INTRODUCTION: Attention is drawn to the spontaneous incidence of twinning, both 
dizygotic and monozygotic in different mammalian species. Conjoined twinning, 
however, only arises when the twinning event occurs at about the primitive 
streak stage of development, at about 13-14 days after fertilisation in the 
human, and is exclusively associated with the monoamniotic monochorionic type of 
placentation. It is believed that the highest incidence of conjoined twinning is 
encountered in the human. While monozygotic twinning may be induced 
experimentally following exposure to a variety of agents, the mechanism of 
induction of spontaneous twinning in the human remains unknown. All agents that 
are capable of acting as a twinning stimulus are teratogenic, and probably act 
by interfering with the spindle apparatus.
DISCUSSION: The incidence of the various types of conjoined twinning is 
discussed. Information from the largest study to date indicates that the 
spontaneous incidence is about 10.25 per million births. The most common 
varieties encountered were thoraco-omphalopagus (28%), thoracopagus (18.5%), 
omphalopagus (10%), parasitic twins (10%) and craniopagus (6%). Of these, about 
40% were stillborn, and 60% liveborn, although only about 25% of those that 
survived to birth lived long enough to be candidates for surgery. Conjoined 
twinning occurs by the incomplete splitting of the embryonic axis and, with the 
exception of parasitic conjoined twins, all are symmetrical and "the same parts 
are always united to the same parts". Fusion of monozygotic twins is no longer 
believed to be the basis of conjoined twinning. Accounts are provided of the 
anatomical features of each of the commonly encountered varieties.