Question: how are Siamese twins formed

Zena Hadjivasiliou answered on 16 Mar 2015:

Siamese twins are identical twins that did not divide properly during pregnancy. Identical twins start off from the same single cell, and normally they split into two different embryos after fertilisation. Siamese twins fail to split into two different embryos properly which means that two babies are being developed during the pregnancy but they never get properly separated.