Determination of Twin Chorionicity: λ- and T-sign at 11–14 Weeks (ISUOG 2025)
λ-sign and T-sign: Ultrasound Criteria
According to ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy, 2025, determination of chorionicity is based on visualization of the site of attachment of the intertwin septum to the placenta.
| Sign | Chorionicity | Ultrasound Appearance |
|---|---|---|
| "Complete lambda" / twin peak sign | Dichorionic diamniotic (DCDA) | Fetuses separated by a thick layer of fused chorionic membranes with two thin amniotic layers (one on each side) |
| T-sign / "empty lambda" (empty lambda sign) | Monochorionic diamniotic (MCDA) | Two thin amniotic layers separating the fetuses, without wedge-shaped infolding of chorionic tissue |
Determination of Chorionicity After 14 Weeks
According to ISUOG 2025, in women presenting for the first time after 14 weeks of gestation, chorionicity is best determined using the same ultrasound signs, in particular by counting membrane layers and assessing sex discordance between fetuses (different sex indicates dichorionic twin pregnancy).
Clinical Significance of Chorionicity for Monitoring
Chorionicity determines the surveillance schedule, key assessed parameters, and complication profile (Clinical Ultrasound in Gynecology and Obstetrics, 2026):
| Type | Frequency | Key Complications |
|---|---|---|
| Dichorionic | Usually every 4 weeks in uncomplicated pregnancy | Growth discordance, preterm birth, hypertensive disorders |
| Monochorionic diamniotic | Typically every 2 weeks from mid-second trimester onwards | TTTS (twin-to-twin transfusion syndrome), TAPS, selective fetal growth restriction |
| Monoamniotic | Very careful specialized surveillance | Cord entanglement, fetal distress, prematurity |
Frequently asked questions
Which sign corresponds to dichorionic twin pregnancy?
The "complete lambda" (twin peak sign): fetuses are separated by a thick layer of fused chorionic membranes with two thin amniotic layers (ISUOG 2025).
What does the T-sign indicate?
The T-sign (empty lambda) is characteristic of monochorionic diamniotic (MCDA) twin pregnancy: fetuses are separated by only two thin amniotic layers (ISUOG 2025).
How is chorionicity determined after 14 weeks?
Using the same ultrasound signs, in particular by counting membrane layers and assessing sex discordance between fetuses (ISUOG 2025).
How frequently should monochorionic diamniotic twin pregnancy be monitored?
Typically every 2 weeks starting from mid-second trimester onward due to the risk of TTTS (twin-to-twin transfusion syndrome), TAPS, and selective fetal growth restriction.