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Twin-peak sign

Dr Yuranga Weerakkody and Dr Frank Gaillard et al.

The twin peak sign (also known as the lambda sign) is an ultrasound finding that is helpful in determining the chorionicity of a multifetal pregnancy. The only absolute sign of dizygosity on ultrasound is documentation of different sex fetuses.

Prenatal diagnosis of chorionicity is important as monochorionic pregnancies have increased rates and severity of all types of obstetric complications when compared with dichorionic pregnancies.

It should be noted that the 'twin' in 'twin-peak' refers not to the presence of two peaks, but that it relates to twins. Also important to realise is that in pregnancies with more than two fetuses, the chorionicity and amnionicity of the each fetus may be different, and therefore this sign only aids in determining chorionicity of adjacent twins.

Radiographic assessment

The sign describes the triangular appearance to chorion insinuating between the layers of the inter twin membrane and strongly suggests a dichorionic 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.

In contrast the T sign refers to the appearance of the intertwin membrane in a monochorionic twin pregnancy. The sign should not be confused with the lambda sign of sarcoidosis.

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