Twin-to-Twin Transfusion Syndrome
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Direct answer
Twin-to-twin transfusion syndrome (TTTS) complicates roughly 10-15 per cent of monochorionic twin pregnancies when unbalanced placental vascular anastomoses shunt blood from one fetus to the other. The donor becomes anaemic, growth-restricted and oligohydramniotic (the "stuck twin"), while the recipient develops hypervolaemia, polyhydramnios, hypertension and hydrops. Quintero stages I to V grade severity; fetoscopic laser ablation of the crossing vessels is the standard treatment for stage II-IV disease between about 16 and 26 weeks, and monochorionic twins are screened with ultrasound every two weeks from 16 weeks precisely to catch it early.
What you must remember
- Pathophysiology: monochorionic placentas share vascular anastomoses — arteriovenous connections unbalanced by arterioarterial or venovenous counterparts create a net transfusion from donor to recipient.
- Recognition: a monochorionic pregnancy with a deepest vertical pocket over 8 cm in one sac (recipient, polyhydramnios) and under 2 cm in the other (donor, oligohydramnios) from 16-26 weeks defines the polyhydramnios-oligohydramnios sequence; presentation is often rapidly increasing fundal height or maternal breathlessness.
- Quintero staging: I — donor bladder still visible with normal Doppler; II — donor bladder absent; III — abnormal Doppler (absent or reversed end-diastolic flow in the donor umbilical artery or reversed a-wave in the ductus venosus); IV — ascites or hydrops in either twin; V — demise of one or both twins.
- Treatment: fetoscopic laser coagulation of anastomoses (the Solomon technique coagulating the entire vascular equator reduces recurrence and TAPS) is standard for Quintero II-IV at about 16-26 weeks; amnioreduction suits stage I, maternal sedation unavailable settings or when laser expertise is absent; selective reduction by cord occlusion is considered with one severely compromised twin.
- After laser: surveillance for recurrence, twin anaemia-polycythaemia sequence (TAPS) and premature rupture of membranes; delivery planning as for complicated monochorionic twins, commonly around 34-37 weeks depending on findings.
- Related monochorionic entities: selective fetal growth restriction with abnormal Doppler, TAPS (chronic slow transfusion with discordant haemoglobin and MCA velocities but no poly-oligo sequence) and TRAP sequence with a perfused acardiac twin treated by laser or radiofrequency ablation of the acardiac twin's cord.
- Postnatal confirmation: neonatal haemoglobin discordance (recipient polycythaemic, donor anaemic) and placental injection studies.
Common confusion
TTTS is confused with selective fetal growth restriction: both affect monochorionic twins with a small twin, but TTTS is defined by the fluid imbalance — polyhydramnios in one sac, oligohydramnios in the other — whereas selective growth restriction shows oligohydramnios or normal fluid without a polyhydramniotic recipient and has different Doppler-based management. TAPS, the anaemia-polycythaemia variant, likewise lacks the fluid sequence. The practical anchor is fluid, not size.
Exam-focused takeaway
Stems give a monochorionic twin description and ask for the diagnosis, the Quintero stage or the treatment — laser ablation for stage II-IV within the 16-26 week window. Screening intervals (two-weekly scans from 16 weeks in monochorionic twins) and the donor-versus-recipient physiology (anaemic stuck donor, plethoric polyuric recipient) are favourite one-liners, as are TAPS and TRAP as recent-concept options.
Frequently asked questions
Which twins develop TTTS?
Monochorionic twins — about 10-15 per cent — because their shared placenta contains unbalanced vascular anastomoses.
What defines the polyhydramnios-oligohydramnios sequence?
A deepest vertical pocket above 8 cm in the recipient's sac and below 2 cm in the donor's sac in a monochorionic pregnancy.
How is TTTS staged?
By the Quintero system, I (donor bladder visible) through V (demise of one or both twins), with Doppler abnormality at stage III and hydrops at stage IV.
What is the standard treatment?
Fetoscopic laser ablation for Quintero stage II-IV between about 16 and 26 weeks; amnioreduction for earlier or milder disease.
What is TAPS?
Twin anaemia-polycythaemia sequence — chronic feto-fetal transfusion producing severe haemoglobin discordance without the fluid imbalance of TTTS.