surviving twin and, in such cases, it is recommended to carry out an ultrasound evaluation a few weeks after the event to exclude potential brain lesions. Additionally, magnetic resonance imaging is recommended in the early third trimester to provide further insight into any neurological damage.
If a single demise occurs in the first trimester of a twin pregnancy, this will usually lead to what is known as the "vanishing twin syndrome", which is generally associated with good pregnancy outcome. In such cases, one of the twins appears to "vanish" from ultrasound imaging due to its miscarriage, while the other twin continues to develop normally (Figure 11).
11
Vanishing twin syndrome: 2D (left) and 3D (right) view of an empty sac and a sac with an embryo.
CONCLUSIONS
Ultrasound has a major role in the prenatal management and surveillance of multiple pregnancies. A standardized protocol for ultrasound assessment can help in early detection of possible complications, and appropriate management to improve pregnancy outcomes.
Determining chorionicity early in the first trimester is essential for predicting potential risks associated with multiple pregnancies. This early identification helps guide appropriate management strategies and expectations.
Moreover, the use of ultrasound as a screening tool can significantly improve the diagnosis of complications in both dichorionic and monochorionic multiple pregnancies. This can facilitate the early referral of complicated pregnancies to specialized centers, where timely intervention can be performed to ensure the best possible outcome.
PRACTICE RECOMMENDATIONS
- Ultrasound determination of chorionicity is of paramount importance for the correct management of multiple pregnancies.
- Ultrasound should be used in both DC and MC twins and higher-order multiple pregnancies with scheduled visits following the diagnosis in the first trimester: every 4 weeks for DC pregnancies after 20 weeks and every 2 weeks in MC pregnancies after 16 weeks.
- Identification of ultrasound markers of pregnancy complications in the first trimester, such as NT ≥95th percentile, CRL discordance ≥10%, and abnormal a-wave flow in the DV, allows clinicians to identify pregnancies at higher risk for structural defects, fetal loss or need for laser treatment before 20 weeks.
- Fetal growth, amniotic fluid and Doppler assessments, including umbilical artery, middle cerebral artery and DV, should be performed at every visit in all MC twins and higher-order MC multiple pregnancies as a screening tool for specific complications associated with shared placentation.
- Ultrasound evaluation of EFW, Doppler indices and amniotic fluid is essential for the prenatal surveillance of both DC and MC multiple pregnancies.
CONFLICTS OF INTEREST
The author(s) of this chapter declare that they have no interests that conflict with the contents of the chapter.
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REFERENCES
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