How should twin discordancy be evaluated and managed in a multiple pregnancy?

Evaluation and Management of Twin Discordancy

Twin discordancy ≥25% requires intensive surveillance with ultrasound every 2-3 weeks, umbilical artery Doppler studies (especially in monochorionic twins), and individualized delivery planning based on chorionicity, gestational age, and Doppler findings. 1

Definition and Clinical Significance

• Severe discordance is defined as an estimated fetal weight (EFW) difference ≥25% between twins, calculated as [(larger twin EFW - smaller twin EFW) / larger twin EFW] × 100 1
• Mild discordance is ≥15%, moderate is ≥20%, and severe is ≥25% 1
• Approximately 75% of twins show <15% discordance (concordant), 20% are mildly discordant (15-25%), and 5% are severely discordant (>25%) 2
• The larger the discordance level, the greater the risk for adverse outcomes including perinatal mortality and morbidity 2
• Severe discordance occurs in approximately 11.6% of monochorionic diamniotic pregnancies 1

First Trimester Evaluation

• Referral to a specialist should occur as early as the first trimester if crown-rump length (CRL) discrepancy is ≥10% or nuchal translucency (NT) discordance is ≥20% 3
• NT discordance ≥20% is found in approximately 25% of monochorionic twins and carries up to 30% risk of severe twin-twin transfusion syndrome (TTTS) or early intrauterine fetal demise 3
• CRL discordance ≥10% is significantly associated with pregnancy loss, though the pooled predictive risk is only 52% 3
• First trimester size disparity increases risk of underlying growth restriction, aneuploidy, congenital anomalies, and subsequent demise 3

Chorionicity-Specific Risk Stratification

Monochorionic Twins

• Monochorionic twins with any growth discordance should undergo umbilical artery Doppler velocimetry even without other complications, as abnormal Doppler patterns can appear as early as 16-20 weeks and predict adverse outcomes including fetal demise 1
• Begin surveillance at 16 weeks with fetal biometry every 2-3 weeks and weekly assessment for TTTS or other monochorionic-specific complications 3
• Fetal echocardiography should be performed in all monochorionic gestations due to substantially increased risk of structural cardiac anomalies (eight-fold higher in monoamniotic twins) 1

Dichorionic Twins

• Umbilical artery Doppler has no proven benefit in uncomplicated dichorionic twins but becomes valuable when growth delay is suspected 1
• Follow every 3-4 weeks without complications 3
• When discordance develops, increase surveillance frequency to every 2-3 weeks 1

Ultrasound Surveillance Protocol

Growth Assessment

• Perform transabdominal ultrasound every 2-3 weeks to evaluate fetal growth, EFW percentiles, and abdominal circumference measurements 1
• Abdominal circumference discordance >10% between 14-22 weeks is the strongest predictor of later adverse outcomes in both monochorionic and dichorionic pregnancies 1
• Use standard singleton growth charts, recognizing that twin growth naturally slows compared to singletons 3

Amniotic Fluid Assessment

• At each scan, assess amniotic fluid volume in each sac 1
• Oligohydramnios may signal TTTS in monochorionic twins or uteroplacental insufficiency in any chorionicity 1

Doppler Studies

• Umbilical artery Doppler velocimetry is essential whenever growth discordance is identified, particularly in monochorionic twins 1
• Absent or reversed end-diastolic flow indicates severe placental insufficiency 1
• Evaluate the ductus venosus for reversal of the A-wave and assess for tricuspid regurgitation, which denote fetal cardiac compromise 1

Antepartum Fetal Testing

• Initiate non-stress testing (NST) or biophysical profile (BPP) surveillance using the same criteria applied to singleton pregnancies with suspected growth restriction 1
• NST and BPP have comparable reliability in multiple gestations as in singletons for detecting fetal compromise 1

Additional Evaluations

• Screen for velamentous cord insertion and vasa previa, which occur in up to 22% of monochorionic twin pregnancies and are more prevalent in all twin gestations 3, 1
• Perform transvaginal cervical length assessment at the time of the routine anatomic survey (18-22 weeks) to stratify preterm delivery risk 3, 1
• Evaluate for placenta previa, which is more common in twin pregnancies, especially dichorionic twins 3

Common Pitfalls and Caveats

• Third trimester ultrasound EFW is a poor predictor of actual birthweight discordance, with area under ROC curve of only 0.70 4
• First trimester CRL differences are no less accurate than third trimester EFW in predicting discordance 4
• Not all discordant pairs represent growth restriction—mild discordance may represent normal variation between siblings 2
• A 15% birth weight discordancy should be used as the safety limit for birth weight disparity, with special attention when the second twin is discordant 5
• Consider other causes of growth restriction including viral infection and chromosomal abnormalities, though unequal placental sharing is most common 3
• The smaller twin in severely discordant pairs faces disproportionate risk for neonatal mortality 2