The first is that a single fertilized egg does not fully split during the process of forming identical twins. If the zygote division occurs after two weeks of the development of the embryonic disc, it results in the formation of conjoined twins. The second theory is that a fusion of two fertilized eggs occurs earlier in development. Partial splitting of the primitive node and streak may result in the formation of conjoined twins. These twins are classified according to the nature and degree of their union. Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. The type of twins formed depends on when and to what extent abnormalities of the node and streak occurred. Misexpression of genes, such as Goosecoid, may also result in conjoined twins. Goosecoid activates inhibitors of BMP4 and contributes to regulation of head development. Over- or underexpression of this gene in laboratory animals results in severe malformations of the head region, including duplications, similar to some types of conjoined twins. Types
Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are:

Thoraco-omphalopagus (28% of cases): Two bodies fused from the upper chest to the lower chest. These twins usually share a heart and may also share the liver or part of the digestive system. Thoracopagus (18.5%): Two bodies fused from the upper chest to lower belly. The heart is always involved in these cases.
Omphalopagus twins John Nelson and James Edward Freeman (Johnny and Jimmy) were born in Youngstown, Ohio, on April 27, 1956. The boys shared a liver but had separate hearts and were successfully separated at North Side Hospital in Youngstown, Ohio, by Bertram Katz. The operation was funded by the Ohio Crippled Childrens Service Society.Recent successful separations of conjoined twins include that of the separation of Ganga and Jamuna Shreshta in 2001, who were born in Kathmandu, Nepal, in 2000. The 97-hour surgery on the pair of craniopagus twins was a landmark one which took place in Singapore; the team was led by neurosurgeons Chumpon Chan and Keith Goh. The surgery left Ganga with brain damage and Jamuna unable to walk. Seven years later, Ganga Shrestha died at the Model Hospital in Kathmandu in July 2009, at the age of eight, three days after being admitted for treatment of a severe chest infection.Infants Rose and Grace Attard, conjoined twins from Malta, were separated in the United Kingdom by court order Re A over the religious objections of their parents, Michaelangelo and Rina Attard. The twins were attached at the lower abdomen and spine. The surgery took place in November 2000, at St Marys Hospital in Manchester. The operation was controversial because Rose, the weaker twin, would die as a result of the procedure as her heart and lungs were dependent upon Graces. However, if the operation had not taken place, it was certain that both twins would die.