Zone of placental tissue
extending into and tapering to a point within twin membranes(transition
zone at the intertwin membrane - placental
Lambda or TwinPeak sign
“T-sign” of MonochorionicDiamniotic Twins
Although it was initially described
at the level of fusion between the two adjacent placentas, recent evidence
suggests that it may be identified at the edge of the placentas located on
opposite sides of the uterus (1,2).
Echo pattern similar to
Triangular in cross section.
Wider at chorion
surface of the placenta.
Absence of "twin
peak" sign neither excludes dichorionic
pregnancy nor implies monochorionicity.
Seen in over 50% of dichorionic twins, especially in the first trimester
(10-14 weeks is optimal). It may disappear by week 20 in 7% of dichorionic pregnancies with fused placentas
(therefore absence after wk 20 should be viewed with caution) (4).
Tends to be focal occurring
in one or more sites along the junction between the membranes with the
Wood SL, Onge R St,
Connors G et.al. Evaluation of the twin peak or
lambda sign in determining chorionicity in
multiple pregnancy. ObstetGynecol 1996;88:6-9.
Sepulveda W, SebireNJ, Hughes K et.al.
The lambda sign at 10-14 weeks of gestation as a predictor of chorionicity in twin pregnancies. Ultrasound ObstetGynecol 1996;7:421-423.
Finberg HJ. The "twin
peak" sign: Reliable evidence of dichorionic
twinning. J Ultrasound Med 1992;11:571-577.
Sepulveda W, SebireNJ, Highes K et.al. Evolution of the lambda or twin-chorionic peak sign in dichorionic