Diagnostic Criteria for Twin-to-Twin Transfusion Syndrome

Essential Diagnostic Criteria

The American College of Obstetricians and Gynecologists recommends that TTTS diagnosis requires both monochorionic diamniotic placentation and discordant amniotic fluid volumes, with oligohydramnios (maximal vertical pocket ≤2 cm) in the donor twin's sac and polyhydramnios (maximal vertical pocket ≥8 cm) in the recipient twin's sac 1, 2
Discordant amniotic fluid volumes, specifically oligohydramnios (MVP ≤2 cm) in one sac and polyhydramnios (MVP ≥8 cm) in the other sac, are required for TTTS diagnosis, representing the 5th and 95th percentiles respectively 1, 2

Oligohydramnios with maximal vertical pocket ≤2 cm is a characteristic finding in the donor twin 1, 2
A small or absent bladder, with nonvisualization of the fetal bladder, is a pathognomonic finding in the donor twin 3, 2
The donor twin may appear trapped against the uterine wall, contained within the collapsed intertwin membrane due to severe oligohydramnios, described as a "stuck twin" appearance 3
Intrauterine growth restriction may be present in the donor twin, but is not required for diagnosis 1, 4

Polyhydramnios with maximal vertical pocket ≥8 cm is a characteristic finding in the recipient twin 1, 2
A large, distended bladder is visible on ultrasound in the recipient twin 3
Cardiomegaly (enlarged heart) may be present in the recipient twin in more advanced stages of TTTS 3
Hydrops fetalis (fluid accumulation) may occur in severe cases of TTTS in the recipient twin 3

In more severe TTTS (stages II-IV), abnormal umbilical artery Doppler findings, including absent or reversed end-diastolic flow in the donor twin, may be present 3, 4
Cardiac dysfunction, including tricuspid regurgitation or reversal of A-wave in ductus venosus, may occur in the recipient twin 3
Cardiac chamber enlargement may be present in the recipient twin 3

Growth discordance alone does not diagnose TTTS, and amniotic fluid discordance is the defining feature, according to the American College of Obstetricians and Gynecologists 1, 4
Subjective fluid differences require close monitoring, as progression to TTTS occurs in 15% of cases, and serial ultrasound surveillance every 2 weeks starting around 16 weeks gestation is recommended 1, 2, 4, 5
The timing of presentation matters, with the most severe cases typically becoming evident before 20 weeks gestation, and milder forms may not appear until 26-28 weeks 3
A differential diagnosis, including selective IUGR or a fetal anomaly causing amniotic fluid abnormality in one twin, must be excluded, as these conditions can mimic TTTS 1, 2

Twin-Twin Transfusion Syndrome (TTTS) is most commonly diagnosed during the mid-trimester, typically between 16 and 26 weeks of gestation, though it can manifest at any point during pregnancy 6
Most cases are diagnosed in the mid-trimester, which aligns with the recommended surveillance protocol that begins at 16 weeks of gestation 6
The presentation timing is highly variable, and while mid-trimester diagnosis is most common, TTTS can present at any gestational age during pregnancy 6

Advanced (Stage III) TTTS presenting before 26 weeks carries a particularly bleak prognosis, with reported perinatal loss rates of 70-100% without intervention 7, 8
Fetoscopic laser photocoagulation is the standard treatment for Stage II-IV TTTS diagnosed before 26 weeks of gestation 6

All monochorionic diamniotic (MCDA) twin pregnancies require serial ultrasound surveillance beginning at 16 weeks of gestation and continuing every 2 weeks until delivery to ensure timely diagnosis regardless of when TTTS manifests 6
This biweekly surveillance protocol has demonstrated effectiveness for timely TTTS diagnosis and low rates of Stage V TTTS (involving fetal demise) 6