Diagnosis and Staging of Twin to Twin Transfusion Syndrome

Placental vascular basis

Most monochorionic placentas contain vascular anastomoses. TTTS develops when net transfusion across deep arteriovenous connections is not adequately balanced by other vascular connections. This produces chronic intertwin volume imbalance rather than a simple difference in fetal size.

Donor physiology

The donor twin loses net circulating volume, develops reduced renal perfusion and urine output, and may show a small or absent bladder, oligohydramnios, fetal growth restriction, abnormal umbilical artery Doppler, or demise.

Recipient physiology

The recipient twin receives excess volume, produces excess urine, and develops polyhydramnios. Cardiovascular strain may lead to ventricular hypertrophy, atrioventricular valve regurgitation, ductus venosus abnormality, hydrops, or demise.

Frequency

TTTS is usually described as affecting approximately 10% to 15% of monochorionic diamniotic twin pregnancies. It is much less common in monochorionic monoamniotic twins, but it can occur.

Core ultrasound criteria

• Monochorionic pregnancy.
• Recipient DVP/MVP ≥ 8 cm.
• Donor DVP/MVP ≤ 2 cm.
• Some older or local protocols use a recipient threshold of ≥ 10 cm after 20 weeks; state the threshold used.

Minimum documentation

• Chorionicity and amnionicity.
• Fluid pocket for each twin.
• Donor bladder visible or not visible.
• Umbilical artery, ductus venosus, and umbilical venous Dopplers when feasible.
• Recipient cardiac findings and hydrops.
• Estimated fetal weight and growth discordance.
• MCA-PSV when evaluating for TAPS overlap.

Without effective treatment

Advanced midtrimester TTTS has a high risk of fetal loss and severe neonatal morbidity. Risk is highest with hydrops, fetal demise, severe Doppler abnormalities, very early disease, and marked cervical shortening.

After laser therapy

For stage II-IV TTTS in the usual midtrimester treatment window, fetoscopic laser therapy is the preferred causal treatment when technically feasible. Contemporary counseling often quotes survival of at least one twin in approximately 80% to 90% of treated pregnancies, with dual survival commonly lower and center-specific.

Neurodevelopment

Neurologic risk is influenced by prematurity, fetal demise, hemodynamic instability, stage, and post-procedure complications. Laser therapy reduces morbidity compared with older amnioreduction-only strategies but does not eliminate risk.

Screening and referral

The review supports ultrasound surveillance for TTTS every 2 weeks beginning at 16 weeks in monochorionic gestations, with more frequent assessment if isolated polyhydramnios, oligohydramnios, discordant fluid, growth discordance, abnormal Dopplers, or maternal symptoms suggest evolving disease. Referral to a center experienced in fetal therapy is appropriate when fluid or growth discordance is detected or when the diagnosis is uncertain.

Dynamic disease

Most TTTS presents before 26 weeks, with a peak around 20 weeks. Because the disease may evolve quickly, a single reassuring examination does not exclude subsequent progression. Trends in fluid, bladder filling, Dopplers, cardiac findings, growth, and cervical length are often more informative than one isolated measurement.

Stage I dilemma

Stable, asymptomatic stage I TTTS is often managed expectantly with close surveillance, but fetal therapy consultation is reasonable when symptoms, short cervix, cardiac dysfunction, or rapid progression are present. Available randomized data have not shown a clear intact-survival advantage for immediate laser in all stage I cases, but progression can occur and surveillance must be active rather than passive.

Laser and Solomon technique

Fetoscopic laser treats the underlying placental vascular problem by coagulating intertwin anastomoses. The Solomon technique coagulates the placental vascular equator after visible anastomoses are treated, reducing residual connections and lowering recurrent TTTS and post-laser TAPS in published trials.

Periprocedural risk

Important counseling modifiers include gestational age at diagnosis, Quintero stage, progression, preprocedural cervical length, coexisting selective fetal growth restriction, Doppler or cardiac abnormalities, membrane complications, and fetal therapy center experience. Preterm prelabor rupture of membranes remains one of the major risks after fetoscopic laser.

Neurologic and cardiac outcomes

Survival is not the only outcome. Recipient cardiovascular dysfunction, donor placental insufficiency, fetal demise, prematurity, and postprocedure complications all influence neurologic outcome. CNS injury may be present before intervention in some cases, so counseling should include both fetal and neonatal follow-up.

When to measure

• Measure transvaginal cervical length when TTTS is diagnosed, when symptoms are present, and before fetoscopic laser evaluation when feasible.
• Document cervical dilation, funneling, exposed membranes, contractions, bleeding, or rupture of membranes separately from the cervical length number.
• Cervical length should not be used to change the Quintero stage; it is a preterm-birth and procedural-risk modifier.

What the data suggest

• In laser-treated TTTS, preoperative cervical length below about 25 to 30 mm is associated with higher risk of early preterm delivery.
• One study identified a preoperative cervical length cutoff of 28 mm for spontaneous preterm delivery before 34 weeks after laser surgery.
• In a NAFTNet multicenter cohort of TTTS pregnancies with cervical length <30 mm undergoing fetoscopic laser photocoagulation, 62% delivered before 32 weeks.

Implications

• Short cervix should increase attention to referral timing, perioperative counseling, membrane risk, neonatal counseling, and post-procedure surveillance.
• A very short cervix, progressive shortening, or cervical dilation may affect whether laser, amnioreduction, cerclage, expectant management, or delivery is technically feasible or clinically appropriate.
• Management should be individualized with the fetal therapy center and neonatal team rather than handled as a routine short-cervix pathway.

Intervention caveat

• Evidence is limited and heterogeneous for progesterone, pessary, or cerclage specifically in TTTS with short cervix.
• The NAFTNet cohort did not show a clear improvement in delivery interval, gestational age at delivery, live birth, or neonatal survival with expectant management, vaginal progesterone, pessary, cerclage, or combination therapy.
• SMFM short-cervix guidance recommends against routine progesterone, pessary, or cerclage for cervical shortening in twin gestations outside a clinical trial.

Routine monochorionic diamniotic surveillance

• Determine chorionicity and amnionicity in the first trimester when possible.
• Begin TTTS screening at 16 weeks.
• Repeat ultrasound at least every 2 weeks until delivery when stable.
• At minimum, assess amniotic fluid on both sides of the membrane and fetal bladder filling.
• Ideally include umbilical artery Doppler; consider MCA-PSV from 16 weeks when feasible.

Pre-TTTS or abnormal screen

• Repeat MFM ultrasound in one week or less.
• Increase urgency with maternal symptoms, rapidly rising fluid, short cervix, abnormal Dopplers, hydrops, or early severe growth restriction.
• Follow until findings normalize, stabilize, or progress to TTTS, TAPS, sFGR, or another diagnosis.

After laser therapy

• Weekly surveillance for 6 weeks is commonly recommended.
• If stable, resume every-other-week surveillance unless concerns persist.
• Screen for recurrent TTTS, post-laser TAPS, fetal growth restriction, membrane complications, and fetal demise.

TAPS screening

TTTS staging does not use MCA-PSV, but MCA Doppler is important because TAPS may coexist with TTTS or occur after laser. Prenatal TAPS criteria include donor MCA-PSV > 1.5 MoM with recipient MCA-PSV < 1.0 MoM, or intertwin MCA-PSV difference > 0.5 MoM.

Why diagnosis is different

• No intertwin membrane means there is no donor sac DVP and recipient sac DVP to compare.
• A single common sac may show polyhydramnios, but this does not localize fluid to one fetus.
• The donor may still show a small or absent bladder, growth restriction, abnormal Doppler, or anemia.
• The recipient may show a large bladder, cardiomegaly, valve regurgitation, ductus venosus abnormality, hydrops, or polycythemia.

Implications

• Quintero staging should not be used without a clear caveat in monoamniotic twins.
• Refer suspected cases to a fetal therapy center because diagnosis and treatment are individualized.
• Assess for competing monoamniotic complications, including cord entanglement, cord compression, TRAP sequence, structural anomaly, selective fetal growth restriction, and TAPS.
• Laser therapy may be considered in selected cases but can be technically more complex because both fetuses and both cords are in one sac.

Selective fetal growth restriction

sFGR may coexist with TTTS. Donor stage III disease may overlap with abnormal umbilical artery Dopplers from placental insufficiency.

TAPS

TAPS produces anemia-polycythemia without classic oligo-polyhydramnios. MCA-PSV discordance is central to screening and diagnosis.

Atypical recipient disease

Recipient cardiovascular compromise may occur without donor oligohydramnios meeting classic stage I criteria. Describe the recipient cardiac and Doppler findings clearly.

Discordant anomaly

Renal disease, bladder outlet obstruction, cardiac disease, genetic syndromes, or structural anomalies can cause discordant fluid or growth.

Membrane rupture or maternal disease

PPROM, infection, placental insufficiency, diabetes, or other maternal disease can alter fluid and growth patterns and should be considered when findings are atypical.

TRAP sequence

TRAP is a monochorionic complication with a pump twin supporting an acardiac twin. It is not TTTS but may coexist in the differential of monochorionic hemodynamic disease.