Obviously this is highly unusual. A person brought in more than 29 cats and kittens and one of the females was pregnant. She gave birth to 5 conjoined kittens. This happened about a month ago so I will presume that they have been separated. The animal rescue, at the time, needed donations to help cover the unexpected veterinary bill.
If you click on the link above the photograph I discuss the causes of conjoined twins which I hope is useful. Essentially, though, I suspect that even today the experts are not sure why this developmental defect occurs.
