Monochorionic-Diamniotic (MCDA) Twin Pregnancy
One yolk sac with twins in separate amniotic sacs signifies a monochorionic-diamniotic (MCDA) twin pregnancy, which results from embryonic cleavage 4-8 days after fertilization and carries significantly higher perinatal risks than dichorionic twins. 1
Understanding the Finding
This ultrasound pattern indicates:
Single placental mass (monochorionic): The single yolk sac suggests one gestational sac with a single peripheral chorionic reaction, characteristic of monochorionic placentation 1
Two separate amniotic sacs (diamniotic): The presence of a thin intertwin membrane dividing the twins confirms diamniotic pregnancy 1
Monozygotic (identical) twins: MCDA twins result from cleavage of the blastocyst between 4-8 days post-fertilization, representing approximately two-thirds of monozygotic twin gestations 1
Critical Caveat About Yolk Sac Counting
The number of yolk sacs is NOT a reliable indicator of amnionicity and should not be used for this determination. 1, 2
- Studies show 9.3% discrepancy between yolk sac number and actual amnionicity in monochorionic pregnancies 3
- Monoamniotic twins can have two yolk sacs 4, 5
- Monochorionic-diamniotic twins may show only one visible yolk sac 2, 6
- The definitive determination of amnionicity requires demonstration of the thin intertwin membrane, ideally by transvaginal ultrasound after 10 weeks gestation 1, 2
Confirming the Diagnosis
To accurately establish MCDA twin pregnancy:
Before 10 weeks: The amnion may not yet be formed or visible, so lack of membrane visualization doesn't confirm monoamnionicity 1
After 10 weeks: Look for the "T sign" - perpendicular attachment of the intertwin membrane to the placenta (sensitivity nearly 100%, specificity 98%) 1
Absence of "lambda sign": No triangular projection of placental tissue into the membrane base (which would indicate dichorionic placentation) 1
Single placental mass: Confirms monochorionic placentation 1
Clinical Significance and Risk Stratification
MCDA twins have substantially elevated perinatal risks compared to dichorionic twins:
- Perinatal mortality rate >2-fold higher than dichorionic twins 1
- Fetal demise rate 7.6% versus 1.6% in dichorionic twins 1
- Increased neonatal morbidity, particularly necrotizing enterocolitis 1
Specific MCDA Complications
- Twin-twin transfusion syndrome (TTTS): Affects 8-12% of MCDA pregnancies due to shared placental vascular anastomoses present in >95% of cases 1
- Twin anemia-polycythemia sequence (TAPS) 1
- Selective fetal growth restriction 1
- Risk of co-twin injury if single fetal demise occurs 1
Management Recommendations
Immediate actions:
- Confirm chorionicity and amnionicity definitively with first-trimester ultrasound, ideally transvaginal after 10 weeks 1, 2
- Refer to maternal-fetal medicine specialist or tertiary center for specialized MCDA twin management 2, 7
- Establish intensive surveillance protocol including regular growth scans and monitoring for TTTS 7
Follow-up imaging:
- Nuchal translucency scan at 11-14 weeks 7
- Detailed anatomic survey at 18-22 weeks 7
- Regular third-trimester growth assessments 7
- Serial monitoring for TTTS development 1
Common Pitfalls to Avoid
- Do not rely on yolk sac number alone to determine amnionicity - this is erroneous in up to 9-36% of cases depending on amnionicity type 1, 2, 3
- Do not assume absence of membrane means monoamnionicity before 10 weeks - the membrane may simply not be visible yet 1, 2
- Do not use single sonographic features - combine multiple findings (membrane appearance, T-sign, single placenta) for accurate chorionicity determination 1, 2
- Do not manage as low-risk pregnancy - MCDA twins require specialized care and intensive monitoring 1, 7