Monochorionic twin pregnancy with suspected TTTS — structured oral station (12 minutes)

Opening prompt — "What is your working diagnosis and how do you stage it?"

Model response, in this order: ^[1][4]

• "This is twin-twin transfusion syndrome complicating a monochorionic diamniotic twin pregnancy. The recipient twin has polyhydramnios with a deepest vertical pool of 9.5 cm, the donor has oligohydramnios with a deepest vertical pool of 1.5 cm, and the donor is stuck against the uterine wall — the classical oligo-poly sequence."
• "By the Quintero system, this is stage II, because the donor bladder is not visible over the observation period. Both twins have positive end-diastolic flow in the umbilical artery, so this is not stage III. Neither twin has ascites or hydrops, so this is not stage IV."
• "I would confirm chorionicity from the first-trimester scan and ensure the structural surveys are complete."^[1][4]

Examiner is listening for: the diagnosis stated as TTTS, the Quintero stage with the defining feature named, and the explicit exclusion of stages III and IV. ^[4]

Probe 1 — "What is the immediate management?"

• "Refer urgently to a fetal medicine centre with fetoscopic laser capability, today. This is a stage II TTTS at 19+4 weeks, and the definitive treatment is fetoscopic laser coagulation of the communicating placental vessels."^[1]
• "Explain that this is a life-threatening complication for both twins with perinatal mortality of 70 to 100 per cent if untreated."
• "Maternal support: analgesia, tocolysis if uterine activity is present, monitoring for preterm labour. Symptomatic polyhydramnios may need an amnioreduction for comfort while awaiting definitive therapy, but this is a temporiser, not the treatment."^[1]
• "I would not recommend serial amnioreduction as definitive therapy for stage II to IV TTTS before 26 weeks. The Senat randomised trial (NEJM 2004) established that laser reduced neonatal death and improved survival without neurological sequelae at six months compared with amnioreduction."^[2]

Probe 2 — "What is the Solomon technique and what does it add?"

• "The Solomon technique coagulates the entire vascular equator of the placenta rather than selectively coagulating only the visible anastomoses. It eliminates the residual anastomoses that selective coagulation leaves behind."^[3]
• "The Slaghekke Solomon randomised trial (Lancet 2014) showed the Solomon technique reduced recurrence of TTTS from around 7 per cent to around 1 per cent, and post-laser twin anemia-polycythemia sequence from around 16 per cent to around 3 per cent."^[3]
• "The price is a slightly longer operative time, but the reduction in recurrence and post-laser TAPS is the dominant benefit, and the systematic review confirms the survival benefit."^[3]

Probe 3 — "What is twin anemia-polycythemia sequence and how do you screen for it after laser?"

• "TAPS is a chronic, slow inter-twin transfusion through small arteriovenous anastomoses, producing severe hemoglobin discordance with normal amniotic fluid volumes. It is distinct from TTTS, in which amniotic fluid volumes are discordant."^[6][8]
• "Antenatally, TAPS is defined by a donor middle cerebral artery peak systolic velocity over 1.5 multiples of the median with a recipient under 1.0 multiples of the median, in the absence of the amniotic fluid criteria for TTTS."^[8]
• "Postnatally, the diagnostic criteria include an inter-twin hemoglobin difference of at least 80 g/L with a reticulocyte count ratio over 1.7, and a placental anastomotic pattern on dye injection."^[6]
• "MCA-PSV is a part of the biweekly post-laser surveillance scan, and any donor MCA-PSV over 1.5 multiples of the median prompts assessment for post-laser TAPS."^[1]

Probe 4 — "At what gestation would you plan birth for this pregnancy if it remained otherwise uncomplicated after laser?"

• "I would plan birth between 34+0 and 37+0 weeks, individualised by complications. The RCOG Green-top Guideline 51 (2024 partial update) recommends 36+0 to 37+0 weeks for uncomplicated monochorionic diamniotic pregnancies, and this would apply if the pregnancy remained otherwise uncomplicated after laser."^[1]
• "Continuing to 39 or 40 weeks is not recommended because of the unacceptable stillbirth risk."^[1]
• "Mode of birth follows the Twin Birth Study (Barrett, NEJM 2013): planned vaginal delivery is appropriate for twin A cephalic, with planned caesarean for breech first twin or other obstetric indications."^[1]

Probe 5 — "What is selective fetal growth restriction in monochorionic twins and how is it classified?"

• "Selective fetal growth restriction is unequal placental sharing producing asymmetric growth in one twin, complicating 10 to 15 per cent of monochorionic pregnancies."^[7]
• "The current ISUOG criteria use an estimated fetal weight below the 3rd centile in the smaller twin or an estimated fetal weight discordancy of 25 per cent or more. The Delphi consensus identifies a broader group, and the Sorrenti study compared their perinatal predictive value."^[7]
• "The type is determined by the umbilical artery Doppler pattern in the smaller twin: type I positive end-diastolic flow, type II persistently absent or reversed, type III intermittent absent or reversed."^[7]
• "Type III carries the highest risk of unexpected intrauterine death despite apparently normal growth velocity, because of large arterioarterial anastomoses that produce cyclic hemodynamic fluctuations. Type III is the type that benefits most from laser."^[7]

Probe 6 — "Tell her what is happening, in language she will understand."

This is scored. Demonstrate it out loud rather than describing it. ^[1]

• Sit down, at her eye level, with her partner or support person present if she wishes.
• Plain words: "Your twins share a single placenta, and the blood vessels between them have become unbalanced. One twin — we call it the recipient — is getting too much blood flow and has too much amniotic fluid around it. The other twin — the donor — is not getting enough, and that is why your abdomen has become so swollen and uncomfortable."
• Stop and acknowledge: "This is a serious complication and it can be life-threatening for both twins if we do not treat it."
• The plan: "The treatment that gives the best chance for both twins is a procedure called fetoscopic laser, in which a specialist uses a small camera and laser inside the womb to seal off the connecting blood vessels on the placenta. We need to refer you to a centre that does this procedure today."
• Outcome honesty: "Even with the laser, we expect both twins to survive in around 50 to 70 per cent of cases, and at least one twin in around 80 to 90 per cent. There is a risk of preterm birth and a smaller risk of long-term developmental problems in survivors, and I will be honest with you about those numbers as we go."
• Offer to call her support person, allocate a named midwife, and commit to a follow-up conversation after the fetal medicine referral.^[1]

Probe 7 — "What would change if one of the twins had already died?"

• "Single intrauterine death in a monochorionic pregnancy is a different conversation. The surviving co-twin is at substantial risk of acute hypotension through the residual placental anastomoses — about 12 to 15 per cent risk of co-twin death and 10 to 20 per cent risk of neurological injury in survivors."^[1]
• "Immediate delivery is rarely the right response, because the harm occurs at the moment of death, and delivering the surviving twin does not reverse it."^[1]
• "Surveillance with fetal MRI for cerebral injury and structured neurodevelopmental follow-up is mandatory. I would be honest with the woman about the risk to the surviving twin and the limits of what we can change."^[1]

Probe 8 — "What are the options if the fetal therapy centre is hundreds of kilometres away?"

• "I would still refer, because laser is the gold-standard treatment and the alternative — serial amnioreduction — is inferior in the Senat trial. The woman may need to be transferred to the fetal therapy centre for assessment and treatment."^[2]
• "Where transfer is genuinely impossible or refused, serial amnioreduction can be used as a temporiser, but the woman should be counselled that this is not the recommended treatment and the outcomes are worse."^[1]
• "I would document the conversation, including the recommendation to transfer and the reasons for any decision not to, and ensure ongoing senior obstetric and midwifery support."^[1]