Why does chorionicity matter clinically in twin pregnancy?

Chorionicity (not zygosity) determines the risks and management. Monochorionic twins (sharing a placenta) have markedly higher rates of miscarriage, growth restriction, preterm birth, and perinatal death than dichorionic, plus unique complications (TTTS, TRAP, co-twin death effects).

So monochorionic pregnancies need more intensive surveillance and specialist care.

Clinical relevance. Chorionicity determines the risk profile and surveillance intensity in twin pregnancy - monochorionic twins need closer specialist care.

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Clinical relevance of chorionicity

Chorionicity is the key factor determining twin pregnancy risk and care.

Why it matters more than zygosity

• Chorionicity (number of placentae), not zygosity, determines the risk profile - because monochorionic twins share a placenta with vascular connections.

Higher risks in monochorionic vs dichorionic

• Miscarriage (e.g. ~12% vs ~2% in 12-24 weeks), IUGR (~21% vs ~12%), preterm birth (<32 weeks, ~9% vs ~5%), and perinatal death (~2-3% vs ~1.6%) - all higher in monochorionic.

Unique monochorionic complications

• Twin-twin transfusion syndrome, TRAP/acardiac twin, selective growth restriction, and serious effects on the co-twin if one dies.

Implication for care

• Monochorionic twins need more intensive surveillance (e.g. 2-weekly scans), specialist fetal medicine input, and earlier delivery.