Lambda sign - describes the triangular appearance to chorion insinuating between layers of the inter-twin membrane and is an important indicator of dichorionic twin pregnancy. Also known as the twin-peak sign, it is best seen in the first trimester between 10 - 14 weeks. Chorionicity status has major management implications for multifetal pregnancies which makes assessment for this sign essential. The analogous sign in monochorionic twin pregnancy is the T-sign where no chorion extends between the edges of the inter-twin membrane.
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