"The twin peak sign, also known as the lambda (λ) sign, is the triangular appearance of the chorion insinuating between the layers of the intertwin membrane and strongly suggests a dichorionic diamniotic twin pregnancy. It is best seen in the first trimester (between 10-14 weeks) 5. While the presence of a twin peak sign is a useful indicator of dichorionicity, its absence, however, is not that useful in confidently excluding it."
"It should be noted that the "twin" in "twin peak" refers not to the presence of two peaks, but that it relates to twins. In pregnancies with more than two fetuses, the chorionicity and amnionicity of each fetus may be different, and therefore this sign only aids in determining the chorionicity of adjacent twins."
"T sign: refers to the appearance of the inter-twin membrane in a monochorionic twin pregnancy"