Chorionicity is set in the first trimester (lambda vs T sign) and decides surveillance: monochorionic twins need two-weekly scans from 16 weeks for twin–twin transfusion (Quintero stage), selective FGR and TAPS.
Orient first
- Dichorionic: thick membrane with a lambda (twin-peak) sign; monochorionic diamniotic: thin membrane with a T sign.
- TTTS: polyhydramnios in the recipient (deepest pocket ≥ 8 cm) with oligohydramnios in the donor (≤ 2 cm) — verify local cut-offs.
- TAPS: discordant MCA peak systolic velocity without amniotic fluid discordance.
Acquire the study
- Transabdominal ultrasound; label twins consistently; deepest vertical pocket per sac; biometry and Doppler (UA, MCA-PSV) per twin.
The manoeuvre
- First trimester: membrane insertion — lambda vs T sign; number of placentas; amnionicity.
- Each visit (monochorionic): deepest vertical pocket in cm per sac; bladders visible?
- Biometry per twin and EFW discordance in %.
- Umbilical artery Doppler per twin; MCA-PSV in MoM for TAPS.
- Stage TTTS (Quintero I–V) when criteria met.
What confirms it
- Chorionicity documented in the first trimester; complications staged with the stated criteria.
What licenses you to exclude it
- Chorionicity cannot be reliably assigned after ~16 weeks if not recorded earlier — say so.
The classic misread
- Mislabelling the twins between visits.