Monochorionic twin pregnancy

Definition

• A monochorionic twin pregnancy is a twin pregnancy with a single shared placenta; up to one third of all twin pregnancies are monochorionic.

Classification

• Monochorionic diamniotic twins (one placenta, two amniotic sacs) are distinguished from monochorionic monoamniotic twins (one placenta, one amniotic sac).

Aetiopathogenesis

Aetiology and pathogenesis

• Monochorionic twins are monozygotic; diamniotic twins arise from splitting between days 3 and 8, monoamniotic twins between days 8 and 13 after fertilisation.
• In nearly all monochorionic placentas, vascular anastomoses connect the two fetal circulations; their angioarchitecture determines the risk profile.

Clinical features

Specifics of monochorionicity

• Specific risks in monochorionic pregnancies are twin-to-twin transfusion syndrome (TTTS), TAPS, selective fetal growth restriction and the TRAP sequence.
• In monoamniotic pregnancies, an additional serious risk is umbilical cord entanglement.
• If one fetus dies in a monochorionic pregnancy, an acute transfusion into the deceased fetus occurs via the anastomoses, which can lead to hypotension with death or neurological damage in the surviving twin.
• Approximately one in every four to five monochorionic twin pregnancies develops twin-to-twin transfusion syndrome.

Clinical features and complications

• Complications unique to monochorionic pregnancy are twin-to-twin transfusion syndrome (TTTS), twin anaemia-polycythaemia sequence (TAPS), TRAP sequence, monoamnionicity and conjoined twins.
• After intrauterine death of one twin in a monochorionic pregnancy, the co-twin also dies in about 15%, and postnatal cranial imaging is abnormal in about 34%.

Diagnosis

• On ultrasound, the T sign indicates monochorionicity and the lambda sign dichorionicity; chorionicity is best determined in the first trimester.
• According to Quintero, TTTS is present with a deepest vertical pocket of at most 2 cm in the donor and at least 8 cm in the recipient.
• TAPS is recognised prenatally by discordant middle cerebral artery Doppler findings, for example a peak systolic velocity of at least 1.5 MoM in the donor and at most 0.8 MoM in the recipient.

Keep learning in the app

Further reading (selection)

1. Ultrasound Obstet Gynecol 2025: ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy (PubMed Central)
2. StatPearls: Twin-to-Twin Transfusion Syndrome (NCBI Bookshelf)
3. DocCheck Flexikon, Mehrlingsschwangerschaft

Cross-references