The exact cause of Siamese twins, more accurately known as conjoined twins, is not entirely known. Since the prevalence of conjoined twins is thought to be higher in southeast Asian and African populations than in Caucasian populations, Sciences 360 suggests that environmental or genetic factors may play some role in the formation of conjoined twins.
Seattle Children's Hospital, Research and Foundation explains that while the risk factors that cause the formation of conjoined twins are not well understood, the process through which they form falls into two prevailing theories. Like identical twins, conjoined twins begin as a single, fertilized egg. During the formation of identical twins, the egg divides into two halves within two weeks of fertilization. According to Mayo Clinic, when separation occurs too late, more than 13 to 15 days after fertilization, the egg is unable to complete the division process. This leaves the two halves partially attached, resulting in conjoined twins.
An opposing theory, explained by Seattle Children's Hospital, Research and Foundation, is that the egg does completely separate, but for unknown reasons joins back together. In some cases, the connection between the twins is relatively simple, with major structures and vital organs developing independently in each twin. For other sets of twins, some or all of the vital organs and bodily structures are shared.