There are various types of Siamese twins, some are born while facing each other, others are born with joined heads and others joined at the chest and abdomen.
In less critical conditions, at three months Siamese twins can be separated. though many survive, some die or one of them with less life dies.
Those joined at parts which cannot be separated are left to grow up as Siamese twins.
Siamese twins are born world wide with most cases reported outside Africa.