Continuing Education Activity

Twin-to-twin transfusion syndrome (TTTS) is a serious complication unique to monochorionic twin pregnancies, caused by unbalanced placental blood flow between fetuses who share a single placenta. There are numerous possible outcomes of this condition, ranging from stabilization to fetal demise. It is important to screen for the development of this condition to ensure appropriate management and treatment can be undertaken. This activity reviews the embryology of twinning, classification of twin gestations, and the pathophysiology underlying TTTS.

Learners will examine how the timing of zygotic splitting determines chorionicity and amnionicity, including dichorionic-diamniotic (DCDA), monochorionic-diamniotic (MCDA), and monochorionic-monoamniotic (MCMA) pregnancies. Given that more than 95% of MCDA twins share intertwin placental circulation, participants will gain insight into why these gestations are at high risk for TTTS. The course emphasizes the diagnostic criteria for TTTS and specialized diagnostic considerations for monoamniotic twins. The value of interprofessional collaboration in optimizing outcomes for TTTS is highlighted. Coordinated roles among maternal–fetal medicine specialists, obstetricians, sonographers, neonatologists, surgeons, nurses, and allied health professionals to support early diagnosis, fetal intervention, perinatal management, and family counseling are explored.

Objectives:

• Identify the underlying etiology and pathophysiologic mechanisms of twin-to-twin transfusion syndrome.
• Apply standardized evidence-based criteria to accurately diagnose and stage twin-to-twin transfusion syndrome across clinical settings.
• Improve clinical decision-making by integrating consensus recommendations and emerging evidence into the management of twin-to-twin transfusion syndrome.
• Collaborate with an interprofessional healthcare team to optimize coordinated management and patient counseling for twin-to-twin transfusion syndrome.

Introduction

Multiple gestations can be either dizygotic (derived from 2 separate ova) or monozygotic (derived from a single ovum). Twin gestations can be classified further as dichorionic-diamniotic (DCDA, containing 2 placentas and 2 amniotic sacs), monochorionic-diamniotic (MCDA, containing 1 placenta and 2 amniotic sacs), or monochorionic-monoamniotic (MCMA, containing 1 placenta and 1 amniotic sac).[1] Monozygotic twins account for approximately 30% of spontaneously occurring twins. In monozygotic twins, two-thirds are monochorionic and share a single placenta. It is important to understand that over 95% of MCDA twins will share intertwin placental circulation, which can lead to the development of twin-twin transfusion syndrome (TTTS).[2] This disorder underscores the importance of determining chorionicity (number of placentas) and amnionicity (number of amniotic sacs) in all twin gestations. 

In the most accepted model of monozygotic twinning, the number of placentas and amniotic sacs depends upon when the splitting of the zygote occurs. DCDA twins result when splitting occurs between days 1 and 3 post-fertilization. MCDA twins result when splitting occurs between days 3 and 8 post-fertilization. MCMA twins result when splitting occurs between days 8 and 13 post-fertilization. Splitting that occurs after day 13 post-fertilization results in conjoined twins.[3] 

Ultrasound findings that help distinguish dichorionic and monochorionic twin gestations include a "lambda sign" in dichorionic and a "T sign" in monochorionic twin gestations.[4] The "T sign" is created by the thin dividing membrane between 2 amniotic sacs when there is only 1 placenta supporting both gestations (monochorionic). As there is only 1 chorion in a monochorionic gestation, the dividing membrane between the twins is only composed of 2 layers (each twin's amnion), and is therefore thin. In contrast, the "lambda" or "twin peak" sign is seen in dichorionic twins when each gestation is supported by its own placenta. In dichorionic gestations, the dividing membrane is made up of 4 layers (each twin's chorion and each twin's amnion) and is therefore thick.[4] The best time to determine chorionicity is in the first trimester. 

The initial diagnosis of TTTS can be made in a monochorionic pregnancy when 1 gestational sac has a maximum vertical pocket (MVP) of amniotic fluid of <2 cm (oligohydramnios) while the other gestational sac has an MVP of >8 cm (polyhydramnios). Growth discordance and fetal growth restriction can occur, but are not required to diagnose TTTS. In monoamniotic twin gestations, TTTS cannot be based on oligohydramnios and polyhydramnios; rather, it is based on polyhydramnios and nonvisualization of the bladder in 1 fetus over a 60-minute observation period.[5]

Etiology

The principal etiology of TTTS is an increased number of arteriovenous anastomoses deep in the placenta. These are vascular connections that occur in the cotyledon portion of the placenta. Unidirectional flow can occur in these arteriovenous anastomoses, resulting in shunting of blood toward 1 twin (the recipient) and away from the other twin (the donor). Arterio-arterial anastomoses and venovenous anastomoses can also occur in the placenta and often have bidirectional flow. These vascular connections are found more superficially on the placental surface. Arterio-arterial anastomoses are considered protective against TTTS and are less common in twin gestations with TTTS. The placenta of MCMA twins typically contains a greater number of protective arterio-arterial anastomoses, which is a theoretical explanation for why TTTS rates are lower in MCMA twins than in MCDA twins.[5]

Epidemiology

It is estimated that twin births account for about 2% to 4% of births worldwide.[6] Based upon data from the National Vital Statistics System of the National Center for Health Statistics of the Centers for Disease Control and Prevention, the prevalence of twin births in the United States in 2023 was about 30.7 per 1000 live births, or about 3% of live births.[11].The twin birth rate increased dramatically from 1.8% in the U.S. in 1980, likely due to the rise in assisted reproductive technologies.[6] However, the twin birth rate declined slightly from 2014 to 2018. Since 2018, the rate has remained relatively stable.[11]

Of twin gestations, an estimated 67% are dizygotic, and 33% are monozygotic. Among monozygotic twins, approximately 66% are MCDA. TTTS occurs at a rate of about 8% to 12% of MCDA twin gestations and about 6% of MCMA twin gestations. It is estimated that 1 to 3 per 10,000 births are affected by TTTS.[1][5]

There is little data regarding the prevalence of each stage of TTTS (see the Staging section below). Based upon data from referral centers, the Society for Maternal Fetal Medicine (SMFM) estimates a prevalence of Stage I: 11% to 15%, Stage II: 20% to 40%, Stage III: 38% to 60%, Stage IV: 6% to 7%, and Stage V: 2%.[1]

Pathophysiology

The hypovolemia experienced by the donor twin causes renal hypoperfusion, which then stimulates the renin-angiotensin-aldosterone system of the donor twin. This effect leads to oliguria and, in turn, oligohydramnios. The hypervolemia experienced by the recipient twin causes cardiac stretch, which increases atrial natriuretic peptide and brain natriuretic peptide release in the recipient twin. These effects inhibit the renin-angiotensin-aldosterone system, leading to polyuria and, in turn, polyhydramnios.[7][8][9] Due to the persistent hypervolemic state in the recipient twin, atrioventricular valve insufficiency, diastolic dysfunction, and pulmonary stenosis or atresia can be seen. In contrast, vascular changes, including increased collagen synthesis and hypertrophy of the vascular media and smooth muscle layers, are evident in the donor twin.[10]

Histopathology

Placental dye studies can be performed to analyze the different vascular connections following delivery of the twins and the placenta. During dye studies, a different colored dye is injected into the arteries and veins of each twin's umbilical cord. The dye's path can then be analyzed to determine which types of vascular anastomoses are present. 

History and Physical

The physical findings of TTTS depend upon the stage of the disease. At the very least, in MCDA twins, ultrasound findings must include oligohydramnios in 1 amniotic sac and polyhydramnios in the other. In cases of significant oligohydramnios, the amnion surrounding the donor twin can appear “stuck” to the twin on ultrasound.[4]

Pregnant patients may experience no symptoms or outward signs that TTTS is occurring. In a survey of women who had a pregnancy complicated by TTTS, symptoms were experienced by almost half of the women before their diagnosis, but were nonspecific and may be confused with pregnancy symptoms such as rapid weight gain, swelling, and pain.[11]

Evaluation

As mentioned previously, early diagnosis of chorionicity of a twin gestation is of the utmost importance, as this will guide the recommended surveillance of the pregnancy. An ultrasound at 10 to 13 weeks to evaluate chorionicity, crown-rump length, and nuchal translucency is recommended for women with twin gestations. Future development of TTTS has been associated with discrepancies in crown-rump length and nuchal translucency between twins. Additional ultrasound findings that may be associated with TTTS include velamentous umbilical cord insertion and intertwin membrane folding, which can be seen in the second trimester. Once a diagnosis of TTTS is established, Doppler studies of the umbilical artery, umbilical vein, and ductus venosus in each twin can help determine the stage.

Once a MCDA twin gestation is established, ultrasound surveillance should begin at 16 weeks. At a minimum, ultrasound surveillance for TTTS should include assessment of amniotic fluid volumes on both sides of the interwin membrane and evaluation for the presence or absence of urine-filled bladders in each twin. Ideally, TTTS surveillance should also include a Doppler study of the umbilical arteries of each twin. More recent guidelines also include a middle cerebral artery (MCA) Doppler study of each twin starting at 16 weeks of gestation. It is recommended that ultrasound surveillance continue at least every 2 weeks until delivery. More frequent monitoring should be completed as indicated by the clinical status of the mother and/or fetuses.[2]

MCA Doppler can be useful for diagnosing twin anemia-polycythemia sequence (TAPS), which results from the transformation of submillimeter intertwin arteriovenous anastomoses, commonly located near the placental edge. The blood flow through these connections is estimated at approximately 5 mL to 15 mL per day, allowing some fetal compensation during the early stages of the disease. Changes in amniotic fluid are not usually seen in TAPS. A MCDA pregnancy that has been affected by TAPS will result in 1 twin that is anemic (donor) and 1 twin that is polycythemic (recipient). TAPS may occur spontaneously or as a result of procedural intervention (laser photocoagulation) for TTTS. Spontaneously occurring TAPS may occur in approximately 2% to 5% of MCDA twin pregnancies during the second and third trimester.[2]

In addition to a Level II anatomy ultrasound for each twin, it is also recommended that fetal echocardiography be performed in MCDA twin gestations due to the increased risk of congenital heart disease in this population. Ultrasound for fetal weight assessment is also recommended at least every 4 weeks throughout the pregnancy.

Treatment / Management

Management of TTTS is dependent on the stage and gestational age at the time of diagnosis. Options for management include expectant management, amnioreduction, intentional septostomy (not commonly performed), fetoscopic laser photocoagulation, selective reduction, and voluntary pregnancy termination. 

As outlined in SMFM Consult Series No. 72, recommended management of TTTS includes the following:

From 16 to 26 Weeks of Gestation

Stage I TTTS, asymptomatic - expectant management with at least weekly fetal surveillance. Expectant management is recommended over amnioreduction and laser photocoagulation at this stage, as outcomes are similar. Only about 25% of Stage I TTTS progresses, and with expectant management, the survival of at least 1 twin occurs in most pregnancies.[12]

Stage I TTTS, complicated by additional factors such as maternal polyhydramnios-associated symptomatology) - consider fetoscopic laser surgery due to maternal complications such as shortness of breath, contractions, etc.

Stage II to Stage IV TTTS - recommend fetoscopic laser surgery as the standard treatment. A multicenter randomized controlled trial conducted by Senat et al demonstrated better outcomes after fetoscopic laser coagulation than serial amnioreductions, including increased survival rates of 1 or both twins, delivery at greater gestational ages, and superior neurological outcomes.[5] It should be noted that this study did not include Stage I TTTS and should not be applied to the management of Stage I TTTS.

Stage V TTTS - No interventions have been evaluated at this stage. Immediate delivery for the surviving twin does not improve outcomes, and preterm delivery should be avoided unless other indications for early delivery exist.[2]

It is recommended that all patients who qualify for laser therapy be referred to a fetal intervention center for evaluation, consultation, and management. Fetoscopic laser photocoagulation is performed under ultrasound guidance, typically between 16 and 26 weeks of gestation. The procedure can be completed outside of this timeframe, but only with expert consultation from a fetal intervention center. When performed less than 16 weeks of gestation, there is a greater risk of premature rupture of membranes. When performed at more than 26 weeks of gestation, there may be greater difficulty in adequately coagulating blood vessels due to their larger size.

It has been recommended that arteriovenous, arterio-arterial, and venovenous anastomoses be selectively coagulated rather than nonselectively coagulated. However, there is concern that some anastomoses may be missed, increasing the risk of recurrence of TTTS and TAPS. Fetoscopic laser photocoagulation follows a general protocol that includes a variation of the Solomon technique, which involves coagulating a line from 1 end of the placenta to the other after identifying and coagulating the anastomoses. This technique results in fewer TTTS recurrences, decreased TAPS development, and increased perinatal survival, but it also carries a greater risk of placental abruption. Based upon the available data, the authors of the 2019 Update on TTTS in Best Practice and Research Clinical Obstetrics and Gynaecology recommend a partial Solomon technique in which anastomoses are coagulated as well as a small area along the division of the placenta to optimize sufficient anastomoses coagulation and salvaging of a healthy placenta.[5]

Following fetoscopic laser photocoagulation, it is recommended that weekly fetal surveillance be continued for approximately 6 weeks. Following the initial 6 weeks, the frequency of fetal monitoring can be extended to every 2 weeks unless there is concern for the development of post-laser TTTS, post-laser TAPS, or selective fetal growth restriction.

Fetoscopic laser surgery should not influence the mode of delivery. Following resolution of TTTS after laser surgery, if no other indications exist for earlier delivery, it is recommended that for MCDA twins in which both twins survive laser surgery, delivery should be completed between 34 and 36 weeks of gestation. In TTTS pregnancies in which there has been demise of 1 twin following laser surgery, delivery at 39 weeks is recommended for the surviving twin unless there are other indications for earlier delivery. 

Differential Diagnosis

The minimal diagnostic criteria for TTTS are oligohydramnios in 1 twin and polyhydramnios in the other. However, when oligohydramnios is noted in a MCDA twin gestation, other causes of oligohydramnios must be entertained before diagnosing TTTS. The differential diagnosis of oligohydramnios would include rupture of membranes. The patient should be asked about leakage of fluid from the vagina, and a physical examination should be completed to rule out rupture of membranes.

Staging

The staging system most utilized for TTTS is the Quintero Staging System, which is based upon 2-dimensional ultrasound and Doppler study findings and is as follows:

• Stage I: Oligohydramnios (MVP <2 cm) and polyhydramnios (MVP >8 cm) sequence, donor twin bladder is visible, Doppler studies of umbilical artery, umbilical vein, and ductus venosus are normal in both twins
• Stage II: Oligohydramnios and polyhydramnios sequence, donor twin bladder is not visible over 60 minutes of observation, Doppler studies of umbilical artery, umbilical vein, and ductus venosus are normal in both twins
• Stage III: Oligohydramnios and polyhydramnios sequence, abnormal Doppler study (only 1 of the following is required in either twin)

  • Absent or reversed end-diastolic flow in the uterine artery, pulsatile flow in the utrine vein, or reversed a-wave flow in the ductus venosus
• Stage IV: Oligohydramnios and polyhydramnios sequence, one or both fetuses have ascites or hydrops
• Stage V: Oligohydramnios and polyhydramnios sequence, fetal demise of 1 or both twins [2]

Prognosis

Prognosis varies depending on stage, disease severity, and gestational age at diagnosis. Factors such as younger gestational age and higher stage at diagnosis are associated with a poorer prognosis. The prognosis is best for Stage I, with an overall survival of 86%. Additionally, about 75% of Stage I remain stable or spontaneously regress with expectant management.[12] There is less information available for Stages II through IV, but the perinatal death rate for Stage III or greater is estimated to be 70% to 100%. Regarding Stage V, following the demise of 1 twin, there is a 10% risk of death and a 10% to 30% risk of neurological complication in the surviving twin. Some research shows an improved neurological outcome in the surviving twin if fetoscopic laser photocoagulation was performed earlier in gestation, as this may provide some protection to the surviving twin.

Complications

As mentioned above, the death of 1 or both twins is a complication of TTTS, with the survival of 1 twin ranging from 15% to 70% and the survival of both twins around 50%. Cardiac complications can also occur in both the recipient and donor twin and include atrioventricular valve insufficiency, diastolic dysfunction, pulmonary stenosis or atresia in the recipient, as well as vascular changes due to increased collagen synthesis and hypertrophy of the vascular media and smooth muscle layers in the donor twin.[10] In general, twin gestations have a higher risk of premature delivery, and TTTS similarly carries an elevated risk of premature delivery. Neurological deficits are complications of TTTS and preterm delivery, with an increased risk of cerebral palsy and long-term neurodevelopmental impairment.[13][14]

Complications also differ depending on management for TTTS. Expectant management carries the risk of progression to a more advanced stage. Approximately 75% of Stage I TTTS will remain stable or spontaneously regress without treatment.[12] Potential complications of amnioreduction include the death of 1 or both twins, with survival rates following this procedure ranging from 50% to 65%. There is also the risk that serial amnioreductions will be necessary, thereby increasing the overall risk of the procedure. Amnioreduction can also be complicated by preterm, premature rupture of membranes (PPROM), preterm labor, placental abruption, infection, and decreased success of potential future fetoscopic laser photocoagulation. Additionally, there is an increased risk of poor neurological outcomes, including cerebral injury, cerebral palsy, and neurodevelopmental impairment after amnioreduction compared to fetoscopic laser photocoagulation.[13] While still the recommended treatment for Stages II to IV, fetoscopic laser photocoagulation has numerous possible complications, which consist of PPROM, preterm delivery, extravasation of amniotic fluid outside the uterus, placental abruption, vaginal bleeding, infection, fetal death, recurrent TTTS, and TAPS.[5] Although associated with a lower risk than amnioreduction, cerebral injury, cerebral palsy, and neurodevelopmental impairment are also potential complications of fetoscopic laser photocoagulation.[13]

Deterrence and Patient Education

Educating patients about what symptoms may be signs of TTTS, including contractions and a sudden increase in abdominal size, and advising them to report them to their clinician quickly, may help with earlier diagnosis of TTTS.[15] However, the mainstay of diagnosis is early surveillance, which can facilitate early detection. In addition, early determination of chorionicity is of utmost importance so that appropriate surveillance can be initiated in the setting of monochorionic twins.

Once TTTS is diagnosed, patient education and counseling regarding the prognosis should be provided, based on the stage, gestational age, management, and available treatment options, along with their respective risks and benefits, and the expected progression of the condition. Additionally, there should also be a discussion about the possibility of long-term complications after birth, including neurological complications.[13]

Enhancing Healthcare Team Outcomes

TTTS is a complication of monochorionic twin pregnancies caused by unbalanced blood flow through shared placental vascular connections. The condition is diagnosed based on ultrasound findings, including amniotic fluid discordance and other standardized sonographic and Doppler criteria. Early recognition, timely intervention, and coordinated interprofessional management are essential to improving maternal, fetal, and neurodevelopmental outcomes.

Teamwork skills, strategy, interprofessional communication, and care coordination are imperative in the antepartum and intrapartum management of multifetal gestations. Team functioning is strengthened by effective communication, structured management, and leadership, which are consistently associated with improved quality of care. Communication consists of inquiry and information sharing; management consists of vigilance and workload management; and leadership consists of assertion, information sharing, and plan evaluation.[16] Preparation and training for difficult deliveries and management of newborns can be undertaken with team simulation scenarios.[17] Prompt referral to fetal intervention centers is critical in the management of advanced-stage TTTS and can improve pregnancy outcomes.

Given that most MCDA twins will deliver preterm, plans should be discussed and implemented early to ensure optimal outcomes.[18] An interprofessional team at an urban academic medical center retrospectively examined the deliveries of late preterm infants and found that transfers to the neonatal intensive care unit (NICU) commonly result from exhaustion of resources and the development of complications in the preterm neonate. These transfers lead to the separation of the mother from the newborn, which is not an optimal situation. Pre-delivery planning for the delivery mode and location can help prevent unnecessary separations. Communication with parents regarding their goals of care is imperative, especially for preterm infants.[17]

After a stay in the NICU, it is important to ensure interprofessional follow-up, parent education, and early therapeutic interventions to improve outcomes.[19] Facilitating connections to different levels of care and linking parents with local community resources and potential funding sources is also important. Short and long-term counseling should be provided for families.[18][19]

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van Klink JM, Koopman HM, Rijken M, Middeldorp JM, Oepkes D, Lopriore E. Long-Term Neurodevelopmental Outcome in Survivors of Twin-to-Twin Transfusion Syndrome. Twin Res Hum Genet. 2016 Jun;19(3):255-61. [PubMed: 27137794]

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Sueters M, Middeldorp JM, Lopriore E, Oepkes D, Kanhai HH, Vandenbussche FP. Timely diagnosis of twin-to-twin transfusion syndrome in monochorionic twin pregnancies by biweekly sonography combined with patient instruction to report onset of symptoms. Ultrasound Obstet Gynecol. 2006 Oct;28(5):659-64. [PubMed: 16969785]

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Disclosure: Maya Merriweather declares no relevant financial relationships with ineligible companies.

Disclosure: Jaimie Maines declares no relevant financial relationships with ineligible companies.

Disclosure: Anthony Shanks declares no relevant financial relationships with ineligible companies.