Conjoined twin births are extremely rare
What makes conjoined twins so interesting is not only their bodily malformation, but also the rarity of their occurrence. Conjoined births are seen once every 50,000 to 100,000 births, meaning they make up about 1% of identical twin births. When identical twins do not fully separate from a single egg, conjoining happens. Scientists are uncertain why a full separation does not occur in such cases, and it happens so infrequently as to make it difficult to study in depth.
Conjoining is not limited to humans; it has been observed in fish, reptiles, primates, birds, and other mammals as well. Nor is the phenomenon new to the human race: accounts of identical, conjoined twins have been found dating as far back as pre-Colombian ceramics made by ancient Peruvians, and one of the first written cases of raciphagus conjoined twins (joined back to back from the pelvis to the shoulders) was found in Isle-Brewers, England, according to the American Journal of Medical Genetics.