Chorangiopagus parasiticus twins.

Chorangiopagus parasiticus twins (CAPP) occur due to asymmetric abnormality of monozygous, twinning. It is the commonest form of asymmetric twinning, occurring in 1% of monozygous twins. The asymmetric twin survives by parasitizing the more normally developed co-twin by connecting with the chorionic circulation, hence, chorangiopagus parasiticus. CAPP twin is a markedly edematous foetus with relatively well-developed legs, incomplete pelvis and lower spine. The body cavity contains some incomplete abdominal viscera but usually no thoracic organs. The upper portion of the twin consists of edematous cystic tissue. This parasitic perfused twin has no placental vascular connection and its cord vessels are conjoint with those of the parasitized pump twin on the surface of the placenta or somewhere along the cord. This malformation can be identified by ultrasound as early as 12 weeks gestation.