Twin-to-twin transfusion syndrome

Last revised by Joshua Kogan on 24 May 2024

Twin-to-twin transfusion syndrome (TTTS), less commonly known as stuck twin syndrome, is a potential complication that can occur in a monochorionic twin pregnancy (either MCDA or MCMA). 

This complication can occur in ~10% (range 15-25%) of monochorionic pregnancies, giving an estimated prevalence of ~1:2000 of all pregnancies.

Maternal abdominal distension has been described as a clinical feature.

TTTS results from unbalanced vascular (arteriovenous and arterioarterial) anastomoses in the placenta - that is, placental circulation is directed predominantly towards one twin and away from the other.

The resultant hypovolemia and hypoperfusion in one twin and hypervolemia and hypertension in the other create a cascade of hormonal changes including the renin-angiotensin system. This in turn leads to chronic tubulopathy and oliguria in the hypovolemic (donor) twin with consequent oligohydramnios, and polyuria and consequent polyhydramnios in the hypervolemic (recipient) twin.

The extent of the syndrome can be staged according to severity. One method proposed by Quintero et al. is as 3:

* Almost always in the recipient; rarely in the donor if there is coexistent TAPS (see below).

Twin anemia-polycythemia sequence (TAPS) is a described subset of TTTS, which is discussed separately.

Monochorionicity is a prerequisite at any stage of gestation. 

Growth discordance is often but not invariably present, and does not contribute to diagnostic criteria.

The following findings on a first-trimester ultrasound of a monochorionic pregnancy have been associated with an increased risk of developing TTTS, although predictive value remains poor 8:

Features that may be noted individually in each twin include:

  • recipient twin

  • donor twin 

    • oligohydramnios (deepest vertical pocket <2 cm), causing the twin to appear pinned to the edge of the chorion, the so called "stuck twin"

    • small or non-visualized urinary bladder

    • Doppler abnormalities: predominantly umbilical artery +/- venous

    • evidence of fetal anemia

Serial sonographic monitoring is common practice. In an uncomplicated monochorionic twin pregnancy, TTTS screening should start from 16 weeks gestation with assessment of fetal growth, deepest vertical pockets, and umbilical arterial pulsatility index (UA-PI) performed every two weeks. Detailed morphology ultrasound is routine at 20 weeks. Additional fetal middle cerebral arterial-peak systolic velocity (MCA-PSV) is recommended from 22 weeks.

Untreated, TTTS generally carries a poor prognosis, with up to 90% perinatal mortality 8. Laser coagulation of the chorionic plate is the treatment of choice and significantly improves the prognosis for both twins - although both morbidity and mortality remain considerably higher than in non-TTTS monochorionic pregnancies.

Other management options include:

  • conservative management with surveillance for Quintero stage I TTTS

  • serial amnioreduction, where laser treatment is not available 8

General imaging differential considerations include:

  • other causes of oligohydramnios and/or polyhydramnios, e.g. related to fetal structural anomalies

  • isolated placental insufficiency in one of the twins (will not produce polyhydramnios in the other twin)

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