Twin-twin transfusion syndrome at 20 weeks — structured SAQ (15 marks)

(a) Diagnosis, Quintero stage and the defining feature (4 marks)

Two marks for the diagnosis and stage, one for naming the feature that places it at this stage, one for excluding the next stage explicitly. ^[1][4]

• Working diagnosis: twin-twin transfusion syndrome complicating a monochorionic diamniotic twin pregnancy. The combination of recipient polyhydramnios (deepest vertical pool 11 cm, over the 8 cm threshold), donor oligohydramnios (deepest vertical pool 1 cm, under the 2 cm threshold) and a stuck donor is the classical oligo-poly sequence.^[4]
• Quintero stage II. The donor bladder is not visible over the observation period, which is the stage II criterion. The polyhydramnios and oligohydramnios alone would be stage I if the donor bladder were visible.^[4]
• The feature that places it at this stage rather than the next: both twins have positive end-diastolic flow in the umbilical artery and neither has ascites or hydrops, so stage III (critically abnormal Dopplers) and stage IV (hydrops) are excluded.^[4]
• A mark is available for stating that the maternal dyspnoea and abdominal distension reflect the symptomatic polyhydramnios and that the symphysis-fundal height of 28 cm at 20+4 weeks confirms significant uterine overdistension.^[1]

(b) Immediate management and counselling (4 marks)

One mark each for referral, maternal support, the counselling content, and the explicit statement that serial amnioreduction is not the definitive treatment. ^[1][5]

• Refer urgently to a fetal medicine centre with fetoscopic laser capability within days, not weeks. Explain that severe TTTS is a life-threatening complication for both twins with perinatal mortality of 70 to 100 per cent if untreated.^[1]
• 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]
• Counsel explicitly on outcomes: after laser, at least one twin survives in roughly 80 to 90 per cent of cases, both twins in 50 to 70 per cent, and severe neurological injury in survivors in around 5 to 10 per cent. The risk of preterm prelabour rupture of membranes after fetoscopy is 5 to 10 per cent.^[1][2]
• State that serial amnioreduction is not the definitive treatment for stage II to IV TTTS before 26 weeks. It is reserved for symptomatic polyhydramnios where laser is contraindicated, unavailable, or refused, and as an adjunct when polyhydramnios persists after laser.^[1][2]
• Discuss the option of selective reduction by radiofrequency ablation in selected cases, although in this case with both twins structurally normal and stage II disease, this is not the first-line approach.^[1]

(c) Why laser and what the Solomon technique adds (4 marks)

Two marks for the Senat trial evidence, two for the Solomon technique and the Slaghekke trial. ^[2][3]

• Fetoscopic laser coagulation of the communicating placental vessels is the gold standard for severe TTTS before 26 weeks. The Senat randomised trial (NEJM 2004) compared endoscopic laser surgery with serial amnioreduction for severe TTTS before 26 weeks and showed better neonatal survival and fewer neurological complications at six months in the laser arm.^[2]
• The mechanism is to disconnect the communicating anastomoses on the chorionic plate that drive the unidirectional transfusion. This treats the cause rather than the symptom of polyhydramnios.^[5]
• The Solomon technique coagulates the entire vascular equator of the placenta rather than selectively coagulating only the visible anastomoses. The Slaghekke Solomon randomised trial (Lancet 2014) showed it 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) by eliminating the residual anastomoses that selective coagulation leaves behind.^[3]
• Counsel on the trade-off: the Solomon technique takes slightly longer than selective coagulation, but the reduction in recurrence and post-laser TAPS is the dominant benefit. The systematic review and meta-analysis by Shamshirsaz confirms the survival benefit.^[3]

(d) Post-treatment surveillance and timing of birth (3 marks)

One mark each for the surveillance plan, the timing of birth, and the mode of birth and place of birth. ^[1][6]

• Post-treatment surveillance: continue biweekly scans with deepest vertical pool, MCA-PSV (to detect post-laser TAPS), EFW and umbilical artery Doppler. A fetal MRI for cerebral injury is considered if there is concern about the co-twin's wellbeing or after single intrauterine death.^[1][6]
• Timing of birth: planned birth at 34+0 to 37+0 weeks, individualised by complications. RCOG Green-top 51 (2024 partial update) recommends 36+0 to 37+0 weeks for uncomplicated MCDA, and this would apply if the pregnancy remained otherwise uncomplicated after laser.^[1]
• Place of birth: tertiary centre with neonatal intensive care, with the anaesthetic and neonatal teams informed of the fetal therapy history. 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]