Twin Pregnancy: Ultrasound Surveillance and Common Complications | Article

This chapter should be cited as follows:
Palmrich P, Khalil A, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.419323

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

Twin Pregnancy: Ultrasound Surveillance and Common Complications

First published: September 2024

AUTHORS

Pilar Palmrich

Department of Obstetrics and Gynecology, Division of Obstetrics and Feto-maternal Medicine, Medical University of Vienna, Vienna, Austria

Asma Khalil, MD(Res), MRCOG, MSc(Epi), MBBCh

The Royal College of Obstetricians and Gynaecologists, London, UK; Fetal Medicine Unit, Department of Obstetrics and Gynaecology, St George's University Hospitals NHS Foundation Trust, London, UK

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INTRODUCTION

The incidence of twin pregnancy has increased substantially over the past decades (33.2 per 1000 live births in 2009),¹ secondary to emerging use of assisted reproduction techniques.² However, according to more recent reports, there has been a decline in twin birth rates since 2014.³

Twin pregnancies are associated with a high risk of adverse outcomes, particularly perinatal morbidity and mortality.⁴^,⁵^,⁶

Ultrasound assessment of fetal anatomy, biometry, Doppler blood flow investigation and amniotic fluid volume is used to identify and monitor twin pregnancies at risk of adverse outcomes including complications such as twin-to-twin transfusion syndrome (TTTS) and fetal growth restriction (FGR).

DATING OF TWIN PREGNANCY

Twin pregnancies should ideally be dated when the crown–rump length (CRL) measurement is between 45 and 84 mm (11 + 0 to 14 + 0 weeks of gestation). In pregnancies conceived spontaneously, it is recommended to use the larger of the two CRLs to estimate gestational age.⁷ After 14 weeks of gestation, the larger head circumference should be used to date the pregnancy. Twin pregnancies conceived via in-vitro fertilization (IVF) should be dated using the oocyte retrieval date or the embryonic age from fertilization.⁷

DETERMINING CHORIONICITY AND AMNIONICITY

Chorionicity describes the number of placentas in a pregnancy. Chorionicity should be determined before 14 weeks of gestation, when the amnion and chorion have not yet fused, based on the intertwin membrane thickness at the site of insertion of the amniotic membrane into the placenta, and identifying the T-sign or lambda sign and the number of placental masses.⁷^,⁸

In dichorionic diamniotic (DCDA) twin pregnancies, the twins are separated by a thick layer of fused chorionic membranes with a thin amniotic layer on each side, appearing as a “full lambda” (Figure 1a), while in monochorionic diamniotic (MCDA) twin pregnancies, the fetuses are separated by two thin amniotic layers (T-sign) (Figure 1b).

Ultrasound images in the first trimester of: (a) dichorionic diamniotic twin pregnancy, in which the twins are separated by a thick layer of fused chorionic membranes (lambda sign); (b) monochorionic diamniotic twin pregnancy in which the twins are separated by two thin amniotic layers (T-sign).

At the time at which chorionicity is determined, amnionicity (describing whether or not the fetuses share the same amniotic sac) should also be determined and documented. When the intertwin membrane cannot be visualized, the pregnancy should be considered to be monochorionic monoamniotic (MCMA). Additionally, cord entanglement can be visualized, using color and pulsed-wave Doppler ultrasound, in MCMA twin pregnancies (Figure 2). Approximately 5% of monochorionic twin pregnancies are monoamniotic (MCMA).⁹^,¹⁰ MCMA pregnancies are at increased risk of intrauterine death compared to MCDA and DCDA twin pregnancy¹¹ and should be referred to a tertiary center with expertise in their management.²

(a) Non-visualization of intertwin membrane indicating monochorionic monoamniotic (MCMA) twin pregnancy. (b) Visualization of cord entanglement in MCMA using color Doppler ultrasound. (c) Cord entanglement using pulsed-wave Doppler on 3D ultrasound.

LABELING OF TWIN FETUSES

There are different options to label twin fetuses, including labeling according to their site (left/right or upper/lower), or mapping according to the cord insertion in relation to the placental edges and membrane insertion in the first trimester. Most importantly, antenatal labeling of twins should follow a consistent strategy. Using multiple features to describe each twin is advisable (e.g. “Twin A (male) is on the maternal right with an anterior placenta and central cord insertion”). The information should be clearly documented in the woman’s notes. In case of discordance, additional information such as “Twin A, recipient”, should be added to the description. Labeling in MCMA pregnancies is less accurate, especially in the first trimester.

ROUTINE ULTRASOUND MONITORING OF TWIN PREGNANCIES

Women with an uncomplicated dichorionic twin pregnancy should have a first-trimester scan, a detailed second-trimester anatomy scan, followed by 4-weekly ultrasound assessments. Complicated dichorionic twins should be scanned more frequently, depending on the condition and its severity (Figure 3).
Uncomplicated monochorionic twins should have a first-trimester scan and ultrasound assessment every 2 weeks after 16 weeks to detect TTTS and twin anemia–polycythemia sequence (TAPS) promptly (Figure 4). Complicated monochorionic twins should be scanned more frequently, depending on the condition and its severity.
Ultrasound assessment should include fetal biometry, and measurement of amniotic fluid volume and Doppler indices, as shown in Figures 3 and 4.
Discordance in estimated fetal weight (EFW) should be calculated and documented every time from 20 weeks onwards.
In monochorionic twin pregnancies, middle cerebral artery (MCA) peak systolic velocity (PSV) should be recorded from 20 weeks (in order to screen for TAPS).
In MCDA twins, the amniotic fluid volume (deepest vertical pocket, DVP) should be assessed at each ultrasound (in order to screen for TTTS).
Cervical length assessment to identify women at risk of extreme prematurity is recommended at the time of the anomaly scan.⁷

Ultrasound monitoring pathway in uncomplicated dichorionic twin pregnancy according to the ISUOG practice guideline: role of ultrasound in twin pregnancy.⁷

Ultrasound monitoring pathway in uncomplicated monochorionic twin pregnancy according to the ISUOG practice guideline: role of ultrasound in twin pregnancy.⁷ DVP, deepest vertical pocket of amniotic fluid; MCA, middle cerebral artery; PI, pulsatility index, PSV, peak systolic velocity; UA, umbilical artery

SCREENING FOR CHROMOSOMAL ABNORMALITIES

First-trimester combined screening for trisomy 21, including maternal age, nuchal translucency (NT) measurement and serum beta human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein-A (PAPP-A) levels, or alternatively a combination of maternal age and NT, can be performed between 11 + 0 and 13 + 6 weeks of gestation.² In the case of a vanishing twin, first-trimester screening for trisomies should include a combination of maternal age, NT and serum free β-hCG without the use of PAPP-A.¹²

The risk of trisomy 21 in dichorionic twin pregnancy is calculated per fetus while in monochorionic twin pregnancy the risk is calculated per pregnancy based on the average risk of both fetuses.

Cell-free DNA (cfDNA) analysis of maternal blood is well established in singleton pregnancies for the detection of common trisomies (trisomies 21, 18, 13). A large multicenter study confirmed that cell-free DNA testing is the most accurate screening method for trisomy 21 in twin pregnancies, demonstrating similar screening performance to that in singleton pregnancies with a very low failure rate. Although the predictive accuracy for trisomies 18 and 13 may be lower, a low false-positive rate supports the use of cell-free DNA as a first-line screening option for women with twin pregnancies.¹³ A recent large systematic review indicated that non-invasive prenatal testing (NIPT) can also effectively detect common autosomal aneuploidies in pregnancies with a vanishing twin, though with a higher false-positive rate.¹⁴

INVASIVE PRENATAL DIAGNOSIS

Early diagnosis of aneuploidy is crucial in twin pregnancy, particularly in view of the lower risk of pregnancy loss and preterm delivery of selective termination in the first trimester compared to the second trimester (7% risk of loss of the entire pregnancy, and 14% risk of delivery before 32 weeks).¹⁵

Chorionic villus sampling (CVS) is preferred in dichorionic twin pregnancy because it can be performed earlier than can amniocentesis.

If monochorionicity is confirmed, it is expected that both fetuses have an identical karyotype. It is therefore justifiable to perform a single CVS in the first trimester or single amniocentesis at a later gestational age. However, cases of discordant karyotype in monozygotic twins have been reported. In the case of discordant ultrasound findings of growth or anatomy, it is therefore essential to sample both fetuses. This is preferably performed via amniocentesis of both amniotic sacs as it cannot be guaranteed that both fetuses are sampled with CVS.¹⁶

ULTRASOUND SCREENING FOR STRUCTURAL ABNORMALITIES

First-trimester ultrasound examination should be performed between 11 + 0 and 14 + 0 weeks of gestation and routine second-trimester ultrasound screening for anomalies in twins should be performed at 18–22 weeks of gestation.⁷

The risk of fetal anomalies is higher in twin compared with singleton pregnancy. In dichorionic twins, the prevalence per fetus is the same as in singletons (2%). The risk of at least one fetus being affected by an anomaly is therefore twice as high as that in singleton pregnancy. In monochorionic twins, the reported prevalence of a structural defect per fetus is twice as high as in singletons (4%), leading to an 8% risk of fetal abnormality of at least one fetus.¹⁷^,¹⁸

MANAGING TWIN PREGNANCY DISCORDANT FOR FETAL ANOMALY

In around 1–2% of twin pregnancies, only one fetus is affected by an anomaly. Even in monozygotic twin pregnancies the majority of cases are discordant for a structural anomaly. However, discordant aneuploidy in monochorionic twins is very rare. Expert ultrasound with invasive fetal chromosomal or genetic testing is indicated for these cases.

For lethal conditions at high risk of causing intrauterine demise, conservative management is preferred in dichorionic twin pregnancies. In a monochorionic twin pregnancy, however, intervention is warranted in order to protect the healthy cotwin from adverse effects of spontaneous demise of the affected twin.

The timing of selective termination in twin pregnancy substantially influences the risk of miscarriage and preterm birth. In dichorionic twin pregnancy, selective feticide is performed by ultrasound-guided intracardiac or intrafunicular injection of potassium chloride or lignocaine, preferably in the first trimester. Selective feticide in monochorionic twins is performed by cord occlusion, intrafetal laser ablation or radiofrequency ablation (RFA). The survival rate of the cotwin is approximately 80% and the risk of preterm birth prior to 32 weeks is approximately 20%.¹⁹ The risk of adverse neurological outcome in the surviving cotwin is increased.¹⁹^,²⁰^,²¹

SCREENING, DIAGNOSIS AND MANAGEMENT OF FETAL GROWTH RESTRICTION (FGR)

Diagnostic criteria for selective FGR (sFGR)

Selective fetal growth restriction (sFGR), conventionally, is defined as a condition in which one fetus has an EFW <10th centile and/or the intertwin EFW discordance is ≥25%.²²^,²³
Screening and diagnosis for sFGR are established by calculation of EFW based on measurement of the fetal head, abdomen and femur length.
Intertwin EFW discordance is calculated using the following formula: ((weight of larger twin – weight of smaller twin) × 100)/weight of larger twin.⁷

Assessment of EFW by ultrasound is less accurate in twin pregnancies compared to singleton pregnancies.²⁴ Pregnancies with established diagnosis of sFGR should receive increased fetal surveillance and delivery planning due to the significantly increased risk of perinatal loss.²⁵

Classification of sFGR in monochorionic twin pregnancy

Classification of sFGR in monochorionic twins is based on the umbilical artery Doppler end-diastolic velocity pattern²⁶ (Figure 5).

Type I: positive end-diastolic flow in the umbilical artery.

Type II: absent or reversed end-diastolic flow in the umbilical artery (AREDF).

Type III: cyclical/intermittent pattern of AREDF in the umbilical artery.

Classification of selective fetal growth restriction in monochorionic twins according to umbilical artery Doppler end-diastolic velocity pattern

Managing twin pregnancy complicated by sFGR

In DCDA pregnancies, sFGR should be followed as in growth-restricted singletons, monitoring umbilical artery, middle cerebral artery (MCA) and ductus venosus (DV) Doppler indices approximately every 2 weeks, depending on severity.

Follow-up of monochorionic twin pregnancies affected by sFGR should include assessment of fetal growth every 2 weeks and fetal Doppler assessment of umbilical artery and MCA at least weekly. If umbilical artery Doppler flow is found to be abnormal, the examination should also involve assessment of DV blood flow. If ultrasound assessment suggests a substantial risk of fetal demise of one of the twins before 26 weeks of gestation, the option of selective termination is warranted in order to protect the normally grown fetus in case of in-utero demise of the smaller fetus.

The timing of delivery should be decided upon based on assessment of fetal wellbeing, fetal growth, biophysical profile, DV waveform and/or computerized cardiotocography (CTG) (if available).

COMPLICATIONS UNIQUE TO MONOCHORIONIC TWIN PREGNANCY

Screening, diagnosis, staging and management of twin-to-twin transfusion syndrome (TTTS)

TTTS is a result of arteriovenous anastomoses in the monochorionic placenta which cause an imbalance in the fetoplacental circulation leading to a transfusion from one twin to the other.¹⁸^,²⁷^,²⁸ The diagnosis is made on the ultrasonographic finding of an amniotic fluid imbalance, showing oligohydramnios in the donor twin (deepest vertical pocket, DVP <2 cm) and polyhydramnios in the recipient twin (DVP >8 cm before 20 weeks of gestation, DVP >10 cm after 20 weeks of gestation). Severity of TTTS is defined according to the Quintero staging system (Table 1), which takes into account fetal bladder filling on ultrasound, Doppler indices of the umbilical artery and ductus venosus as well as presence of fetal hydrops²⁹^,³⁰ (Figure 6).

Quintero staging system.²⁹

Stage	Classification
I	Polyhydramnios–oligohydramnios sequence: DVP >8 cm in recipient twin and DVP <2 cm in donor twin
II	Bladder in donor twin not visible on ultrasound
III	Absent or reversed umbilical artery diastolic flow, reversed ductus venosus a-wave flow, pulsatile umbilical venous flow in either twin
IV	Hydrops in one or both twins
V	Death of one or both twins

(a) Polyhydramnios (deepest vertical pocket, DVP >8 cm) in recipient twin. (b) Oligohydramnios (DVP <2 cm) in donor twin. (c) Twin pregnancy with TTTS Quintero Stage I, showing both fetal bladders. (d) Absent end-diastolic umbilical artery Doppler flow in twin pregnancy with TTTS Quintero Stage III.

Treatment of TTTS

For TTTS Quintero stage I, conservative management can be considered as an option. Laser ablation is the treatment of choice for TTTS Stage II and above if diagnosed before 26 weeks of gestation.³¹ If laser-surgery expertise is not available, amnioreduction can be performed in pregnancies diagnosed after 26 weeks of gestation.³¹

Following laser treatment, the recurrence rate of TTTS is up to 14%, which likely results from anastomoses missed at the laser treatment.³² The risk of recurrence of TTTS and occurrence of post-laser TAPS is reduced by use of the Solomon technique compared with the highly selective technique.³³^,³⁴ Another option for the management of severe TTTS is selective termination of pregnancy by performing cord occlusion of one of the umbilical cords.⁷

Screening, diagnosis and management of twin anemia–polycythemia sequence (TAPS)

The incidence of spontaneous TAPS in MCDA pregnancies is up to 5%. It can occur as a complication after laser ablation in cases of TTTS in up to 13%.³² It results from small arteriovenous anastomoses (<1 mm) allowing for slow transfusion of blood from the donor to recipient twin leading to highly discordant hemoglobin concentrations at birth.

Prenatal diagnosis, defined according to Delphi consensus criteria³⁵ is based on discordant MCA Doppler findings (Figure 7), defined as an MCA-PSV ≥1.5 multiples of the median (MoM) suggestive of fetal anemia in the donor, combined with MCA-PSV of ≤0.8 MoM as a sign of polycythemia in the recipient. Alternatively, the intertwin MCA-PSV discordance can be calculated to diagnose TAPS. The agreed optimal threshold for intertwin MCA-PSV discordance is ≥1.0 MoM.³⁵ Postpartum diagnosis of TAPS is made on the following criteria: difference in hemoglobin concentration between the twins of more than 8 g/dL and reticulocyte count ratio >1.7, or the finding of small vascular anastomoses (<1 mm in diameter) in the placenta.³⁶^,³⁷

Staging of TAPS is summarized in Table 2. Additional ultrasound findings seen in TAPS include differences in placental echogenicity and thickness and possibly a ‘starry sky’ appearance of the liver in the polycythemic twin (Figure 8).

Discordant middle cerebral artery peak systolic velocity (MCA-PSV) Doppler findings (donor (a): MCA-PSV >1.5 multiples of the median (MoM) suggestive of fetal anemia, recipient (b): MCA-PSV <1.0 MoM as a sign of polycythemia).

Antenatal and postnatal staging of TAPS.⁷^,³⁶^,³⁷

Stage	Antenatal staging	Postnatal staging: intertwin Hb difference (g/dL)
1	Donor MCA-PSV >1.5 MoM and recipient MCA-PSV <1.0 MoM, without additional signs of fetal compromise	>8.0
2	Donor MCA-PSV >1.7 MoM and recipient MCA-PSV <0.8 MoM, without additional signs of fetal compromise	>11.0
3	Stage I or II and cardiac compromise in donor twin (UA-AREDF, UV pulsatile flow, DV-PI increased or reversed A-wave)	>14.0
4	Hydrops of donor twin	>17.0
5	Death of one or both fetuses, preceded by TAPS	>20.0

AREDF, absent or reversed end-diastolic flow; DV, ductus venosus; Hb, hemoglobin; MCA, middle cerebral artery; MoM, multiples of the median; PI, pulsatility index; PSV, peak systolic velocity; UA, umbilical artery; UV, umbilical vein

(a) Differences in placental echogenicity; (b) ‘starry sky’ appearance of the liver pattern in the polycythemic twin.

Management of TAPS requires individualization according to gestational age, severity of the disease, technical feasibility of intrauterine treatment options and parental choice. Options include conservative management, early delivery, laser ablation or intrauterine blood transfusion (IUT) of the anemic twin, or IUT of the anemic twin combined with partial exchange transfusion of the polycythemic twin.³⁶^,³⁸

Twin reversed arterial perfusion (TRAP) sequence

TRAP sequence is a rare complication, occurring in approximately 1% of monochorionic twin pregnancies. It is characterized by the presence of a TRAP or acardiac mass that is perfused by an apparently normal (pump) twin³⁹ (Video 1). In TRAP sequence there is retrograde perfusion through arterioarterial anastomoses (Video 2), in most cases via a common cord insertion site. It causes a hyperdynamic circulation and progressive high-output cardiac failure in the pump twin.³⁹^,⁴⁰ Different minimally invasive techniques, including cord coagulation, cord ligation and photocoagulation of the anastomoses, as well as intrafetal methods, such as radiofrequency ablation (RFA) and intrafetal laser therapy, can be performed to prevent demise of the pump twin.⁴¹^,⁴²^,⁴³ The survival rate of the pump twin using these treatment modalities is approximately 80%, while the risk of demise of the pump fetus in conservatively managed TRAP sequence is up to 30% by 18 weeks’ gestation.⁴⁰

Monochorionic monoamniotic twin pregnancy complicated by twin reversed arterial perfusion sequence

Arterioarterial anastomoses and common cord insertion site in twin reversed arterial perfusion sequence

MONOCHORIONIC MONOAMNIOTIC (MCMA) TWINS

Approximately 5% of monochorionic twin pregnancies are MCMA.⁹^,¹⁰ MCMA pregnancies are at increased risk of intrauterine death compared to MCDA and DCDA twin pregnancy.¹¹ Current recommendations on timing of delivery vary between 32 to 36 weeks of gestation.⁸^,⁴⁴ Recent evidence, showing that the risk of IUD after 32 + 4 weeks of gestation is greater compared to the risk of neonatal complications, however, suggests delivery by Cesarean section between 32 and 34 weeks of gestation to be the safest option.¹¹

PRACTICE RECOMMENDATIONS

Routine ultrasound monitoring in twin pregnancy

Dating of twin pregnancy should ideally be performed between 11 + 0 and 14 + 0 weeks of gestation using the crown–rump length (CRL).
Chorionicity should be determined before 14 weeks of gestation by identification of the T-sign in monochorionic diamniotic (MCDA) twin pregnancies or the lambda sign in dichorionic diamniotic (DCDA) twin pregnancies, and the number of placental masses.
Amnionicity should be determined and documented at the same time as chorionicity. In the absence of an intertwin membrane, the pregnancy should be categorized as monochorionic monoamniotic (MCMA).
MCMA pregnancies should be referred to a tertiary center for management.
Antenatal labeling of twins should follow a consistent strategy throughout pregnancy and this information should be documented clearly.
Routine ultrasound monitoring of uncomplicated DCDA twin pregnancies should include a first-trimester scan and detailed second-trimester scan, followed by 4-weekly ultrasound assessments.
Routine ultrasound monitoring of uncomplicated monochorionic twin pregnancies should include a first-trimester scan and ultrasound assessment every 2 weeks after 16 weeks.
Complicated twin pregnancies should be scanned more frequently, depending on the condition and its severity.
Discordance in estimated fetal weight (EFW) should be calculated and documented from 20 weeks onwards.
In monochorionic twin pregnancies, middle cerebral artery (MCA) peak systolic velocity (PSV) should be recorded from 20 weeks to screen for twin anemia–polycythemia sequence (TAPS).
In MCDA twins, the amniotic fluid volume should be assessed at each ultrasound examination to screen for twin-to-twin transfusion syndrome (TTTS).

Screening for structural anomalies, chromosomal abnormalities and genetic syndromes in twin pregnancy

First-trimester combined screening can be performed between 11 + 0 and 13 + 6 weeks of gestation.
The risk of fetal anomalies is higher in twin compared with singleton pregnancy. In about 1–2% of twin pregnancies, only one fetus is affected by an abnormality. Expert ultrasound is essential and invasive chromosomal or genetic testing should be offered in these cases.
It is important to establish diagnosis of aneuploidy early in twin pregnancy, due to the lower risk of pregnancy loss and preterm delivery of selective termination in the first trimester compared to the second trimester.
CVS should be preferred in dichorionic twin pregnancy because it can be performed earlier than can amniocentesis. In case of monochorionicity, it is expected that both fetuses have an identical karyotype, and a single CVS in the first trimester or single amniocentesis at a later gestational age can be performed.

Selective fetal growth restriction

Selective fetal growth restriction should be defined as one fetus with an EFW <10th centile and/or an intertwin EFW discordance of ≥25%.
Pregnancies with established diagnosis of sFGR should receive increased fetal surveillance and delivery planning. The timing of delivery should be decided based on assessment of fetal wellbeing, fetal growth, biophysical profile, DV waveform and/or computerized cardiotocography (CTG).

Complications unique to monochorionic twin pregnancy

Twin-to-twin transfusion syndrome (TTTS) is a result of arteriovenous anastomoses in the monochorionic placenta which cause an imbalance in the fetoplacental circulation leading to transfusion from one twin to the other.
The diagnosis of TTTS should be made on the sonographic finding of an amniotic fluid imbalance, showing oligohydramnios in the donor twin (DVP <2 cm) and polyhydramnios in the recipient twin (DVP >8 cm before 20 weeks of gestation, DVP >10 cm after 20 weeks of gestation).
Severity of TTTS should be defined according to the Quintero staging system.
For TTTS Quintero stage I, conservative management can be considered as an option. If available, laser ablation is the treatment of choice for TTTS Stage II and above if diagnosed before 26 weeks of gestation.
Twin anemia–polycythemia sequence (TAPS) results from small arteriovenous anastomoses allowing for slow transfusion of blood from donor to recipient twin leading to highly discordant hemoglobin concentrations.
The prenatal diagnosis of TAPS should be based on discordant MCA Doppler findings, defined as an MCA-PSV ≥5 multiples of the median (MoM) in the donor (anemic), combined with an MCA-PSV of ≤0.8 MoM in the recipient (polycythemic).
Treatment options for TAPS include conservative management, early delivery, laser ablation or intrauterine blood transfusion.
MCMA pregnancies are at increased risk of intrauterine demise compared to MCDA and DCDA twin pregnancy. Delivery via Cesarean section should be recommended between 32 and 34 weeks.

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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