Twin-peak sign | Radiology Signs

Answer:

The twin-peak or lambda sign is a triangular wedge of chorionic tissue extending from the placental surface into the base of the intertwin membrane, and it establishes a dichorionic twin pregnancy. Its counterpart is the T sign, in which the membrane meets the placenta at a right angle with no intervening wedge, indicating a monochorionic pregnancy. Chorionicity should be determined in the first trimester, ideally between 11 and 14 weeks, when these signs are most reliable; the twin-peak sign becomes progressively less visible with advancing gestation and cannot be relied on to exclude monochorionicity later. Additional discriminators are the number of placental masses, discordant fetal sex, which proves dichorionicity, and membrane thickness, with a thick membrane of four layers favouring dichorionic and a thin two-layer membrane favouring monochorionic pregnancy. The determination drives everything that follows: monochorionic pregnancies carry the risk of twin-to-twin transfusion syndrome, selective growth restriction, twin anaemia-polycythaemia sequence and twin reversed arterial perfusion, and require fortnightly surveillance from 16 weeks with assessment of deepest vertical pockets and middle cerebral artery Doppler, while dichorionic pregnancies are monitored less intensively. Monoamnionicity, with no dividing membrane and cord entanglement, is a further high-risk subgroup.

Why is it called so?

The wedge of chorion projecting into the membrane forms two peaks, or the shape of the Greek letter lambda, at the placental insertion.

Pathophysiology

In a dichorionic pregnancy each fetus has its own chorion, and where the two placental discs abut, chorionic villi and extraembryonic tissue are drawn up between the two amnions at their insertion, filling the potential space with a wedge of proliferating chorionic tissue. In a monochorionic pregnancy the two amnions arise from a single chorion, so no chorionic tissue lies between them and the membrane inserts flush against the single placenta, producing the T configuration. As pregnancy advances the chorionic wedge in a dichorionic gestation atrophies and the layers fuse, which is why the sign fades after the first trimester. The shared placental circulation of monochorionic twins, with unbalanced arteriovenous anastomoses, is the substrate for twin-to-twin transfusion syndrome.

Alternative names: Lambda sign; delta sign of chorionicity

Other associated named signs: T sign of monochorionic twins and the stuck twin appearance of twin-to-twin transfusion syndrome

References

1. Wu TWJ, Li YL, Wong KH. The lambda (twin peak) sign. Abdom Radiol (NY). 2018;43(7):1841. PMID: 29043401.