Signs of Twin-to-Twin Transfusion Syndrome
TTTS is diagnosed by ultrasound when two specific criteria are met: (1) confirmed monochorionic diamniotic twin pregnancy, and (2) oligohydramnios (maximal vertical pocket ≤2 cm) in the donor twin's sac combined with polyhydramnios (maximal vertical pocket ≥8 cm) in the recipient twin's sac. 1
Essential Diagnostic Criteria
The diagnosis requires both of the following findings on prenatal ultrasound 1:
- Monochorionic diamniotic (MCDA) placentation - This must be confirmed, as TTTS only occurs in pregnancies sharing a single placenta 1
- Discordant amniotic fluid volumes - Specifically oligohydramnios (MVP ≤2 cm) in one sac AND polyhydramnios (MVP ≥8 cm) in the other sac, representing the 5th and 95th percentiles respectively 1
Key Ultrasound Findings by Twin
Donor Twin Signs
- Oligohydramnios with maximal vertical pocket ≤2 cm 1
- Small or absent bladder - nonvisualization of the fetal bladder is a pathognomonic finding 1
- "Stuck twin" appearance - the donor appears trapped against the uterine wall, contained within the collapsed intertwin membrane due to severe oligohydramnios 1
- Intrauterine growth restriction may be present but is not required for diagnosis 1
Recipient Twin Signs
- Polyhydramnios with maximal vertical pocket ≥8 cm 1
- Large, distended bladder visible on ultrasound 1, 2
- Cardiomegaly (enlarged heart) in more advanced stages 1, 3
- Hydrops fetalis (fluid accumulation) in severe cases 1, 3
Advanced Stage Findings
In more severe TTTS (stages II-IV), additional Doppler and cardiac findings emerge 1:
- Abnormal umbilical artery Doppler - absent or reversed end-diastolic flow in the donor twin 1
- Cardiac dysfunction - tricuspid regurgitation or reversal of A-wave in ductus venosus 1
- Cardiac chamber enlargement in the recipient twin 1
- Atrioventricular valve regurgitation in the recipient 3
Important Clinical Caveats
Growth discordance alone does NOT diagnose TTTS - while growth differences ≥20% often accompany TTTS, discordance or intrauterine growth restriction by themselves are not diagnostic criteria 1. The amniotic fluid discordance is the defining feature.
Subjective fluid differences require close monitoring - if amniotic fluid appears different between sacs but doesn't meet the strict MVP criteria (≤2 cm and ≥8 cm), progression to TTTS occurs in 15% of cases 1. These pregnancies need serial ultrasound surveillance every 2 weeks starting around 16 weeks gestation 1.
Timing of presentation matters - the most severe cases typically become evident before 20 weeks gestation, while milder forms may not appear until 26-28 weeks 1. TTTS generally arises between 15-26 weeks gestation 2.
Differential diagnosis - selective IUGR or a fetal anomaly causing amniotic fluid abnormality in one twin can mimic TTTS and must be excluded 1.