λ Sign in 8 week pregnancy By performing an obstetric ultrasound at a gestational age of 10–14 weeks,<unk> -diamniotic twins are discerned from dichorionic twins. The presence of a "T-sign" at the inter-twin membrane-placental junction is indicative of<unk> -diamniotic twins (that is, the junction between the inter-twin membrane and the external rim forms a right angle), whereas dichorionic twins present with a "lambda (λ) sign" (that is, the chorion forms a wedge-shaped protrusion into the inter-twin space, creating a rather curved junction). The "lambda sign" is also called the "twin peak sign". At ultrasound at a gestational age of 16–20 weeks, the "lambda sign" is indicative of dichorionicity but its absence does not exclude it. In contrast, the placentas may be overlapping for dichorionic twins, making it hard to distinguish them, making it difficult to discern mono- or dichorionic twins on solely the appearance of the placentas on ultrasound.
Monochorionic twins
Monoamniotic twins
Locked twins
Conjoined twins
Vestigial twin
Parasitic twin
Craniopagus twins
Vanishing twin