What complication is specific to monochorionic (mono-chorionic) twins?

Twin-Twin Transfusion Syndrome is the Complication Specific to Monochorionic Twins

Twin-twin transfusion syndrome (TTTS) is the complication that is specific to monochorionic twins, as it results from shared placental circulation that only occurs in monochorionic placentation.1

Pathophysiology of Monochorionic Twin Complications

Monochorionic twins share a single placenta with intertwin vascular anastomoses connecting their fetal circulations. These vascular connections are present in over 95% of monochorionic twins and are the anatomical basis for complications unique to monochorionic twins 1. The key distinguishing feature is:

• Monochorionic twins have a common placental mass with shared intertwin placental circulation
• These vascular communications allow blood and fluid exchange between twins
• This shared circulation is the foundation for TTTS development

Diagnostic Criteria for TTTS

TTTS is diagnosed by the following ultrasound criteria:

1. Presence of monochorionic diamniotic (MCDA) placentation
2. Oligohydramnios (defined as maximal vertical pocket ≤2 cm) in one twin sac
3. Polyhydramnios (defined as maximal vertical pocket ≥8 cm) in the other twin sac 1, 2

This imbalance occurs when there is unequal blood flow between the twins through placental anastomoses, resulting in one twin becoming the "donor" and the other the "recipient."

Why Other Complications Are Not Specific to Monochorionic Twins

• Intrauterine growth restriction (IUGR): While selective IUGR can occur in monochorionic twins due to unequal placental sharing, IUGR itself can occur in any pregnancy type, including singletons and dichorionic twins 1.

• Premature birth: All twin pregnancies have increased risk of preterm birth regardless of chorionicity, though monochorionic twins do have higher rates 1.

• Gestational hypertension: This is a maternal complication that can occur in any pregnancy, including singleton and all types of twin gestations.

Surveillance for Monochorionic Twin Complications

Due to the risk of TTTS and other monochorionic-specific complications:

• Ultrasound surveillance should begin at 16 weeks of gestation
• Monitoring should continue every 2 weeks until delivery
• Assessment should include fetal biometry, amniotic fluid volumes, bladder filling, and Doppler studies 1

Additional Monochorionic-Specific Complications

Besides TTTS, other complications specific to monochorionic twins include:

1. Twin Anemia-Polycythemia Sequence (TAPS): Characterized by anemia in the donor twin and polycythemia in the recipient twin, diagnosed by middle cerebral artery Doppler peak systolic velocity measurements 1, 3

2. Twin Reversed Arterial Perfusion (TRAP) sequence: A rare complication where one twin lacks a functioning heart and receives blood supply from the other twin 4

Clinical Significance

TTTS occurs in approximately 10-15% of monochorionic twin pregnancies and carries significant mortality and morbidity if untreated:

• Without treatment, advanced TTTS has a 70-90% perinatal mortality rate 5, 3
• With fetoscopic laser ablation treatment, survival of at least one twin can be expected in up to 88% of cases 3
• Long-term neurological impairment may affect up to 11% of survivors 3

The unique pathophysiology of shared placental circulation makes TTTS the defining complication specific to monochorionic twins, distinguishing it from other complications that can occur in all types of twin pregnancies.