Early prenatal diagnosis of conjoined cephalopagus twins.

One in every 50,000±100,000 births is a conjoined twin [1]. It has long been postulated that conjoined twinning is due to fission of the developing embryo. However, based on an animal model in Triton embryos and the study of 1,200 case reports of conjoined twins, Spencer [2,3] recently concluded that conjoined twinning is due to fusion of two embryos rather than fission. The union occurs either ventrally or dorsally at sites where the surface ectoderm is absent or is programmed to be fused or disrupted (primordia of heart, oropharyngeal membranes and cloacal membranes). The prognosis for conjoined twins depends on the presence of other anomalies, the extent of union of the intracranial, intrathoracic and intraabdominal structures, and abnormal vascular connections.