When and how is chorionicity determined?

Chorionicity is best determined by ultrasound in the first trimester (ideally 11-14 weeks), when it is most reliable.

The key signs: the 'lambda' (twin-peak) sign indicates dichorionic (a triangular projection of placenta into the inter-twin membrane), whereas the 'T-sign' indicates monochorionic (membranes meet the placenta at a right angle). The number of placentae and fetal sexes also help.

Clinical relevance. Determining chorionicity early (lambda vs T-sign at 11-14 weeks) allows appropriate risk-based surveillance of twin pregnancies.

More Detail

Determining chorionicity

Early ultrasound reliably determines chorionicity, which guides management.

When

• Best determined by ultrasound in the first trimester - ideally at 11-14 weeks (it becomes harder to assess reliably later).

Ultrasound signs

• Lambda (twin-peak) sign - a triangular tongue of placental tissue extending into the base of the inter-twin membrane -> dichorionic.
• T-sign - the inter-twin membrane meets the placenta at a right angle (no placental tissue projecting in) -> monochorionic.
• Number of placentae (two separate placentae -> dichorionic).
• Discordant fetal sexes -> dichorionic (and dizygotic).
• Membrane thickness (thicker membrane in dichorionic).

Why it matters

• Determining chorionicity early allows appropriate risk stratification and surveillance (more intensive monitoring for monochorionic twins).