Evaluation and Management of Twins with Discordant Congenital Anomalies
For twins discordant for congenital anomalies, establish chorionicity in the first trimester (10-13 weeks) as this single determination dictates all subsequent management decisions, surveillance intensity, and counseling about outcomes. 1
Initial Diagnostic Evaluation
Chorionicity Determination
- First-trimester ultrasound (10-13 weeks) is the definitive and only reliable method to establish chorionicity and amnionicity—do not infer chorionicity from fetal gender or number of placental masses, as this leads to critical management errors. 2
- Monochorionic (MC) twins account for ~20% of twin pregnancies but represent ~30% of complications, with overall perinatal mortality of ~10%. 2
Anomaly Detection and Characterization
- Detailed first-trimester ultrasound (12-13 weeks) can detect structural abnormalities in 27.3% of twin pregnancies; anomalies most commonly involve the cranial vault, midline brain, and abdominal wall. 1
- Monochorionicity and increasing intertwin discrepancy in crown-rump length (CRL) and nuchal translucency (NT) measurements are associated with fetal structural abnormalities. 1
- Perform comprehensive fetal anatomic survey at 18-22 weeks for all twin pregnancies, as congenital anomalies occur in roughly 1 in 25 dichorionic (DC) pregnancies. 1
Genetic and Cardiac Evaluation
- Sample both fetuses during invasive diagnostic testing even when only one appears at risk—this prevents missed diagnoses in the apparently normal co-twin. 1
- Twins with increased NT (≥3 mm) or confirmed aneuploidy require detailed second-trimester anatomic survey and fetal echocardiography. 1
- First-trimester combined screening (including NT at 11-14 weeks) detects >85% of chromosomal abnormalities in DC twins. 1
Surveillance Protocols by Chorionicity
Dichorionic Twins with Discordant Anomalies
- Schedule routine ultrasound surveillance every 3-4 weeks beginning after the anatomy scan for uncomplicated DC pregnancies. 1
- At the anatomy scan, obtain transvaginal cervical-length measurement to evaluate preterm-birth risk. 1
- Screen for vasa previa and velamentous cord insertion, which occur more frequently in multiple gestations. 1
- If concerns arise regarding fetal growth or anemia, perform appropriate Doppler evaluation. 1
Monochorionic Twins with Discordant Anomalies
- Initiate ultrasound surveillance every 2 weeks starting at 16 weeks and continue until delivery—MC twins require approximately twice the frequency of scans compared to DC twins. 1, 2
- Each surveillance visit must include:
- Increase surveillance frequency immediately when complications arise (cervical shortening, growth disturbance, abnormal amniotic fluid). 1
Management Options and Outcomes
Expectant Management
- In monochorionic twins with discordant structural anomalies managed expectantly (excluding TTTS, TAPS, TRAP, or selective growth restriction), livebirth rate of the healthy co-twin is 82.7%. 3
- Spontaneous in-utero demise of the anomalous twin occurs in 17.3% of expectantly managed MC cases and is followed by spontaneous demise of the healthy co-twin in 44.4% of these cases. 3
- Expectant management does not increase risk of premature delivery in twins discordant for major anomalies; median gestational age at delivery is similar to twins without anomalies (254 vs. 257 days). 4
- In MC twins discordant for anencephaly managed expectantly, 21.1% of anencephalic fetuses die at 20-32 weeks, and in three-quarters of these cases the normal co-twin also dies. 5
Selective Feticide
- Selective feticide does not significantly alter survival of the healthy co-twin compared to expectant management (88.5% vs. 82.7% livebirth rate). 3
- In MC twins undergoing selective feticide, 11.5% of healthy co-twins die 5.3 ± 3.1 days after the procedure. 3
- For dichorionic twins, selective termination before 18 weeks is associated with the lowest pregnancy loss (3.1%) and highest 28-day survival (96.9%); intermediate procedures (18-23 weeks) have lower survival (86%) and higher loss (12%). 6
- Late selective termination (after 23 weeks) in DC twins achieves 100% survival but carries 27% risk of preterm delivery before 32 weeks. 6
Cord Occlusion for Monochorionic Twins
- Ultrasound-guided bipolar cord coagulation in MC pregnancies (mostly for TRAP or severe TTTS) achieves 77.2% survival rate with 31.0% early preterm delivery rate. 5
- For MC twins discordant for anencephaly, it remains uncertain whether expectant management or cord occlusion is superior—both approaches carry substantial risks to the normal co-twin. 5
Delivery Timing Recommendations
| Pregnancy Type | Recommended Timing | Citation |
|---|---|---|
| DC twins (uncomplicated) | 37-38 weeks | [1] |
| MC twins (uncomplicated) | 34-36 weeks | [1,2] |
| MC twins post-selective feticide | 34-36 weeks if both survive | [2] |
| MC twins with single fetal demise | 39 weeks (full term) unless other indications | [1,2] |
Critical Management Pitfalls
- Do not delay referral to a fetal-intervention center when TTTS (stage II-IV) or TAPS (stage II before 32 weeks) develops in MC twins with discordant anomalies—timely intervention improves outcomes. 1, 2
- Do not rely on umbilical-artery Doppler alone in DC twins without growth restriction; comprehensive assessment is required. 1
- Do not assume the apparently normal co-twin is genetically normal—a wide spectrum of major discordant structural anomalies can occur in monochorionic pregnancies despite genetic identity, with overall survival of the anomalous twin only 32%. 7
- In MC twins with discordant anomalies, 26% are complicated by selective intrauterine growth restriction and 5% by TTTS, requiring intensified surveillance protocols. 7
- Administer a single course of corticosteroids for fetal lung maturation between 24 and 33 6/7 weeks if preterm delivery is anticipated, particularly in pregnancies undergoing invasive fetal procedures. 8