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2010-11-08-13 Lamba sign and "black lambda" sign © Reuss www.TheFetus.net
Lamba and "black lambda" sign

Annette Reuss, MD.

Praxis Central, Willy-Brandt-Platz 4 - 45127 Essen, Germany.
Department of  Obstetrics and Gynaecology, Radboud University Nijmegen Medical Centre, Nijmegen, Netherlands.

In dichorionic diamniotic twin pregnancy, there is a triangular projection of placental tissue which extends from the placenta between the layers of amniotic and chorionic membranes of each fetus. This ultrasound finding is called "lambda" or "twin peak" sign. With advancing gestational age, there is regression of the chorion laeve and the lambda sign becomes difficult to identify. Thus by 20 weeks only 85% of dichorionic pregnancies demonstrate this sign.

In monochorionic diamniotic twin pregnancy, the inter-twin membrane consists only of the two layers of amnion. The chorionic tissue within the septum is missing. The ultrasound examination shows so called "T" sign at the placental side.

Furthermore, in-utero two thin walled amniotic sacs can be seen creating a black triangular space between the outline of the amniotic sacs and the uterine wall and/or chorion in the extraembryonal coeloma called the "black lambda" sign. This "black lambda" sign is present at monochorionic twin pregnancies and should not be confused with the lambda which is a characteristic sign for dichorionic twins.

Images 1,2: Dichorionic diamniotic pregnancy; image 1 shows typical lambda sign. Image 2 shows a 3D-image of the 
inter-twin membrane with the triangular projection of the placental tissue between the layers of chorio-amniotic membranes.



Images 3,4: Monochorionic diamniotic pregnancy; image 1 shows a "T"-sign, thin inter-twin membrane without projection of the placental tissue. Image 4 shows "black lambda" sign, triangular space between the outline of the amniotic sacs.

 

Images 5,6: Another example of "black lambda" sign at monochorionic diamniotic twin pregnancy. It is a red triangular space between the outline of the amniotic sacs and the uterine wall.



Image 7: 3D-image of the monochorionic twins.


 
References:

1. Lopriore E, Sueters M, Middeldorp JM, Klumper F, Oepkes D, Vandenbussche FP: Twin pregnancies with two separate placental masses can still be monochorionic and have vascular anastomoses. Am J Obstet Gynecol. 2006 Mar;194(3):804-8.
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