What is the recommended evaluation and management for significant twin growth discordance (>25% estimated fetal weight difference)?

Management of Significant Twin Growth Discordance (>25%)

For twin pregnancies with ≥25% estimated fetal weight (EFW) discordance, initiate intensive surveillance with transabdominal ultrasound every 2-3 weeks, umbilical artery Doppler studies, and antepartum fetal testing (nonstress test or biophysical profile), with management intensity stratified by chorionicity—monochorionic twins require more aggressive monitoring due to substantially higher risk of adverse outcomes. 1

Defining Severe Discordance

• Discordance ≥25% represents severe growth discrepancy and is associated with significantly increased mortality and morbidity in twin pregnancies 1
• The most widely accepted method for determining discordance is calculating the percentage difference in EFW between twins, with thresholds defined as: mild (15%), moderate (20%), and severe (≥25%) 1
• In monochorionic diamniotic pregnancies, isolated twin weight discordance ≥25% occurs in 11.6% of cases 1

Chorionicity-Specific Risk Stratification

Monochorionic Twins (Higher Risk)

• Monochorionic twins with ≥20% discordance face dramatically elevated risks: 65% preterm delivery <34 weeks (vs 26% in concordant), 35% preterm delivery <28 weeks (vs 4%), and 68% NICU admission (vs 23%) 2
• The relative risk of preterm delivery <28 weeks is 8.8-fold higher in discordant monochorionic twins compared to concordant monochorionic twins 2
• Monochorionic twins require umbilical artery Doppler velocimetry even without other complications, as abnormal Doppler findings can appear as early as 16-20 weeks and predict adverse outcomes including fetal demise 1

Dichorionic Twins (Lower Risk)

• Dichorionic twins with discordance ≥25% show independent association with maternal vascular malperfusion pathology (adjusted RR 2.18), indicating true placental insufficiency of the smaller twin 3
• Discordance <25% in dichorionic twins is primarily driven by small-for-gestational-age status rather than discordance itself 3
• However, dichorionic twins with ≥18% discordance detected between 24-31 weeks still have twice the rate of small-for-gestational-age infants (51% vs 29%) and increased neonatal morbidity 4

Surveillance Protocol

Ultrasound Monitoring

• Perform transabdominal ultrasound every 2-3 weeks to assess fetal growth, EFW percentiles, and abdominal circumference measurements 1
• Measure amniotic fluid volume in each sac—oligohydramnios may indicate twin-twin transfusion syndrome (TTTS) in monochorionic twins or uteroplacental insufficiency in either chorionicity 1
• Abdominal circumference discordance >10% between 14-22 weeks is the single best predictor of subsequent adverse outcomes in both monochorionic and dichorionic pregnancies 1

Doppler Studies

• Umbilical artery Doppler velocimetry is essential when growth discordance is detected, particularly in monochorionic twins where it has proven benefit 1
• Assess for absent or reversed end-diastolic flow in the umbilical artery, which indicates severe placental insufficiency 1
• Evaluate ductus venosus for reversal of A-wave and assess for tricuspid regurgitation, which indicate cardiac compromise 1
• While umbilical artery Doppler has no proven benefit in uncomplicated dichorionic twins, it becomes valuable when growth delay is suspected 1

Antepartum Fetal Testing

• Initiate nonstress testing or biophysical profile (BPP) surveillance using the same indications as singleton pregnancies with suspected growth restriction 1
• Nonstress testing and BPP are as reliable in multiple gestations as in singletons for detecting fetal compromise 1
• The American College of Obstetricians and Gynecologists recommends individualized management based on consensus and expert opinion, considering the welfare of both fetuses 1

Critical Pitfalls to Avoid

• Do not assume discordance <25% is benign in monochorionic twins—even 20% discordance significantly increases adverse outcomes in this population 2
• Do not rely solely on EFW discordance percentage—always assess absolute EFW percentiles, as the smaller twin may have true growth restriction even if discordance is <25% 3, 4
• Do not delay Doppler assessment—abnormal findings can appear in the second trimester (16-20 weeks) and predict fetal demise 1
• Discordant growth alone is not an indication for preterm delivery—timing should be based on Doppler findings, biophysical testing, and gestational age 5

Additional Considerations

• Evaluate for velamentous cord insertion and vasa previa, which are more common in twin pregnancies and contribute to growth discordance 1
• Perform transvaginal cervical length assessment to stratify preterm delivery risk 1, 6
• Consider fetal echocardiography in monochorionic twins due to 8-fold increased risk of structural cardiac anomalies in monoamniotic twins and elevated risk in all monochorionic gestations 1
• Early detection matters: discordance identified between 24-31 weeks predicts earlier delivery (34 vs 36 weeks in dichorionic, 34 vs 36 weeks in monochorionic) and greater neonatal morbidity including respiratory distress syndrome 4