Monochorionic twins have a shared placenta and can be either mono- or diamniotic.
Abstract: Twin pregnancies are categorized based on chorionicity and amnionicity. Monochorionic twins have a shared placenta and can be either mono- or diamniotic. Multifetal gestations carry an increased risk for fetal and maternal morbidity. Monochorionic twins have a higher risk of developing several complications and have their own unique challenges including twin-to-twin transfusion syndrome, twin anemia polycythemia sequence, twin reversed arterial perfusion sequence, selective fetal growth restriction, and conjoined twins, all of which increase mortality in these twins. It is crucial to determine chorionicity early in pregnancy to identify high risk individuals and refer these patients to a Fetal Medicine specialist. These patients will require close monitoring throughout the pregnancy. Surveillance includes measurement of fetal growth, amniotic fluid volume, umbilical artery Doppler and middle cerebral artery Doppler in addition to detailed anatomy scans, cervical length measurement, and fetal echocardiogram. Standardized serial monitoring protocols have been established in clinical practice guidelines but may vary slightly based on jurisdiction, case presentation and concern for specific complications.
Keywords: monochorionic twins, surveillance, ultrasound
Authors: Abigail Christmas1, Katherine Huebner1, Marcos Cordoba MD2,3,4, Vivian C Romero MD2,3,4
1. Michigan State University, College of Human Medicine, Grand Rapids MI, USA
2. Department of Obstetrics, Gynecology and Reproductive Biology, College of Human Medicine, Michigan State University, Grand Rapids, MI, USA
3. Department of Obstetrics, Gynecology and Women’s Health, Corewell Health, Grand Rapids, MI, USA
4. Division of Maternal Fetal Medicine, Department of Obstetrics, Gynecology and Women's Health, Corewell Health, Grand Rapids, MI, USA
Reviewers: Karen Fung-Kee-Fung, Yinka Oyelese
Monochorionic twins are a classification of twins that occur in monozygotic pregnancies. In these pregnancies, a single fertilized oocyte divides into two separate embryos after the third day of normal embryonic development . Approximately 20% of twin pregnancies are monochorionic, and these twins often have significantly more complications than their dichorionic counterparts . These complications can be potentially life threatening and are further discussed in this chapter. The complications occur because of the development of vascular anastomosis between the fetuses that can affect the fetal blood supply of either twin, leading to increased fetal morbidity and mortality [2-4]. For this reason, monochorionic pregnancies need more intense surveillance and management than single gestation and dichorionic pregnancies [3-4]. In this chapter, we will review dating, establishing chorionicity, complications and recommended surveillance of monochorionic pregnancies.
Although the guidelines suggest dating between 11 weeks and 13 weeks and 6 days, chorionicity can often be established as early as 8 weeks gestational age to correctly identify high risk pregnancies and promptly direct their care. Ideally, nuchal translucency should be measured when the crown-rump length (CRL) measures between 45 and 84 mm . It is generally recommended that the dating estimate in twin pregnancies should be based on the larger of the two fetuses' crown-rump length (CRL). . Some studies suggest using the CRL of the smaller fetus or calculating the mean CRL as they have been shown to be more accurate in both prospective and retrospective analyses [4-7]. However, the concern is that these two methods can lead providers into erroneously classifying the larger twin as large for gestational age (LGA) which can deceive them into falsely believing that the smaller twin is an adequate size . This error could potentially have detrimental effects leading to missed surveillance and inadequate management of the pregnancy. When a woman presents after 14 weeks gestational age, the head circumference of the larger fetus should be used to date the pregnancy . For pregnancies conceived by in-vitro fertilization (IVF) dating is based on the age of the embryo .
Chorionicity refers to the number of placentas identified in a pregnancy. The ideal time to establish chorionicity is early on in gestation, specifically it is most accurate before 13 + 6 weeks of gestation . A retrospective study done by Lee et al of 410 twins found that establishing chorionicity by sonographic signs at or before 14 weeks gestational age demonstrated a good sensitivity (89.9%), specificity (99.5%), and positive and negative predictive value (97.8% and 97.5% respectively) [3,8]. If transvaginal or transabdominal ultrasound clearly demonstrates two placentas or fetal sex discordance, then the pregnancy is said to be dichorionic [3-4].
When only one distinct placenta is visualized, chorionicity is determined by ultrasound examination of the thickness of the amniotic membrane that inserts into the placenta between the two fetuses. In dichorionic diamniotic twins, there is a thick projection of this echogenic membrane that is at least three layers [3,4,8]. The layers consist of a fused chorion with a thin layer of amnion on both sides separating the two fetuses [3,4,8]. This is said to give the appearance of a “lambda” and is known as the “lambda or delta sign” [2-4]. Monochorionic diamniotic (MCDA) twins are only separated by the two thin layers of the amnion which is said to give the appearance of a “T” and is known as the “T-sign” and is less than 2 millimeters thick [2,4].
Measuring the number of placental masses is not a reliable method to determine chorionicity because dichorionic placentas can be next to each other and falsely appear as one placenta on ultrasound . Additionally, a study that examined 109 monochorionic placentas found that 3% of them had two placental masses potentially leading to false interpretations of the pregnancy [4,9]. These patients had bipartite placentas that were proven to be monochorionic on macroscopic examination and demonstrated the classic “T-sign” on ultrasound . Chorionicity of women who present after 14 weeks of gestation is evaluated by the same ultrasonographic signs described above, along with determining if there is discordance of fetal sex suggesting dizygotic twins . It is crucial to refer all patients in whom chorionicity cannot be established to a tertiary center for comprehensive evaluation. These pregnancies carry a significant risk of maternal and fetal morbidity and mortality, necessitating specialized care and monitoring . Pregnancies where chorionicity is unknown should be managed as monochorionic .
When determining chorionicity it is also important to document if the fetuses share the same amniotic sac, known as amnionicity. If there is an absence of a membrane between the twins, they are deemed monochorionic monoamniotic (MCMA) twins . However, the examiner must be cautious because it is possible to not visualize a membrane when imaging quality is poor, when the pregnancy is less than 8 weeks, when the amnion is not developed enough to be a specular reflector, or when there is a “stuck twin” in twin-to-twin transfusion syndrome. Other methods for determining amnionicity included looking for cord entanglement on color and pulsed-wave Doppler ultrasound . In MCMA twins pulsed-wave Doppler demonstrates two different arterial waveforms and heart rates at the same sampling gate .
Multifetal gestations carry increased risk for stillbirth, fetal anomalies, preterm delivery, growth issues and short-term and long-term neonatal and infant morbidity [10-12]. Additionally, women with multifetal gestations are at an increased risk of gestational diabetes, anemia, hemorrhage, cesarean delivery, postpartum depression, and hypertensive disorders of pregnancy [3,13-19].
Monochorionic pregnancies have a higher risk of developing complications in pregnancy including fetal and congenital anomalies, prematurity, fetal growth restriction, and stillbirth [20-22]. Complications unique to monochorionic pregnancies include selective fetal growth restriction, twin-to-twin transfusion syndrome, twin anemia polycythemia sequence, and twin reversed arterial perfusion sequence [20,23-24]. Cord entanglement is a risk unique to MCMA pregnancies however studies have concluded that it does not contribute to prenatal morbidity and mortality [25,26]. Conjoined twins are a rare complication that exclusively arises in MCMA pregnancies. Additionally, MCMA twins are at an increased risk of intrauterine demise when compared to other twin types .
The most common fetal anomalies in twin pregnancies include central nervous system, renal, and cardiac malformations . The prevalence of congenital anomalies in monochorionic twins is nearly twice that of dichorionic twins . Four congenital malformations associated with twinning are perimembranous ventricular septal defect (VSD), atrial septal defect (ASD) secundum type, hypoplastic left heart syndrome (HLHS), and omphalocele . MCDA twins have a 7% chance of having one twin with a congenital heart defect (CHD) . If one twin is affected, the risk of both twins having a CHD in a MCDA pregnancy is 26% . The risk of CHD in MCMA twins is 57% . For monochorionic pregnancies, there is increased concordance for the flow lesions including HLHS, congenital stenosis of aortic valve, coarctation of the aorta, and ostium secundum type ASD .
Selective Fetal Growth Restriction
Selective fetal growth restriction (sFGR) occurs in 10-15% of monochorionic pregnancies. Selective fetal growth restriction is defined as a condition when one fetus has an estimated fetal weight (EFW) or abdominal circumference (AC) < 10th percentile and an intertwin discordance > 25th percentile . Selective FGR is most commonly due to unequal sharing of the placental mass and vasculature in monochorionic twin pregnancies . Other causes can include fetal anomalies and velamentous cord insertion. The definition of sFGR varies amongst clinicians and recommending societies. An intertwin discordance of 20% distinguishes pregnancies with an increased risk of adverse outcomes . The American College of Obstetricians and Gynecologists considers discordant fetal growth to be a difference of 15 to 25% . Other studies suggest an EFW discordance of 18% to be an accurate predictor for adverse outcomes .
Screening and diagnosis for sFGR are established by calculation of EFW by measurement of the fetal head, abdomen, and femur length. EFW discordance is calculated by the following formula: (weight of larger twin – weight of smaller twin) × 100)/weight of larger twin . However, it is important to consider that assessing EFW using ultrasound is less accurate in twin pregnancies compared to singleton pregnancies . Once diagnosis is established, these twins should receive increased fetal surveillance, fetal Doppler studies, and delivery planning due to the significantly increased risk of perinatal loss [34,35].
Selective FGR in monochorionic twin pregnancies are further classified based on umbilical artery Doppler waveforms. In Type I, the umbilical artery Doppler waveform for the growth restricted fetus has continuous positive end-diastolic flow. For Type II, there is a persistently absent or reversed end-diastolic flow (AREDF). Type III is categorized by a cyclic or intermittent pattern of AREDF . Type I has a high survival rate greater than 90%. Type II and type III have an increased risk of intrauterine demise (IUD) of the growth restricted twin. Type III is associated with a 10-20% risk of unpredictable sudden death of the growth restricted fetus. If IUD occurs due to sFGR, the surviving co-twin has an elevated risk of neurological damage [36,37]. Additionally, twins complicated by sFGR have an incidence of severe cerebral injury up to 10% . Management of sFGR can include conservative management with early delivery, laser ablation, or cord occlusion of growth restricted fetus [4,39].
Twin-to-twin Transfusion Syndrome
Twin-to-twin transfusion syndrome (TTTS) occurs in approximately 10-15% of MCDA pregnancies. It results from arteriovenous anastomoses in the monochorionic placenta. This leads to an imbalance in the fetal-placental circulation leading to one twin transfusing the other [40-43]. Diagnosis is made with ultrasonography indicating amniotic fluid imbalance. The donor twin will have oligohydramnios with a deepest vertical pocket (DVP) < 2cm and the recipient twin will have polyhydramnios with a DVP > 8cm. In the United States, a DVP > 8 cm is used as the polyhydramnios cut off throughout gestation. In Europe, polyhydramnios is defined as a DVP > 8 cm prior to 20 weeks gestation and DVP > 10 cm after 20 weeks gestation . TTTS is staged with the Quintero Staging System which also considers ultrasound evaluation of the bladder, Dopplers of the umbilical artery, and evaluating for the presence of hydrops. Other classification systems that consider cardiac parameters of the donor and recipient twin have been proposed . These include the CHOP-score, cardiovascular profile score, and Cincinnati score that consider different cardiac parameters including cardiothoracic ratio, ventricular wall thickness, atrioventricular regurgitation, myocardial performance index, and others [45-48]. Early diagnosis can allow for intervention with fetoscopic laser coagulation, amnioreduction, selective reduction by radiofrequency ablation or voluntary pregnancy termination [49,50]. If TTTS progresses and is left untreated, it can lead to fetal demise in up to 90% of cases [51,52].
Twin Anemia Polycythemia Sequence
Twin anemia polycythemia sequence (TAPS) occurs in up to 5% of pregnancies and may be present in up to 13% of cases of TTTS following laser ablation . It results from arteriovenous anastomoses allowing for slow transfusion of blood from donor to recipient twin. The antenatal diagnosis and staging are based on discordant middle cerebral arterial (MCA) Doppler peak systolic velocities, indicating anemia in the donor twin and polycythemia in the recipient twin . The outcomes differ based on syndrome severity and gestational age of presentation but can lead to intrauterine demise and neurodevelopmental delay . Management also varies based on severity but can include laser ablation or intrauterine blood or exchange transfusion . A trial is currently underway to evaluate if fetoscopic laser therapy will improve neonatal outcome by prolonging the pregnancy .
Twin Reversed Arterial Perfusion Sequence
Twin reversed arterial perfusion sequence (TRAP) which may also be known as acardiac twin pregnancy occurs in 1% of monochorionic twins . It results from perfusion of an acardiac twin by the normal twin who serves as the pump through arterioarterial anastomosis [58,59]. The pump twin can develop high-output cardiac failure . Management varies from cord coagulation, cord ligation, photocoagulation of the anastomoses, and intrafetal laser therapy .
Conjoined twins occur in 1% of monochorionic twin pregnancies . Ultrasound is used to establish the diagnosis. Many classifications exist including cephalopagus, pygopagus, cephalothoracopagus, and thoracopagus which is the most common . The diagnosis is associated with significant morbidity and mortality.
Due to the increased risk of complications with monochorionic twin pregnancies, there is an increased need for surveillance in order to detect potential abnormalities and guide management. Additionally, the early determination of chorionicity is important for proper patient counseling and guidance. Clinical practice guidelines suggest a first trimester scan followed by a scan every 2 weeks after 16 weeks for uncomplicated monochorionic twins .
During the 11–14-week period patients should receive an initial ultrasound to establish dating, labeling of twins, and chorionicity to aid in proper surveillance, counseling, and pregnancy planning. First trimester ultrasound should also look for any major anomalies and measure nuchal translucency . This is also the ideal time to consider screening for fetal chromosomal abnormalities including Trisomy 21 . Multiple options are available for screening including the combined test and non-invasive prenatal testing . Importantly, screening tests for chromosomal abnormalities are not as accurate for twin gestations as they are for singleton gestations . There is a higher risk for inaccurate test results if a fetal demise, vanishing twin, or anomaly is identified in one fetus .
In the 16–18-week period, fetal growth, deepest vertical pocket (DVP), and umbilical artery Doppler pulsatility index (UA-PI) should be recorded to assist in the early detection of TTTS and fetal growth restriction . Screening should continue every 2 weeks thereafter if normal parameters are present . If there is evidence of abnormal DVP, then weekly UA and middle cerebral artery (MCA) Dopplers are warranted . Additionally, documentation of fetal bladder on ultrasound may be warranted to determine Quintero Staging of TTTS.
Between 18-20 weeks, a detailed anatomy scan should be performed to screen for any evident anomalies including but not limited to neural tube defects, cardiac defects, gastrointestinal abnormalities, craniofacial defects, and brain abnormalities [63, 34]. Due to the increased risk of congenital heart disease, all monochorionic twins should receive a fetal echocardiogram by a skilled sonographer at 18-22 weeks gestation . At the time of the anomaly scan during the second trimester, cervical length should also be measured to screen for preterm birth, with the cutoff being 25mm .
Serial growth ultrasounds with fetal biometry should continue to be performed every 2-4 weeks to screen for fetal growth restriction (FGR) . Due to the concern of discordant growth and selective fetal growth restriction, discordance should be calculated and documented starting at 20 weeks . Additionally, measurement of DVP and UA-PI should continue every 2 weeks to screen for TTTS and FGR. Starting at 20 weeks gestation, middle cerebral artery peak systolic volume (MCA-PSV) should be measured to screen for TAPS and every 2 weeks thereafter .
If sFGR is identified during screening, fetal growth should then be assessed at least every two weeks with biometry. The frequency of fetal Doppler with UA-PI and MCA-PSV should be increased to at least once a week. If UA Dopplers are abnormal, this may warrant measurement of the ductus venosus blood flow to further assess the severity. Management and delivery timing of twins with sFGR should be determined based on assessment of fetal well-being, interval growth, biophysical profile, gestational age and other factors .
If sFGR leads to intrauterine demise, there is increased risk of demise, preterm delivery, and neurodevelopmental impairment in the surviving co-twin [67-69]. The surviving co-twin should undergo assessment of fetal Doppler and fetal biometry every 2-4 weeks . Additionally, the fetal brain should be imaged 4-6 weeks after the demise of a co-twin to look for evidence of cerebral damage. This can be conducted with neurosonography or fetal MRI with findings that have been reported including infarctions, multicystic encephalopathy, cerebral atrophy, intraventricular hemorrhage, cortical necrosis, subdural hematoma, and others [70,71].
From 22-36 weeks, fetal growth should continue to be monitored every 2-4 weeks . DVP, UA-PI, and MCA-PSV should be performed every 2 weeks to continue monitoring for development of FGR, TTTS, and TAPS .
For MCDA twins, antenatal testing (fetal cardiotocography or biophysical profile) should begin at 32 weeks [72-74]. For MCMA twins, antenatal testing should be individualized [75-77]. Many clinicians recommend early inpatient management with daily fetal surveillance starting between 24-28 weeks. Frequency of surveillance while inpatient is individualized, and optimal management of these patients remains uncertain [75-77]. The timing of delivery for uncomplicated MCDA twins is 36 weeks and mode of delivery depends on twin presentation . MCMA twins should deliver between 32-34 weeks by cesarean section to avoid cord complications [75-77].
In conclusion, multifetal gestations carry increased risk for fetal and maternal morbidity. Monochorionic twins have a higher risk of developing several complications and have their own unique complications including TTTS, TAPS, TRAP sequence, sFGR and conjoined twins. Hence, early determination of chorionicity during pregnancy is vital to identify individuals at high risk and provide appropriate counseling and referrals to Maternal Fetal Medicine. This allows for timely intervention and management of potential complications associated with specific chorionicity types, ensuring the best possible outcomes for these patients. Surveillance includes measurement of fetal growth, DVP, UA-PI, MCA-PSV, detailed anatomy scans, cervical length measurement, and fetal echocardiogram. Standardized monitoring intervals have been established but may vary based on case presentation and concern for specific complication. Early detection and intervention for TTTS and TAPS results in decreased morbidity and mortality. Therefore, it is extremely important that patients receive frequent surveillance throughout their pregnancy.
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The article should be cited as: Christmas A, Huebner K, Cordoba M, Romero VC: Surveillance of Monochorionic Twins, Visual Encyclopedia of Ultrasound in Obstetric and Gynecology, www.isuog.org, September 2023.
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