Ultrasound Monitoring of Twins: Timing, TTTS, sFGR, and TAPS According to ISUOG 2025
Key Principle: Chorionicity First
The updated ISUOG 2025 guidelines on the role of ultrasound in twin pregnancies base the entire monitoring schedule on chorionicity and amnionicity. These should be determined as early as possible, preferably at 11–13+6 weeks, using the number of placental masses, thickness of the inter-amniotic membrane, lambda and T-signs, number of yolk sacs, and fetal sex. If chorionicity is unclear, the pregnancy should be managed as monochorionic until expert review.
During the first examination, fetal labeling must be documented: location, relation to the placenta, side of the uterus, and sex if possible. This nomenclature should be maintained in all protocols; otherwise, comparisons of biometry and Doppler studies lose clinical reliability.
Basic Ultrasound Schedule for Twins
| Type of Twins | Minimum Schedule According to ISUOG 2025 | Mandatory Assessments |
|---|---|---|
| Dichorionic Diamniotic | 11–13+6 weeks; anatomical study around 20 weeks; then every 4 weeks, usually starting at 24 weeks | Growth of each fetus, EFW discordance, amniotic fluid, Doppler as indicated |
| Monochorionic Diamniotic | 11–13+6 weeks; then every 2 weeks from 16 weeks until delivery | DVP in each sac, bladder of each fetus, signs of TTTS, TAPS, and sFGR, Doppler |
| Monochorionic Monoamniotic | Expert monitoring; serial ultrasounds from 16 weeks, usually every 2 weeks | Viability of both fetuses, growth, Doppler, anomalies, cord entanglement; the latter alone is not a sufficient predictor of demise |
For monochorionic pregnancies, a 2-week interval is chosen for early detection of TTTS and other inter-fetal hemodynamic complications. Increasing the interval raises the risk of missing the transition from a normal picture to a clinically significant syndrome.
What Should Be in Every Protocol
The protocol should indicate chorionicity, amnionicity, fetal labeling, heart rate, presentation, biometry of each fetus, estimated fetal weight, percentile or centile assessment, EFW discordance, maximum vertical pocket of fluid in each amniotic sac, and bladder status. For monochorionic twins, add Doppler studies: umbilical artery, middle cerebral artery with MCA-PSV, and ductus venosus as clinically indicated.
EFW discordance is calculated as the difference between the larger and smaller estimated weights, divided by the larger weight, in percentage. A threshold of ≥25% is used as a clinically significant risk marker, including in the diagnosis of selective growth restriction.
TTTS: Ultrasound Diagnosis
Twin-to-twin transfusion syndrome occurs only in monochorionic pregnancies. The ultrasound diagnosis is based on the combination of oligohydramnios in the presumed donor and polyhydramnios in the recipient: DVP <2 cm in the donor and DVP ≥8 cm before 20 weeks or ≥10 cm after 20 weeks in the recipient. Also important are a small or non-visible donor bladder, overfilled recipient bladder, cardiomegaly, valve regurgitation, hydrops, and abnormal Doppler.
| Quintero Stage | Ultrasound Criterion |
|---|---|
| I | Oligohydramnios/polyhydramnios; donor bladder is visible |
| II | Donor bladder is not visible |
| III | Critical Doppler abnormalities in one or both fetuses |
| IV | Hydrops in one or both fetuses |
| V | Intrauterine demise of one or both fetuses |
If TTTS is suspected, the pregnancy should be referred to a fetal medicine center. For stages II–IV between 16–26 weeks, the standard is fetoscopic laser coagulation of placental anastomoses; for stage I, management depends on gestational age, severity of signs, cervical status, and availability of expert monitoring.
sFGR: Criteria and Types
Selective fetal growth restriction in twins is not just a “small fetus.” According to ISUOG, important factors include the size of the smaller fetus, EFW percentile, abdominal circumference, umbilical artery Doppler, and mass discordance. A practically significant discordance threshold is ≥25%.
| Type of sFGR in Monochorionic Twins | Umbilical Artery Doppler in the Smaller Fetus | Clinical Significance |
|---|---|---|
| Type I | Positive end-diastolic flow | Most stable variant; requires serial monitoring |
| Type II | Persistently absent or reversed end-diastolic flow | High risk of deterioration and preterm delivery |
| Type III | Intermittently absent or reversed end-diastolic flow | Unpredictable course due to large arterio-arterial anastomoses |
In sFGR, not only the smaller fetus but also the risk to the normal twin, especially in monochorionicity, is assessed. Protocols should separately document MCA-PSV, ductus venosus, and amniotic fluid dynamics, as sFGR may coexist with TTTS or TAPS.
TAPS: When to Measure MCA-PSV
Twin anemia–polycythemia sequence can develop spontaneously or after laser treatment for TTTS. Screening is based on the peak systolic velocity in the middle cerebral artery. The classic antenatal criterion is MCA-PSV >1.5 MoM in the anemic donor and MCA-PSV <1.0 MoM in the polycythemic recipient. Updated approaches also consider inter-twin MCA-PSV differences, especially with borderline absolute values.
| TAPS Stage | Ultrasound Sign |
|---|---|
| 1 | Donor MCA-PSV >1.5 MoM, recipient MCA-PSV <1.0 MoM, no signs of cardiac decompensation |
| 2 | More pronounced MCA-PSV discrepancy: donor >1.7 MoM, recipient <0.8 MoM |
| 3 | Stage 1 or 2 plus critical Doppler abnormalities |
| 4 | Hydrops of the donor fetus |
| 5 | Demise of one or both fetuses |
MCA-PSV should be measured with technical precision: insonation angle close to 0°, proximal third of the MCA, without probe pressure and without active fetal movements. Technical errors can easily lead to a false TAPS diagnosis.
TRAP and Single Fetal Demise
In the twin reversed arterial perfusion sequence, an acardiac twin and a “pump” twin are identified. Ultrasound determines blood flow in the umbilical cord of the acardiac twin, growth of the acardiac component, signs of cardiac overload in the pump twin, polyhydramnios, and Doppler abnormalities. Such pregnancies require referral to a specialized center.
Single intrauterine demise in monochorionic twins is more dangerous than in dichorionic twins due to shared vascular anastomoses and the risk of acute transfusion to the surviving twin. After confirming the diagnosis, the viability of the second twin, Doppler, signs of anemia by MCA-PSV, and subsequently neurosonography and/or MRI according to gestational age and availability are assessed.
Practical Algorithm for the Ultrasound Office
- At 11–13+6 weeks, establish chorionicity, amnionicity, gestational age, and consistent fetal labeling.
- For dichorionic twins, plan an anatomical study around 20 weeks and growth monitoring every 4 weeks starting at 24 weeks.
- For monochorionic twins, schedule ultrasounds every 2 weeks starting at 16 weeks.
- At each visit for monochorionicity, document DVP, bladders, EFW discordance, umbilical artery, and MCA-PSV.
- If DVP <2 cm in one fetus and DVP ≥8/≥10 cm in the other, stage TTTS and urgently refer to a fetal center.
- If EFW discordance is ≥25% or there is a small fetus, exclude sFGR and classify the type by blood flow in the umbilical artery of the smaller fetus.
Frequently asked questions
How often should ultrasounds be performed for uncomplicated monochorionic diamniotic twins?
Every 2 weeks from 16 weeks until delivery. The goal is early detection of TTTS, TAPS, and selective growth restriction.
What ultrasound criteria should be used for TTTS?
Oligohydramnios in the donor with DVP <2 cm and polyhydramnios in the recipient: DVP ≥8 cm before 20 weeks or ≥10 cm after 20 weeks. The syndrome is then staged according to Quintero I–V.
What mass discordance threshold is considered significant in twins?
An EFW discordance threshold of ≥25% is clinically significant and used in risk assessment, including the diagnosis of sFGR.